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High Idle Issue

sroth004

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Hello all, I am chasing an idle issue on my recently purchased ‘94 which now has approx. 69,000 miles and am hoping the group can point me in the right direction. I did own a near exact vehicle in the 90s so I know the behavior is not normal. The symptoms and what I’ve already done to try and resolve the issue are as follows:

Symptoms
- When purchased in July I noticed there was no high idle at cold start up and that on occasion the truck was slow to return to normal idle when coasting down to 0 mph.
- Being Summer, the lack of high idle did not affect drivability too badly however now in Winter the truck will not run/idle on its own until warm and is still very slow to return to normal idle when letting off the gas. When warm, it hangs at about 1,200 rpm and eventually drops to about 600-700 rpm 15-20 seconds after releasing the accelerator pedal.
- I have noticed that resetting the ECU by disconnecting the battery seems to resolve the issue temporarily however over a few days the symptoms reappear incrementally, almost as if a sensor is faulty and the ECU is adjusting for the faulty sensor which then causes the starting/idle issue to occur.

Having researched the heck out of the issue on this Forum and elsewhere I and my local Ford dealer have done the following:

- I inspected, cleaned and ultimately replaced the Idle Air Control Valve with an OEM (Hitachi) unit. I did reset the ECU and as described above this resolved the issue for only a few days.
- I adjusted the idle stop screw and ended-up with it where it was originally; no change
- Dealer replaced the IACV again with a Motorcraft part as it believed the IACV I installed was defective; no change
- Removed and soaked the Throttle Body to clean; no change
- Neither I or the dealer tech can hear a vacuum leak, though it was not smoke tested and I cannot guarantee there is no leak
- There is no check engine light and both I and the dealer separately confirmed there are no Ford-specific or OBD codes

Any suggestions on next steps? MAF? Coolant Temp Sensor?

Thanks in advance.

Steve R
 
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Centaurious

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Look for a plugged passage behind the EGR valve. It should be almost 1/2" in diameter. If you can pull the codes and see if it goves any clues.
 
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Roadrunner777

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He said there were no codes...

But EGR is still in play because EGR activates with a warm engine.

There is a plastic tube from the MAF to the throttle body. Take that off and inspect it closely for any hidden cracks.

There are on-line test procedures for the MAF, you just need a multimeter.

And, there is a temp sensor for the ECU. When I hear engine temp related issues... so many threads have ended up replacing these, so it's always a suspect, and it may not throw a code.

You're close to fixing this, I think.
 
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sroth004

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Thanks guys, I forgot to mention that I had the dealer replace the EGR Valve which had no affect but will check for plugs as suggested and the MAF as well. No CEL but will also recheck for stored codes to make sure I didn’t miss any and that the dealer actually checked.

Also, it may or may not be relevant but the truck sat for about 18 months prior to my purchase with little to no use.

I am away until after the New Year and will report back. Thanks again.
 
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410Fortune

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is this a stick or auto?

You need to check for vacuum leaks
You can do a visual inspection around the base of the upper intake plenum and see if there is excessive dirt/dust/soot buildup. You are looking for leaks in the intake gaskets.

As @Roadrunner777 pointed out any un metered air entering the system can cause this. Un metered air is any air that was not "counted" on by the mas air flow sensor...so leaks in the intake tube between MAS and throttle body are very common on these old trucks, the intake tubes develop hard to spot cracks.
The computer will see a high idle and use the IAC to compensate, try to bring it back down.... this is why it may take time for the idle to "settle"
You ARE on the right path
Smoke test is your friend as is a very thorough visual inspection with flashlight. The PCV must be fully seated, you cannot have any cracked or missing vacuum lines, the automatic has a vacuum modulator that can cause vacuum leaks.....check for any air leaks! high idle = vacuum leaks 98% of the time.
After that the IAT sensor is suspect as is the coolant temp sensor for the engine (not the dash.......)

Happy New year see you in 2020!!!!!
 
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sroth004

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Thanks all for the suggestions. This is an automatic. It may be some time however I will report back.
 
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sroth004

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Guys,

Resurfacing a recent thread with an update and to seek additional guidance on my "no cold high idle, high warm idle and slow to settle issue". Spent a bunch of time capturing sensor data (MAF, ECU coolant temp, etc...) looking for vacuum leaks including the at the trans modulator, rechecking the throttle body adjustment, etc...and came up empty. All vacuum lines are connected and the lines look very good even though they are original (only 67K miles, garaged, zero chassis rust). Brought it to a shop that has experience with Fords in particular which was recommended to me, the shop owner followed all of the same paths with feedback to me that all sensors are working correctly, IACV is doing what it is being told, etc...but does not have a solution at this point. He's had the Ex for almost two weeks and it is a back-burner project for him which I knew so I am going to pick it up today and spend some quality time over the weekend and next week and see what I can see in particular to retrace my and his steps and follow Roadrunner and 410Fortune's recommendations again.

One additional item - just remembered and this may matter. This Ex was ordered for and lived in Salt Lake City with its one owner until I purchased it in July and moved it to PA - there is reference to "High Altitude Principal Use" on the window sticker received with the vehicle's documentation but I have also read on the forum and assume to be correct that the ECU will recognize the altitude/air density change and adjust accordingly.

Couple of questions:
  • Is my assumption about the altitude change being compensated for by the ECU correct or is a different MAF or other change required?
  • Would the "no high idle when cold" problem be caused by the items each of you have recommended I investigate or am I chasing two different issues?
  • The shop noticed a small coolant leak in the rear, likely a head gasket - could a head gasket leak cause the "no high idle when cold, too high and slow to settle to idle when warm" problem?
  • If I cannot find vacuum leaks including vacuum lines, the intake tube, etc...and decide to start throwing parts at the Ex, any suggestions on order of parts to replace?
Sorry for the long note, this project vehicle has taken on a life of its own and right now I am hoping for 500 trouble-free miles.

Thanks

Steve R
 
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MrQ

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Ahh, trials and tribulations of 1st gen ownership.

Bad head gaskets usually are internal leaks, however the freeze plugs on the heads can rust out causing coolant leaks.(Had that on my '94)

ECU's have different calibrations, but I highly doubt it could not compensate for the denser air at lower altitude. The MAFs are all the same. Throwing parts at the problem rarely fixes the issue and usually leaves you looking at an empty wallet.

410 is right about unmetered air; that can cause a big problem. Bad lower intake gasket can cause a vacuum leak and a coolant leak.

I had a similar issue. Found the CTS was out of spec, belt had slashed the passenger O2 wires, and the computer was bad causing it read low voltage off the MAF making it run like crap. Took 3 days of diagnostic work with a multimeter and my wiring guides. :banghead:
 
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Roadrunner777

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That's right, MAF's are the same, which is not to say that your's is ok. I don't know if you ever tested it. That said, here are some insomnia induced words on MAFs:

Mass Airflow Sensor (MAF) - The throttle plate allows a certain volume of air into the engine. But the actual amount of useful air (oxygen) varies with altitude or even weather variations in barometric pressure and air temperature. In an older carbureted engine, we just lived with that, or changed the carburetor jets to compensate. Then came computers and fuel injection. The computer, of course, allowed inputs such as throttle plate position sensors, air temperature sensors, coolant temp sensors and the like to calculate the amount of fuel to be injected. It would be handy to know how much actual usable air is coming into the engine too, so they added the MAF. If you examine an MAF, you will see two little rods strung across the inside of the body of the MAF. These are the heart of the MAF, so I'm going to ramble on about those for a while.

The MAF applies a certain amount of electrical current through those elements, and the elements are temperature sensitive. So, passing air cools the elements, and the MAF has to increase current flow to keep them at a certain temperature. The amount of current is proportional to the useful air (oxygen) passing through the MAF body. So, the MAF has an electronic package that handles all of this and sends the actual airflow, density compensated by design, to the ECU.

Which means... there are not different MAFs for altitude, the design of the MAF is intended to compensate for altitude.

Now, what can happen to a MAF? The design of the MAF assumes the two little elements are sparkling clean. If they are not, it throws the calculations off because it takes more current than it should to maintain their temperature. They can get dirty from the lack of air cleaner sealing, or some past problem where the engine was misfiring, causing fuel vapor to backtrack from the intake back to the sensor. You can clean them! You would need to buy a spray can of MAF cleaner (you need this exact stuff, other cleaners will damage the elements, it's available at car parts stores). Remove the MAF to a clean well-lit ventilated place, and examine the elements, ideally with some magnification. Use the solvent and a small soft brush to clean them off.

What else? the little electronic package in the MAF can fail. You can test all this by measuring the output of the MAF with the engine running, and there are procedures for testing Ford MAFs on the internet, such as this one: Part 1 -How to Test the Ford Mass Air Flow (MAF) Sensor

I don't think your issues are the high/low altitude business. The calibration change is most likely to fine tune emissions.

The IAC was a good candidate for this, I understand why it was changed twice, so we can rule that out.

Has the plastic intake tube been inspected for cracks? What testing has been done on the MAF? The ECU could be an issue. There are parts in it (capacitors) that age and cause problems.

You know what would be a novel cause for this... I know you have inspected vacuum lines. There's one though, it would be easy to diagnose and it lines up nicely with the cold/warm thing. This line ends up going to the air cleaner box, if I recall. A temperature sensitive element in the box sends the vacuum on to a dashpot when it is cold. The dashpot controls whether it is getting cold air from the front of the vehicle, or warm air from around the exhaust manifold. Something with the element, vacuum line to the dashpot, or the dashpot itself could fail in such a way that it creates a vacuum leak only when it is cold, of perhaps there is some obstruction in the tube from the exhaust keeping it from getting warm air. I don't know, that just occurred to me sitting here. You could plug the two vacuum lines at that sensor and see if it changes things.

Good Luck... here endeth the rambling, back to bed!
 
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sroth004

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Guys - thank you all for the insight and suggestions, certainly hope I did not cause your insomnia Roadrunner777. I've noticed in using the Ex over the past few days some refinement in my symptoms. It has (finally) gotten cold here in the NE and the Cold Start Idle appears to have started functioning with air temps in the low 30s, cold start idles at about 1200, and even more so as air temps dropped to the 20s with cold start idle at 1400-1500. I recall this is about what my first Ex did back in the 90's so I am wondering if the cold high idle issue is the coolant temp sensor and unrelated to the high and slow to drop warm idle.

When warm, the return to normal idle remains slow, essentially holding at about 1100 and dropping to about 900 and eventually 600-700 in 12-15 seconds while in Drive. The car will pull itself at about 8 mph if I release the brake so I know this is not right. What I've also noticed in putting the Ex in neutral when stopping causes the engine speed to jump to about 1400 and it then drops over 15 seconds or so to about 800. I know idle speed will be higher in Park and Neutral and assume 800 is about the normal level when not in gear but the return to idle remains the issue. The reason I've started shifting into neutral when I park is that the trans bangs into drive from reverse or reverse into drive if I do not give it the time described above for the engine speed to drop to a normal level. The transmission was rebuilt after I bought it over the summer and is shifting up and down well other than the above and is smooth if I allow the engine to drop to idle by sitting in neutral for a bit. Here's the question which may be ridiculous - does the transmission modulator have any affect on bringing down the engine speed to idle when the accelerator is released or does it only affecting shifting based on vacuum provided to it? As mentioned, shift quality is good when cold and warm.

I will be jumping back on this in the next couple of days and will report back. Thanks again for everyone's insight and assistance.

Steve
 
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massacre

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It’s been 10-12 degrees here and truck starts same as it always did.

Listen for prime, turn key goes up to 1100 or so then comes back down to 5-6000
When it’s cold it does take a little longer but only like 30 seconds
 
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sroth004

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Thanks guys, I'll be back at it a bit tonight.
 
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MrQ

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The 3 capacitors circled are the ones to look for (ignore the ceramic one, the electrolyte spewed out from the cap and just landed around it making it look bad). Pretty easy to replace. There is a "gummy" sealant over the circuit board. I used the soldering tool to melt it out of the way.

IMG_20191231_194209.jpg


I bought this kit off Amazon. It has the two types of capacitors you need.

The two lower capacitors have the 5 volt reference circuit run through them so if they go bad or break it will play holy hell with your computer reading the sensors. The upper one has a 12 volt signal. Don't know where that ran to.

My caps were so bad the legs broke off the board. And this all happened after a severe cold snap. Once I replaced them I managed to get my manual PCM to work (It was giving a no sensor reference value P code. Don't remember which one it was), the auto that was in it was un-salvageable.
 
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sroth004

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Thanks MrQ, will add this detail to the mix.

Steve R
 
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sroth004

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Gents - thanks again for the suggestions on my no cold high idle/warm too high idle issue; found small cracks and a hole in the air intake between the MAF and Throttle body and a broken tab on the MAF plug. The plug itself seemed to be correctly seated but who know, ordered both the plug and (plastic) air intake and am awaiting delivery. Also smoke tested the Ex and found no other air leaks; I have not ruled out any of the other suggestions and will circle back on once the air intake and MAF plug are replaced.

Regarding the coolant leak - found coolant leaking from the rear of both head gaskets, can definitely see runoff and sometimes active dripping. I have arranged for a shop to which I was referred and which the owner is "a Ford guy" for replacement of both head gaskets ($$$, ouch). The Ex has only 69k miles and zero chassis rust so I am hoping this expense proves worthwhile.

Will update on both issues in a week or so.
 
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MrQ

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Head gaskets aren't too bad on the OHV. Probably only take a few hours if you choose to do so yourself and save some money.

  1. remove the fan and shroud
  2. pull the upper intake, you don't have to pull the fuel rail, just leave it attached and disconnect the two quick disconnect fuel lines and the injector connectors (careful) and be careful with the egr tube
  3. drain the coolant and remove the upper radiator hose.
  4. remove the accessories and accessory brackets from both heads (you should be able to just unbolt the compressor without disconnecting the lines)
  5. pull the lower intake (might be a good time to replace the thermostat)
  6. pull off the valve covers
  7. unbolt the rocker rail (remember which side is which)
  8. pull the pushrods out and maintain the positions they were removed from (good time to inspect the rockers, purshrods, and lifters for wear. With 69k miles I doubt you will see any though)
  9. unbolt the exhaust headers from the collector and the head (pulling the fender shields makes this pretty easy)
  10. unbolt the heads from the engine (inspect for cracks or have magnafluxed) (You will probably get coolant in the cylinders and oil valley, so make sure you change the oil out after you run it for a little bit)
  11. scrape and clean all the old head gasket material off both surfaces
  12. Install new gasket with head (use new head bolts, they are torque to yield) following the tightening sequence and the correct torque values
  13. Install is reverse of removal, replace any old seals you took off. Make sure that you put a decent amount of rtv on the back and front of the oil valley sealing surface before installing lower intake use the proper torque sequence for reinstall (all the information is on here, I can also provide it as well).
 
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sroth004

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MrQ - Thanks for the additional details on head gasket replacement, will decide soon whether I will give it go or have the work completed by a shop. Although I have never done this before, having time is the biggest challenge right now as my 200k mile daily driver ('01 Nissan Maxima) suddenly started leaking a lot of oil, all of it trans gear oil, and brake fluid, all within a couple of days so I need to get this one done pronto.

By the way, the hole in the air intake was on the bottom in one of the crevices near where it would connect to the Throttle Body. It was very small, did not see it until the Ex was smoke tested.
 
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MrQ

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Those intake tubes are notorious for cracking or getting sliced by the fan. In my experience the holes are usually not enough to throw the AF mixture off too bad since it's unmetered air, but it's better to fix it.
 
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sroth004

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Gents,

I have been focused on things other than my X over the last few months (COVID, etc...) and am resurfacing my thread from a few months ago regarding a high idle issue on my '94 X, now at about 70K miles. I have followed the suggestions made in this and other threads (thank you again everyone for your suggestions), replaced the heads gaskets due to the coolant leak (fixed) and replaced the purge valve solenoid due a CEL code received during a sustained highway speed drive (also fixed). Also, I smoke tested the engine again and confirmed all vacuum leaks are resolved.

I am now focused on replacing the ECU, even as a trial in resolving the high idle speed issue. I did test the various sensors related to operation (MAF, IACV, etc...) and am get just over 5V to each (5.01, 5.02). I tested but did not replace the MAF however I did replace the MAF plug as it was was loose on the MAF (did not resolve the issue).

Here's the question: I found and purchased a Grade-A ECU with part number F47F-12A650-NB before checking the part number in my X which I found today is part number F47F-12A650-MB (I didn't realize how easy it was to remove the kick panel to access the ECU and did things a little backwards; I also got a "deal" on the NB unit). Both units are identified on RockAuto as Automatic Transmission, Federal Emissions for the '94 X. My questions are:
  1. Can I use the NB unit even though mine is an MB?
  2. What is the difference between the MB (mine) and the NB unit? My X has was originally from Utah and has "high altitude" configuration; it is now in PA. I assume the NB is an update to the MB but the related components, switches, solenoids, etc... do not change mid-model year so shouldn't the NB work in my X?
  3. Also, Roadrunner777 suggested I look at the vacuum lines/dashpot for cold/warm mix to the air filter housing - can someone either post a photo or let me know which line connects to which port on the top-rear of the air filter housing? I am wondering if I reinstalled the lines on the wrong port when disassembled to replace the air filter when I purchased the X last Summer. Also wondering if it would make a difference.
As an FYI, the NB unit I purchased was very inexpensive ($30) and I can use it as the core replacement for a rebuilt MB unit if I need to stay with the MB unit in my X so it would not be wasted money spent on the NB unit, only some lost time.

Any thoughts and suggestions would be appreciated.

Thanks

Steve
 
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