Tell me when to blow it up :) Idle issue Still

[MadExplorer93]

A little more of an update, nice out this morning warm outside so i decided to test the MAF connector and the Relay Pump connector wires and ECC Relay Connectors.

I never started truck and battey Volt was 11.75v

This was KOEO:

The Relay Pump Connector wires in power box and the ECC wires showed proper voltage 11.75v for the power and ground part of it. The other two connections never showed anything, nut sure why, but maybe they don't till enigine was running, but how to test it afterwards when running?

I check the MAF wires and the Power was 11.68 RED wire and i grounded to Battery Ground at BAT terminal, The Ground wrie = Black Wire also showed similar result when i touched the RED/Black probing with my Volt Meter. The Tan/Black stripe wire showed 5.75V and the Blue/Black showed 0.5V.

I do believe those readings are correct therefore the MAF itself must be either malfunctioning or i may have a PCM issue not reading right. Remember i get a Relay Issue Code and a MAF reading under minimum amount.

If anyone can verify my readings please let me know?

Now i no battery was a low reading 11.75 even though vehicle starts fine it is a DEAD battery in the world of backyard mechanics as it should maintain at least 12.65v no prior starting.

 

[MadExplorer93]

Ok since no one has answered my last post before this post.

Can anyone tell me the proper Voltage for the MAF?

I for the heck of it cleaned the MAF again two days ago even though it looked clean already.

------------------

Also i have now "TODAY" changed the plugs to the MotorCraft FineWire Platinum Plugs that are for this vehicle and i changed the Air-Filter with a MotorCraft Air-Filter all was bought right from Ford Dealer. The Old Plugs seemed to be fine and with in a proper gap except two of them were a little more then .056, but other then that those Old Champions were still OK.

Now get this my Temp has be off for sometime now where the gauge only went just over the C on the gauge and halfway when hot, then back to C again. I chagned the Thermostat about a montn or so ago and one day the vehicle overheated cause i had a small RAD leak which allowed to much to leak out. I did not know until i filled it up again that it had such a leak. Anyhoo since then the TEMP gauge has never been at Halfway which normally before all this the (OVERHEATING) happened it would reach halfway then go to 3/4 way when hot and cool down to halfway, never, never has it been at C until that day when it OVERHEATED. I am thinking my new thermostat maybe no good now or the sensor may have gotton damaged.

What do you guys think?

-------------------

Ok back to the engine now and rough idle issue:

The Rough Idle is still there, but not as bad as when then Champions were in. The new plugs are all pre-gapped to .054 which i checked prior to putting them in.

I have not yet changed the fuel filter as it is to cold today to lay under the truck, so i hope it is nice out tommorow to change that.

So if it is not Plugs, Wires and Fuel Filter issue or COIL which i have not changed yet as it appears to be ok when testing, but i am not ruling it out. If it is not these itens then i am certain it maybe a Weak Fuel Pump or Bad Ground to it or maybe a bad Engine Cylinder Cmpression, but i get no BLOW BY, no burning OIL so Cylinders should be fine as i checked then last summer and yeah i know it can go bad at anytime and i will when it is California weather here in CANADA again check the compression unless i decide to freeze to do so..

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The new plugs no doubt made this Bat of Hell Truck go even better, smoother acceleration which it had before, but now i hit 80KM in no time at all, where as before i thought it was fast and it was, but now nicer then before... I despise champion plugs for FORDS..

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Also the Exhaust leak is still not fixed and remember it is before the O2 sensor on the passenger side where the Y-PIPE mates the exhaust manifold. This is a tiny leak, but can/could it cause a rough idle?

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[daves97sport]

but i am 100% certain it is not a Vaccum issue, but infact a Fuel or Spark related issue, but Spark would be pushing it. I am certain it is a Fuel Issue, but not certain 100% what or which it is, FPS or Injectors

I'm 100% certain that you're100% uncertain. (sorry-I couldn't resist...:scratch: )

Are the codes gone now that the Motorcraft plugs are in?
What did the old plugs look like? can you take a picture and post? Reading plugs is very helpful.

Yes the exhaust could cause an errant idle.

You still have not tested the fuel pressure... With the age of the vehicle, the o-rings on the injectors are most likely leaking. This is very common. IF you're wary of bad or clogged fuel injector, before you do the o-rings, try running a GOOD fuel injector cleaner such as Chevron Techron - run 2 successive tankfuls. Seafoam is good for carbon cleaning the intake system, but not as effective on injectors.

 

[MadExplorer93]

I'm 100% certain that you're100% uncertain. (sorry-I couldn't resist...:scratch: )

Are the codes gone now that the Motorcraft plugs are in?
What did the old plugs look like? can you take a picture and post? Reading plugs is very helpful.

Yes the exhaust could cause an errant idle.

You still have not tested the fuel pressure... With the age of the vehicle, the o-rings on the injectors are most likely leaking. This is very common. IF you're wary of bad or clogged fuel injector, before you do the o-rings, try running a GOOD fuel injector cleaner such as Chevron Techron - run 2 successive tankfuls. Seafoam is good for carbon cleaning the intake system, but not as effective on injectors.

I have not gotten any codes today, but it has taken 1-3 days before a CEL happens.

I can't test fuel pressure until i can get a gauge, so i am doing what regular maintenace it should have anyways = fuel filter and plugs/wires/air filter..

I did not do fuel filter yet as it is DANG cold out here in the north.

Yes O-Rings won't hurt to replace, i don't see any visible gas leaking on the intake, but can the bottom O-Ring leak more gas into the engine without a visible sign?

And yes your right it is a guessing, game so 100% i am not certain to what it is, but i am certain it is a Fuel Related issue..

Or

Like My Title Of This Thread States "tell me when to blow it up" ..

 

[MrShorty]

Can anyone tell me the proper Voltage for the MAF?

I can't remember what I posted, but I posted readings for my MAF for somebody else on here in the last month or two. He was having problems with his MAF. You might see if you can find that post and see if it helps you.

 

[MadExplorer93]

I can't remember what I posted, but I posted readings for my MAF for somebody else on here in the last month or two. He was having problems with his MAF. You might see if you can find that post and see if it helps you.

Ok i think i found the thread your refering too.

http://www.explorerforum.com/forums/showthread.php?t=202616&page=2

This is what member "btexpress" posted in there:

1 is battery voltage to the MAF
2 is MAF ground
3 is reference voltage (5.0V) 5.66 is a little high but acceptable
4 is the MAF signal, with the sensor disconnected or
the engine off this should be 0. With the engine running
it will read between .5V and about 4.0V
depending on engine rpm (and the sensor connected!).


And here is what i posted my readings were the other day and this is with engine off, key on and not started at all in morning.

A. Power was 11.68 RED wire and i grounded to Battery Ground at BAT terminal,

B. The Ground wire = Black Wire also showed similar result when i touched the RED and Black wire in the connector probing with my Volt Meter.

C. The Tan/Black stripe wire showed 5.75V and

D. the Blue/Black showed 0.5V.

------------

So basically i am getting good readings except that 5.75v reading like member "btexpress" mentioned to the other persons issue that 5.66v wa a little high, but acceptable.

The thing is is this proper readings correct or will it have an effect on MAF air/fuel mixture values coming from the PCM. Now if the MAF connector is fine and the MAF is clean and i get a %^&ing code again... .. i will then think about replacing the sensor.

I may beforehand check the PCM wires as i never have had a need to do so, but it would not hurt to have a look see for any corrosion.

 

[daves97sport]

but can the bottom O-Ring leak more gas into the engine without a visible sign?

The top o-ring will leak fuel if they are bad - but will usually evaporate too fast to notice. The lower is a "vacuum" and will suck air (and dirt) causing a lean condition, sending a signal to pump more fuel for the whole bank (left or right side). Did you examine your sparkplugs removed?

 

[MadExplorer93]

The top o-ring will leak fuel if they are bad - but will usually evaporate too fast to notice. The lower is a "vacuum" and will suck air (and dirt) causing a lean condition, sending a signal to pump more fuel for the whole bank (left or right side). Did you examine your sparkplugs removed?

I could not find any leaks when i tested the upper intake and lower portion which has the injectors in it. I used propane gas to see if there was any issues.

The plugs when i removed them all looked fine no oil on them, no coolant loss, not imporper burning, no rich mixture look, but two were past .056 gap and the rest were still at .054.

The new plugs (ford Finewire Plugs) are pre-gapped at .054 and i still have a tiny rough idle at 1500rpm idle speed, then as it reduces idle it becomes more noticible even when you sit in truck you can feel the little to rough idle like your in a rocket engine vehicle, more like a muscle car rumble idle and its not Loud and not severly terrible, but noticible rough idle.

So new wires (not fords) but good wires, new plugs, cleaned MAF again even though it looked clean from when i did it last time last summer, New Air Filter and still i have this idle issue, running hot or cold it is worse when engine is cold.

Now i am hoping to change the fuel filter today as it is a nicer day then yesterday and i will see if this limits any issues.

I have another theory that the tiny exhaust leak where the Y-Pipe mates the Manifold maybe getting extra air sucked into the last Cylinder on passenger causing the Dunk. Dunk sound i hear in exhaust when idling at 1500rpm and is not a consistant sound, but more or less wait 50 secs hear it, then nothing for 20 secs, then wait 60 secs hear it again, VERY LOW dunk, dunk and it is a Twice Dunk, Dunk sound not dunk, dunk, dunk, dunk, but only twice.

It can in worse case be a bad valve or pitted valve seat or valve, remember when i SEAFOAMED the intake it seemed to run fine, no roughness can be seen or felt, but 12 hours later on it showed signs again of this idle issue and stalling when first started in morning or even if you let sit for 2-3 hours and start it up it stalls out right away or at times after 2-3 min of idling it stalls out, but once started again it will continue to idle.

It is a guessing game, but i am doing over due maintence right now to see if it clears this little issue up. The past owner did not do so in a proper time frame and all these things i have changed thus far is routine maintence.

 

[MadExplorer93]

Ok so a day after doing the plugs and clearing codes, plus two days ago cleaned MAF again like i noted. Remember i stated it can take 1-3 days before i get a CODE to show, well again 24 hrs is when it did so.

I just got a CEL on again... i did not get to change my fuel filter today beautiful day it was, but i had other things to attend to that was more important.

I hope to try this fuel filter this weekend or when i have some free time this week.

I will try to pull whatever code(s) it is throwing today and post back when i do so.

 

[MadExplorer93]

These are the codes i pulled this morning after doing what i did two days ago (from today) with NEW motorcraft plugs and with NEW motorcraft air filter and MAF cleaning again even though it was clean.


These TWO are NEW CODES NOW:

118
ECT Sensor Circuit Above Maximum Voltage

116
Engine Coolant Temperature Higher Or Lower Than Expected

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These codes are like last time i checked and i got them again this time round:

172
HO2S System Indicates Lean

176
System Indicates Lean (Bank No. 2)


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These codes I ruled out from my relay testing i did:

543
Fuel Pump Circuit Open–Battery To Powertrain Control Module (PCM)

556
Fuel Pump Relay Primary Circuit Failure



-----------------------------

This code I ruled out as i pulled it off when running
and i raised idle a bit before hand:

114
Intake Air Temperature/Air Charge Temperature Higher Or Lower Than Expected

------------------------------

So basically by changing to the proper plugs from ford the temp will want to get to proper temp and like i mentioned earlier in this post that i had a coolant temp issue not working (to i know) this truck should be at.

It maybe possible the code 157 beforehand was the PCM with the champions plugs in wanted to steal voltage from the MAF and send it to the ECT, thats where with the NEW PROPER speced plugs from FORD the Finewire Double Platinum plugs the ECT has a high reading for voltage as the PCM tried to give it more to read right, but it failed to do so and the engine was WARMER/HOTTER from the new plugs, but the TEMP sensor was failing or had failed. The Champions being a COLDER plug would not get the engine warm enough to trip the ECT sensor code.

Basically after cleaning the MAF again even though it was cleaned before and still looked cleaned and i checked voltage at the connector while engine was off and key on, the voltage were OK and within range or acceptable range.

Ok, now i have not changed the ECT yet as i will check connection there first for any corrosion, but i suspect it is bad sensor based upon the overheating i had about two months ago, but never started to get any CEL's until this past few weeks for anything. I may even change the ThermoStat with a new Motorcraft one as the one in it now is not one from Ford, but it an after market one speced for this motor, but i want FORDS now in it.

I have not changed the fuel filter yet, but i am hoping to do so like i stated this week or weekend.


Here is something i found on line about what a ECT helps with when at proper temperature reading and quoted it from a PDF:

"The ECT responds to change in the engine coolant temperature. By measuring engine coolant temperature, the ECT knows the average temperature of the engine. The ECT is usually loacated in a coolant passage
just before the thermostat. The ECT is connected to the THW terminal on the ECM.

The ECT sensor is critical to many ECM functions such as fuel injection, ignition timing, variable valve timing, transmission shifting, etc. Always check to see if the engine is operating at temperature and the ECT is accurately reporting the temperature to the ECM."

So my lean BANK issue could very well be the ECT going bad or gone bad or bad connection to it or thermostat and ECT both no good...

 

[MrShorty]

A common "cause" of KOEO 116/KOER 114 is to perform the test on a "cold" engine. Were the 116/118 KOEO or CM codes?

It's unclear to me, are you still getting CM 543/556? Are you also getting them as KOEO codes? I just had the fuel pump die on my Suburban (which doesn't monitor the fuel pump circuit) and the codes I did get from the computer were O2 sensor lean codes. CM fuel pump codes in the absence of KOEO fuel pump codes could indicate an intermittent break in the fuel pump circuit. An intermittent break in the fuel pump circuit couls cause the fuel pump to cut out, which could cause fuel pressure to be too low, which triggers the lean condition. My '92 would occasionally give me a CM 542/543 that didn't make any sense to me until the morning it wouldn't start. I then found that there was a loose connection between the fuel pump fuse and the relay. Such a loose connection might not be apparent in a one time test of the fuel pump relay. If those are CM codes, and you've cleared CM and they have come back, I wouldn't rule out the possibility that you have an intermittent loose connection in the fuel pump circuit.

I also note that you don't report equivalent KOER O2 sensor codes. This suggests to me that whatever is causing your problem is an intermittent problem. This suggests to me that whatever diagnosis you do needs to be set up to account for the fact that everything looks right some of the time, and not other times.

When you've cleared codes and waited for the CEL to come back on, have you tried pulling codes before the CEL comes back on? Those fuel pump codes in particular won't trigger the light to come on, but they may show up in CM before the other problems do. It also might be useful to know if you get all pass codes (111's) before the CEL comes back on.

ECT could be contributing to the problem, yes. ECt is a simple thermistor and is pretty easy to check with an ohmmeter/voltmeter.

 

[MadExplorer93]

I did all testing with KOEO.

The relay codes is because i was going to due a fuel filter change and it to cold outside, but the code is because i pulled the realy out when the engine was running to bleed off pressure in line, which when it died with relay out. I then turned the engine over a few times to help bleed pressure, but it was getting cold outside and i said bugger it for now.


The only other fuel circuit code i ever got was the one where it stated the seconadry curciut was bad groudn at pump or relay. I checked the wires at the relay power BOX for the pump relay and ECC relay and all seemed fine. This then makes me think i have a bad or sticking relay in which i took another in ther power box to try and no codes until i did what i did when i was going to attempt to do a fuel filter change.

-------------------

I have not yet for a few weeks get the 111 code, all pass cause as you cna see i have had codes for such a time and tried eliminating them doing routine maintenance first.

-------------------

Now as for the ECT and temp sending unit sensor. This is my Theory i have a BAD thermostat again, why i think that, well simple of process of elimination and common sense based upon two things. Both the Sending Unit and ECT shows low temp, the GAUGE showing lower then normal and now the ECT complaining by a CODE that it is lower. I know this vehicle is not getting to its proper temp now, but this is because it is getting warmer here where i live and i thought before maybe cause if the colder climate the LOW temp reading was cause of it, but when it is WARM here like today was a nice again. The temp is still showing low and the ECT complained by a CODE to tell me so.

I have that ONE overheating about a month ago and i think the thermostat is not functioning 100% properly.

As i noted and your aware of it yourself that the ECT - PCM signal needs to have the proper TEMP sent to it in order to have the correct fuel mixture for the injectors.

Remember my rough idle, stalls still at a cold start (dies within 30-50secs) then i can start it again and dies, then start again and idles at 1500rpm and then down to 1200, then to 800, but once warmed up and i use the word warmed up loosely. The idle hits 600rpm, i get that surge i was talking about from 500-650rpm, it steadies out, then surges again.

This is where i was thinking clogged filter or injector and i used SEAFOAM in fuel tank and i also used it down the Intake. This was all OK until 12 hours later back to same old thing.

---------------------------

I have not tried to pull any codes before the CEL comes on, i may do so to see if any maybe lurking around, but i will have to clear the codes again before i must try it.

Now the thing is i want to do my filter first and what a PAIN the butt it is as i tried today and i can't get the line off. I need three hands, so i may get a shop to do it for me ... for $20.00 to do it thats not bad.

 

[Firestorm]

I could not find any leaks when i tested the upper intake and lower portion which has the injectors in it. I used propane gas to see if there was any issues.

Spraying ether, etc or using propane around the intake manifold doesn't necessarily rule out the gaskets if your engine's idle speed doesn't increase. Really, I don't see any good way of testing these gaskets.

Remember my rough idle, stalls still at a cold start (dies within 30-50secs) then i can start it again and dies, then start again and idles at 1500rpm and then down to 1200, then to 800, but once warmed up and i use the word warmed up loosely. The idle hits 600rpm, i get that surge i was talking about from 500-650rpm, it steadies out, then surges again.

These were the same symptoms I had back when I was having my nasty idle problem, I attempted to spray carbuerator cleaner around the upper intake. No idle increase. Have you attempted to re-torque your manifold bolts? Also, I wouldn't rule out replacing the gaskets here. You are receiving one lean code. This replaced most of my rough idle, except I'm idling at 500. Anyone know what could cause this?

I have another theory that the tiny exhaust leak where the Y-Pipe mates the Manifold maybe getting extra air sucked into the last Cylinder on passenger causing the Dunk. Dunk sound i hear in exhaust when idling at 1500rpm and is not a consistant sound, but more or less wait 50 secs hear it, then nothing for 20 secs, then wait 60 secs hear it again, VERY LOW dunk, dunk and it is a Twice Dunk, Dunk sound not dunk, dunk, dunk, dunk, but only twice.

I am having this same problem as well. Seems like the same noise "dunk" 3 times, this causes the Ex to vibrate a tad, then returns to normal. Seems like it may want to stall every once in a while .

-Nathan.

 

[MadExplorer93]

Spraying ether, etc or using propane around the intake manifold doesn't necessarily rule out the gaskets if your engine's idle speed doesn't increase. Really, I don't see any good way of testing these gaskets.



These were the same symptoms I had back when I was having my nasty idle problem, I attempted to spray carbuerator cleaner around the upper intake. No idle increase. Have you attempted to re-torque your manifold bolts? Also, I wouldn't rule out replacing the gaskets here. You are receiving one lean code. This replaced most of my rough idle, except I'm idling at 500. Anyone know what could cause this?



I am having this same problem as well. Seems like the same noise "dunk" 3 times, this causes the Ex to vibrate a tad, then returns to normal. Seems like it may want to stall every once in a while .

-Nathan.

Firestorm

- Your right the intake gaskets can still have a tiny leak not being detected.
- I did re-torque the bolts and no difference.

- The dunk, dunk sound as far as i can tell from my experience and research can be one of three things, more gas being put into the chamber and when it flies on by the exhaust and into the convertor the convertor ignites that left over gas = rich mixture. Two it can be a dirty valve, pitted valve, burned valve. Three an exhaust leak expecially where mine is which is at the manifold Y-pipe connection can send cold air into the side the leak is on, which happens to be the passenger side.

The Bank that the code is stating LEAN mixture is Bank 2 which is the Drivers Side.. werid i know it is.

This leads me to my TEMP issue not getting to proper 195F-197F and the ECT finally after doing the plug change to the proper speced platinum finewire plugs from FORD which run hotter then the crappy champion plugs, the ECT reported the coolant was lower or higher then expected. The ECT like i noted about has a big part in telling the PCM what to do with injectors, timing, valve timing and a few other things at the proper temp and if the vehicle is not getting proper temp, then this can cause a LEAN mixture and a miss like feeling as well as rough idle.

I have to change the driverside valve cover gasket sooner or later and i have all the gaskets for it including the plenum as i will be doing the passengers side as well. I then of course will be changing the plenum gaskets (two of them) and the Injectors O-RINGS as i have purchashed news ones from FORD for them even though there is not apprant leaks, but may as well do them for the CHEAP price they cost. This will also give a look see at the injectors themselves to see if any are clogged bad or damaged.

I still have not yet changed my DANG fuel filter as those lines as a PITA and i may have to bring it into a garage to do so, hey for $20.00 to do ... maybe i will bring it in, but i will be buying NEW CLIPS for the internal LINE connectons, they cost about $4.00 each and ya only need two of them.

Now your 500RPM idle is like mine when the motor kicks out on a warm DAY even today it is a nice warm day WINTER still, but warming up and the vehicle idles at 500-550rpm when in gear and i am not 100% sure if that is the correct IDLE in gear for our vehicles.

I was more like thinking 600-750rpm in GEAR and when in PARK 850-900rpm, but some say 750rpm in PARK and 650rpm in gear. I sure would like to see or for ppl here to tell us what their idle is when the vehicle is all warmed up and the CHOKE has kicked off (yes i know we don't have a choke, but the high RPM when it kicks out).

 

[MadExplorer93]

Just a small update i replaced the thermostat with a ford one and the TEMP seems to be much better so far, but time will tell and i mean within 12-24hrs as i cleared the codes again that may have been in there from previous time of checking.

I will wait it out and see, but yes the rough idle is still present, but like i mentioned we will wait it out and see if any or wahat codes come back. If the ECT code comes back i will look into the sensor more (That dang thing cost $127.00 CDN at FORD), but aftermarket one cost $61.00 hmm not sure if i want the aftermarket one as FORD is known to have certain specs for their vehicles.

If anyone has a good aftermarket brand for a ECT sensor, let me know and thats if i require one yet.

I have not incase some are wondering if i got that fuel filter done today.. NOPE, garage was busy today to do so.

I am plugging away at this and i will continue until i trace the bugger down, but until i can get a test on fuel rail and/or change fuel filter, i am stuck on anything else to do now.

I have pretty done much of all the routine maintence items (except fuel filter of course) and now it will be something else or i wait until filter is done and wait it out more for any codes if any.

Now the other thing i want to mention i have a FORD TECH CD installed on my PC and i looked up the fuel pressure for the 4.0L V6 motor and it states:

The fuel system is normally pressurized to 276 kPa (40 psi) and that all ford 4.0L V6 motors (The pressure should be 30-45 PSI ENGINE RUNNING) and (35-45 KEY ON, ENGINE OFF).

That is not what many have stated in here what they get as some have 20-30 PSI running and 40-60 PSI KEY ON ENGINE OFF for the V6 4.0L engine.

Any ideas exactly what it should be?

 

[MrShorty]

When mine has been working right, it is real close to 40 psi without vacuum applied to the FPR, and usually never below 30 with vacuum. While tinkering around once, I pulled the fuel pump fuse and let the pressure fall until the engine started to stumble, which was generally around 25 psi. From my experience, I would lean towards the stated manual specs.

When you pull codes, be sure to distinguish the KOEO codes (hard fault codes, the first set output) from CM codes (soft faults, second set output). SOP when resolving codes is to resolve the KOEO codes first, which is hard to do if you don't know which codes are the KOEO codes.

 

[Emerica19118]

my old explorer had the same missing problem...i could never figure out what it was...i always assumed i had bad valves...and in the end when i put her to rest i did

 

[morphius7777]

THis might be way off base but has anyone looked at the sparkbox or is this more of 95-01 X issue? I just replaced the box on '98 after getting some related symptoms for sometime before it crapped out completely.

Just a thought...

 

[jim_wrench]

I see this all the time at the dealership
and have seen the suggestions time and time again in this thread

replace the INTAKE MANIFOLD GASKETS......

they are like 30 bux at the dealership

get the ones on either side of the injector rail.....

just doi it and be done with it

disconnect your MAF and run it.... any better or worse

your getting the o2 codes due to unmetered air getting into your engine.....

dont go and waste your time or money on anything else.......

 

[Firestorm]

I have to change the driverside valve cover gasket sooner or later and i have all the gaskets for it including the plenum as i will be doing the passengers side as well. I then of course will be changing the plenum gaskets (two of them) and the Injectors O-RINGS as i have purchashed news ones from FORD for them even though there is not apprant leaks, but may as well do them for the CHEAP price they cost. This will also give a look see at the injectors themselves to see if any are clogged bad or damaged.

This is a good thing to do, preventative maintenance. You know that it is done.

The dunk, dunk sound as far as i can tell from my experience and research can be one of three things, more gas being put into the chamber and when it flies on by the exhaust and into the convertor the convertor ignites that left over gas = rich mixture. Two it can be a dirty valve, pitted valve, burned valve. Three an exhaust leak expecially where mine is which is at the manifold Y-pipe connection can send cold air into the side the leak is on, which happens to be the passenger side.

Luckily for me it sounds like like I have an exhaust leak. Really noisy at the bottom of the motor, I haven't really worried about it too much. Still baffled as to why it only idles at 500 at operating temperature.