cold idle good hot idle bad... (?!?)...

[technopimpninja]

howdy folks, sharing this one with the masses in hopes of getting a little help from something that's stumping me...

97 4.0 sohc
cold starts (after 8 hours of cool down) are fine...

sometimes on hot start, rpm's oscillate 1200-500, 1100-600, 1000-700, then level off around 900 or 1000

sometimes on hot starts it oscillates from 1200 back to zero VERY quickly and dies... will do this over and over, unless i repeated tap the gas pedal lightly...

putting the car in gear in both instances above pretty much kills the motor - UNLESS... i (again) repeatedly tap the gas pedal lightly - raising the rpm's to around 1500 -basically, i have to keep feathering the pedal to keep the engine going or it will die... once i'm up to say 5 mph, i don't have to do any pedal work... but rolling up on a stop sign, definitely requires throwing it in neutral... (too bad it's an automatic)...

ok... so here's what i've done... last three weeks running...

1. installed two new upstream oxygen sensors
2. installed a new iac valve
3. installed a new fuel filter
4. replaced one cracked vaccum hose

next i'm going to:

1. clean the maf (or get a new one if i can find one reasonably priced)
2. replace the spark plugs
3. look again for any other vaccum leaks

keeping in mind that driving through a parking lot or on the highway at 55mph (AFAIK) everything sounds and works just fine... it's only when (again after it's warmed up) if i'm not stepping on the gas pedal it will take a nose dive and die or idle roughly...

any advice?

 

[Howard]

I assume that you are not getting any codes. So my bet would be intake gaskets.

 

[technopimpninja]

Originally posted by Howard
I assume that you are not getting any codes. So my bet would be intake gaskets.

the original codes indicated both banks of upper oxygen sensors were bad... (replaced them and cleared the codes)

also fyi folks, i have cleared codes/computer after each repair as listed in my original post... (can never provide enough info - if u ask me)

i thought it could be the intake gaskets - but i have had that problem before - and this seems different...

typically with the lower intake gaskets you experience 'cold engine' start probs - here i have none... runs great when cold in fact...

also of note is that this problem is intermittant...

meaning that 25% of the time, i start it when it's hot and i can put it in drive/reverse gear without having to immediately start tapping the gas pedal...

again, any help would be appreciated...

many thanks....

 

[Nate1]

Plugs seem like a good idea... I wonder about your injectors....
Mabey check your fuel pressure?

 

[technopimpninja]

Originally posted by nweibley
Plugs seem like a good idea... I wonder about your injectors....
Mabey check your fuel pressure?

i cleaned the maf this evening... had a little brown buildup on the "filament like" wires... still having the idle problem...

i'm going to put in the order for a replacement maf in the morning... and change the plugs on wednesday...

i honestly don't believe either of these fixes will fix the problem...

anybody else have any ideas as to what could be causing this?... anybody?...

 

[Ronin8002]

I had a problem that sounds similar to yours, except mine wasn't cutting out, just idling really low and rough w/black smoke coming out of the tailpipe when it did it. I replaced the fuel pressure regulator and this solved the problem. Don't forget to depressurize your fuel system before replacing it if you try this.

-James

 

[technopimpninja]

Originally posted by MaximumViolence
I had a problem that sounds similar to yours, except mine wasn't cutting out, just idling really low and rough w/black smoke coming out of the tailpipe when it did it. I replaced the fuel pressure regulator and this solved the problem. Don't forget to depressurize your fuel system before replacing it if you try this.

yeah... i think i'm going to go ahead and buy a fuel pressure gauge tomorrow...

this evening i Dead Link Removed on the engine -- and everything appeared to come out ok... that was a huge relief (provided i did the testing correctly)...

i'm really leaning towards either a MAF problem or fuel pressure regulator... and thanks for the reminder on depressurizing... i've taken that shower before!... not fun...

fwiw, tomorrow i'll be changing out the PCV valve and installing a new MAF...

i know i need plugs - but c'mon... if i had a fouled plug, i would have other symptoms besides low rough/low idle when hot wouldn't i? so plugs will get their turn on thursday...

again... any advice/etc., would be appreciated...

hank

 

[gerry]

Hank,

Since the problem is worse when warm, I would start by assuming it is related to going into closed loop mode. A few possible scenarios:

1) "downstream" O2 sensors are either bad or seeing a leak in the exhaust system, giving you a large long term fuel trim value. Resetting the stored values in the fuel tables "MAY" help diagnose this by temporarily eliminating the problem until the tables are rebuilt.

2) Bad injector or fuel pressure regulator failed open causing an injector to leak by. When you're warmed up in closed loop, the O2 sensors try to compensate by leaning out the other injectors. (Is exhaust rich / black when you first start it cold?)

3) Vac leak (What did you mean by "looked OK?") If you can keep it running, try the (ballsy) test of spraying propane around the intake and seeing if RPMs rise.

Make sure you look at the plugs and compare them, both between each other and, overall for a rich/lean condition.

Try to borrow a scan tool to monitor the sensors while driving (not just look for codes). At least try to monitor the O2 sensor voltages, even with a voltmeter if you have too.

Hope this helps.

 

[technopimpninja]

1) i thought downstream O2 sensors can't influence the ECU when it comes to fuel/air mixture... they can? and how do i 'reset the fuel tables'?

2) i'm not having any black exhaust on cold startup... tho i did have 'too lean' on both upper O2 banks (prior to replacing the IAC)... wouldn't i be getting a check engine light if an injectore failed open? or perhaps more symptoms?

3) i did vacuum test that's in haynes book - connected a std vacuum gauge, started up the engine, watched it like a hawk (only VERY minor and slow fluctuations up and down)... warmed engine up... then quickly ran up to 2500-3000 rpms and let off the throttle watching changes in vacuum - changes all along the curve, were within acceptable ranges printed in the book...

i'm almost certain it's not a vacuum problem b/c the engine runs just fine in park/neutral - always when cold and as well as when warm (since cleaning MAF)...



from the top (for those of u just joining us)... here's are the latest details of my X's woes...

1. the engine rpms are typically very low (below 500) while at a stop and in drive/reverse gear AND the engine is warm

2. sometimes when the engine is hot - the rpms oscillate in park/neutral as described in original post - then it dies when i put it in drive/reverse while stepping on the brake (tho this hasn't happened since i cleaned the MAF) .

3. cold idle is just fine - no shakes... hot idles (again, since MAF cleaning) are consitently smooth - around 1000 rpms...

5. it's just when i put it gear, and i'm on the brake, that the rpms go from 1000 to 500 and below... again, sometimes it just nose dives and dies right away... but, if i put it in gear and IMMEDIATELY give it gas, then the engine will keep running... so, any speed from 5mph on up is no problem... but if i roll up on a stop sign, when the engine is hot, keep it in gear, and brake to a complete stop - it will idle REALLY low or just shut off all together...

this is my 7th layer of hell... driving an automatic - like a standard... until i get it fixed...


all input is appreciated!

best,

hank

 

[Howard]

Just a very off thought. Could it be linked to the brake booster causing a vacuum leak when the pedal is pressed. I can't for the life of me think how but like I said it is just a thought.

Edit: I just realised. If the brake booster diaphram had a hole in it that would cause a big drop in vacuum when the brake pedal moves the valve to open and possibly your problem. You can test it with the vacuum gauge.

 

[Nate1]

Dont rule out bad plugs. If one of your plugs has a somewhat damaged electrode, I could see how it would take the engine running in the more efficient closed loop mode to make it show. Then again it seems you would get a few misfires when putting load on the engine, so I cant really pin it down. Definatly check fuel pressure, also check to see how long it sustains when you turn the engine off, if it leaks out after a minute or so, you probably have a fuel leak (I believe that is the allotted time anyways). I don't really know what to tell you... my SOHC is idling rough too, but I'm too lazy/cheap to spend money trying to hunt it down, I'm waiting for a CEL to give me some more info.
Good luck!
-----Nate

 

[TPLYNCH]

From your most recent description of the problem, I'd say your torque converter is bad. It may not be unlocking (I think I'm using the right terminology) and causing the engine to stall, like it would on a standard tranny if you don't press the clutch when coming to a stop in gear. Does that make sense???

 

[gerry]

Hank,

1) The downstream O2 sensors input to the air/fuel ratios that gets stored in the tables in the computer. It is these values that get erased when you disconnect the battery for any length of time. These tables are updated during the first 15 minutes of operation of no load conditions for 1) neutral w/o AC, 2) neutral w/AC, 3) drive w/o AC, and 4) drive with A/C on. If the tables are already full, then each start cycle will only have influence on a small percentage of the existing value. If they are empty (reset by disconnecting the battery) the first start cycle will input 100% to the tables. that's why it's best to reset the computer whenever there is a sensor change. So, if the no-load drive values are saturated at some false value, then going from neutral to drive will have the ECM immediately change its outputs based on the incorrect data in the tables, causing a stall. Since you immediately hit the throttle when going into drive, there is no new "no-load drive" time for the data for the computer to obtain and update the tables. So the bottom line is to disconnect the battery for about 30 minutes, reconnect, and WITHOUT TOUCHING THE ACCELERATOR, start the car. Let it idle for 15 minutes, then WITHOUT TOUCHING THE ACCELERATOR, place it in drive and and let it idle again for 15 minutes. If it dies, restart using the same 15 minute procedures. Just don't touch the gas.

The TPS determines when you're in "no load," it's a long shot, but I would check the closed throttle TPS voltage.

2) No, you might not get a code if an injector was leaking by. Check the fuel presssure while at idle and check the leakdown rate when you shut it off.


After re-reading your summary, I agree that the torque convertor could be locked up. Can you feel it locking up / releasing while driving? Try disconnecting the wire going to the TCC (not sure off hand which one it is) and see if you still have the problem. Make sure it's not stuck mechanically locked up by watching rpms with the wire off while above 50 mph under different throttle positions.

I'm still suspicious of the IAC. Did you clean out the ports in the throttle body when you had the IAC removed? Can you swap out the new IAC with another in case you got a bad one? Also let us know what the new MAF does.

Hope this helps.

 

[drwambo]

I gotta agree. Everything you describe is a symptom of a bad IAC......

I know you replaced it, but bad parts happen to good people!