if your check engine light is on you need to pull the codes..

 

Thanks, but NO CEL's unfortunately. The code through the DLC is giving only 12 (clear), nothing more.

 

Who did the timing belt? From what I've been told, those DOHC engines are a real bear to get right and if the belt is off by one tooth on one of the cam gears you could cause idle issues that can grow into bigger things.

 

Even after doing a bunch of work as detailed in a recent post I too have a slightly rough idle once the motor is nice and warm and I get the occasional random 100RPM or so variance. Mine is a 93 3.2 SOHC.

I've also been dealing with the same kind of starting problem. When cold in the morning it fires right up but when warm I have to crank it and it smells rich when initially starting. It also stumbles a little bit then catches and runs fine. I've also noticed that if I turn it off then start it within ten minutes or so it fires right up. Any longer than that and we're back in hard starting land.

I checked the fuel pressure with the key on and the motor off. I watched the fuel pressure slowly drop until I pinched the return fuel line. The pressure went up and held when I did that. From what I understand that behavior is indicitave of a bad fuel pressure regulator not holding pressure in the fuel rail. A couple of years ago I replaced the pressure regulator to no avail. The mechanic who recently replaced my cat told me to get an OEM one from the junkyard. I pulled one and installed it last week. The problem remains. Hopefully someone here has the answer.

 

Who did the timing belt? From what I've been told, those DOHC engines are a real bear to get right and if the belt is off by one tooth on one of the cam gears you could cause idle issues that can grow into bigger things.

I did, just put the new one in this week, job was really quite easy thanks to the many posts on this site for reference. The tensioner was making the death rattle so was time to swap that out. Was hoping that would sort out the idle issue - no luck.

godzilla - sounds like our engines were built by the same hand. The hard start description is dead on same with my symptoms. I'm starting to lean towards changing out the FPR again (will test pressure drop first). :?

 

See if the erratic idle changes or goes away.

recently (at 105K miles) the EGR on my 99 3.5L would stick fully open....sometimes.

Cleaning the pintle stem seems to have cleared it up. The pintle valve and face were clear and clean, the trouble was at full travel which is not commanded all the time.

Unpluging the wires is an easy check, though not suggested as a cure.

 

See if the erratic idle changes or goes away.

recently (at 105K miles) the EGR on my 99 3.5L would stick fully open....sometimes.

Cleaning the pintle stem seems to have cleared it up. The pintle valve and face were clear and clean, the trouble was at full travel which is not commanded all the time.

Unpluging the wires is an easy check, though not suggested as a cure.

Thanks, but no such luck with the '95's egr, it's mechanical/vacuum operated. It was cleaned, including pipe, new gaskets, and it tests out good.

 

It's possible your TBI throttle positioning senser is faulty bad spot. And maybe the idle control valve is all poo ed up.. After doing work on a trooper i had the idle was high and it ended up being the intake gasket for the butterfly...Maybe your intake has a vacume leek to..Just things to check. My 92 amigo i had with a 2.6 would run rough after shutting it off after driving it those have the bosch style warm up valve though. After a minute of running low RPM it would be back to normal. Sometimes i woundered if it was the brain .....

 

sure sounds like a vacuum leak or egr valve/pipe--pcv

run a compression test , everyone should always run a compression test on the engine to get base line data , even when it runs perfect , in fact thats the best time to test compresion. it will tell you a lot later .

 

I'd check your fuel pressure. I've seen that a lot in 2.6s when the fuel pump is not putting out consistent fuel pressure. I've dealt with that a LOT...

 

"I'd check your fuel pressure. I've seen that a lot in 2.6s when the fuel pump is not putting out consistent fuel pressure. I've dealt with that a LOT..."

Fuel pressure is 42 psi key on and holds within 2 psi for about 40 minutes, 36 psi and stable while at idle. The 42 psi with key on is within spec, but I can't find any information on what it should be at idle - even in the service manual. Anybody know?

"sure sounds like a vacuum leak or egr valve/pipe--pcv"

EGR tests as it should (apply vacuum, engine stumbles and stutters - if it was sticking open while idling, it shouldn't change any when vacuum is applied), and has been cleaned. All vacuum lines have been replaced, IMG's are new and intakes were torqued to spec. PCV is relatively new and seems fine. Also, if it was a vacuum leak, PCV or a compression issue, would it not be present ALWAYS at idle whether or not it was in neutral/park or a drive gear? (as original post notes, stutter is only present when the truck is idling and in park/neutral, idle is stable when in a gear OR in park/neutral and the A/C is activated)

TPS tested and is within spec, also tested for leaks around the TB gasket and didn't find any. It does not idle high, so it isn't getting too much air either, actually seems it is being suffocated briefly, causing the stumble.

You can actually "trick" the ECM into thinking it is in a drive gear by moving the selector half way between neutral and drive WITHOUT the transmission actually engaging -it is still in neutral. When you do this, the idle smooths out immediately and is stable. As soon as you put the gear selector back to neutral, the stuttering begins again, and usually more pronounced for a bit. So the ECM is adjusting something (I'm guessing air or fuel) based on whether it thinks the transmission is engaged or in neutral. I had a mechanic tell me once that the engine would just be rough when not under load, smooth when under load. If that were the case and it was a mechanical issue, than tricking the ECM to think the transmission is engaged when it actually isn't, and the engine not actually under load, shouldn't change anything - yet it does.

I tested this by unplugging the IAC while the ECM THOUGHT the truck was in a drive gear, than put the transmission in neutral - and the idle DID NOT CHANGE, it remained stable as if it was still in drive. Soon as the IAC was plugged back in and the truck was driven for a bit, back to the same symptoms. Doesn't this sound like the ECM is signaling for the wrong air/fuel mixture when the truck is in park/neutral ?

Also, can anyone verify if the oxygen sensor affects the ECM while at idle? The tests in the service manual all start with bringing the rpms to 1200, so I'm thinking it doesn't have an effect at idle. This model has 1 sensor, upstream of the cat.

Thanks for the replies, this issue is driving me crazy.

 

Ok, I'm not a specialist on this particuar vehicle but it sounds similar to a system on the 2.6s. There is a fast idle slenoid that will increase the idle when you turn the A/C on. Even if you don't even have an a/c compressor mounted on your engine the ECM activates the fast idle to help with the load of the A/C compressor. It sounds to me like there is a similar system here that is designed to raise the rpm when engaged into drive in order to offset the load of the torque convertor. My guess is that this solenoid or whatever it uses is causing the problem. Could be a vacuum solenoid that regulates the fuel pressure, or some other device that has a vacuum leak in it... Sorry I know that's not much help, but I agree with your diagnosis thus far.

 

Hi Atypicalv,
I just solved this exact problem in my 93 3.2 sohc Trooper. When in Drive or Rev the idle would be totally smooth with no disruptions or mini misfires in the idle speed. If I would shift into Park, all of a sudden the idle would get rough and start stumbling like it was over-rich. After ruling out the typical vacumn leak locations, I cleaned the IAC valve and it made no difference. I also adjusted the throttle plate clearance setting and still was unable to smooth out the idle in Park. Just to try something different, I went to the junkyard and found a near-new IAC valve and swapped it in and, suddenly the idle smoothed out and ran near perfect. In an attempt to get it as smooth as possible I readjusted the throttle plate clearance to different positions, and I was able to recreate the original idle stumbling condition in Park, and then by adjusting it again I was able to smooth it out totally. So, to recap: In an engine that has been checked out on all the other obvious things like ignition, vacumn leaks, etc, and that still has the very slight misfiring at idle, when hot, in Park, try:

One, replacing the IAC for a new one.

Two, adjusting the throttle plate stop screw for the best and smoothest idle setting.

If you only swap out the IAC without adjusting the throttle plate the idle still may not smooth out. Adjusting the throttle plate is what did the trick.

On further inspection and comparison of the two IAC valves side by side I found that the original valve had much more side to side play in the plunger than the newer one that I got from the junkyard, so that while they were both clear and clean of carbon, the older one was still not holding the correct position on a hot engine, when idling in Park.