Diagnosing my idle problems

[mrl390]

Well I spent a good bit of yesterday afternoon with a guy I work with trying to diagnose why my cars idles so poor, misfires and starts hard every now and then. We checked just about everything we could with the scan tool. Checked the TPS, its fine. Checked the coil packs, they are fine. Its got new plugs and wires, PCV valve and new intake gaskets. Checked the pressure at the fuel rail and it is right in spec. Also opened each injector one by one with the scan tool so they all open. Im not sure if one may be hanging open or not tho. We are getting a rich reading on the LT fuel trim but short term isnt unusually high?? The computer is telling the engine to idle down correctly but it will only go as low as 900rpm when it calls for 800.

So recap, running rich, idling poorly and intermittant hard start.
Our thoughts on problems were either an injector stuck open, a bad O2 sensor, vacuum leak or carbon buildup in the IAC keeping it from closing all the way.

What are your guy's thoughts? Of the above what sounds like it could be the problem or what else can you think of we are missing?

97 monte 3.4 dohc

[walterdude]

Typically a vacuum leak will cause a lean condition, tho it could raise the idle. The other two might act like that.. How about the coolant temp sensor?? Sometimes when they fail it will tell the ECM that the engine is cold and add fuel/rpms..
Hope this helps,,,
Tom....

[mrl390]

Yeah I thought about the CTS as well but what gets me is that the computer is telling the engine to idle down correctly. Its setting the target idle at 800 but for some reason its only going down to 900. I would imagine that if the CTS was telling the computer to add more air/fuel, it would be setting the target idle higher than it should be, but its not. Thats what led me to think maybe the IAC is hanging open slightly. I dont know if that would cause the rich conditon or not tho so thats what make me unsure whether the IAC is the problem or not. God I hate cars. Funny that I chose to work on them for a living. At least I do bodywork and dont have to do this kind of stuff all day.

[mrl390]

Cleaned the IAC and changed the rear O2 sensor today. No change. Im stumped.

[walterdude]

Do you know if there's a procedure for resetting the IAC on a OBD2 car?? I've had to do a reset on my 93 Z34, which has OBD1. I "think" it was something like a key on/off type of sequence... Most of my experience has been with OBD1, altho I also have a 99 Lum with OBD2... Hopefully somebody else chimes in... I just got home,, maybe later I'll dig out the Manual and see if there is a reset sequence for OBD2... If you need that is.... If you feel like looking through my posts I DID post the "How-To" in the past..
Keep us posted, you're getting me interested!!!
Luck,,
Tom...

Maybe you should contact pocket-rocket, Jon, He's one of the die-hard LQ1 fans on the site and usually has some good input...

[mrl390]

Thanks for the info. The IAC is operating correctly, we tested it the other day. The scan tool we were using has the ability to cycle the IAC open and closed. I was just concerned that it may have had carbon build up keeping it from closing completely. Unfortunately there wasnt anything holding it open but I gave it a good cleaning anyway. I wont have time to mess with the car for the rest of the weekend. Hopefully pocket -rocket jon will stumble across this thread. If not maybe Ill message him toward the end of the weekend. Thanks for the help walter. I appreciate it.

[pocket-rocket]

Honestly, this one has me stumped

I've been trying to think of anything I can that would give this kind of situation and comparing it with what's been described and can't come up with anything. But now that I think about it, you say you changed the rear O2. When you say rear O2, do you mean the one after the cat? I'm assuming no, since it looks like you guys are doing a good job of troubleshooting the problem and have a good grasp on how the system works as a whole. The reason I ask is the rear O2 has very little to do with fueling like the front one. Also what kind of plugs did you use? The vacuum leak still sounds like a possibility, except for the running rich part... If it makes you feel better (which it probably won't), my car rarely hits the exact target idle while warming up (sits around 100 off, like yours). I mention warming up since it idles fairly smooth and then starts the 13* lope idle as it warms up. Also, was the CTS reading in the scanner on par with what it should be as far as coolant/gauge temp?

[mrl390]

I changed the rear O2 sensor (downstream) only because of how it was reading on the scanner. It was jumping around while the upstream sensor was reading pretty steady. Im not completely sure how they are supposed to look on the scanner tho I dont think either of them are bad honestly. For what I payed for the sensor, it was worth it just for shits and giggles. I didnt check the CTS on the scanner, but I did see when we first hooked it up, it was reading a temp of 125*. I didnt look at it after it was warm only because the computer was telling the engine to idle down. We reasoned that if it were reading a low temp it would still have been telling the engine to idle high. I can always check it with the specs on ShopKeys. BTW where is the CTS if it does end up being the problem? Am I in for a treat if I go to replace it? On a side note, I pulled the vacuum line off the FPR while the car was running and didnt get any fuel leakage. Just a healthy sucking from the vacuum line.

[pocket-rocket]

It was jumping around while the upstream sensor was reading pretty steady. BTW where is the CTS if it does end up being the problem? Am I in for a treat if I go to replace it?

A good O2 will swing up and down rapidly as the PCM is constantly adjusting the A/F ratio to try to maintain it's target ratio. A steady reading tells me it's time to be changed. The CTS is behind the t-stat in the lower intake.

[gtptuner]

I have a freshly rebuilt 3.4 and I have almost the same idle issue . its gotten better with all the vac lines and air temp sensors being reinstalled but still only drops to an idle for 1300 to 1100 rpm and burns rich ...