"Return to Idle" Issue

[nascar24jd]

I am having an unusual problem with my 1993 3.4L Firebird.

A few days ago while slowing down for a red light the car felt like it was still accelerating. I put it in neutral and sure enough the engine idle was up around 1800 rpm. After I stopped at the red light it took about 10 seconds for the car to return to a normal idle of around 900 rpm in neutral.

The car repeats this same problem on and off several times per day. So far I have fully cleaned the throttle body so there is no carbon build up in there. It was actually pretty clean to begin with. Last night I replaced both the IAC and the TPS (I had spares at home) and at first I though it was fixed. I drove it last night and it ran fine. However, this morning on my way to work it did it again. I have also checked the throttle plate itself as well as the linkage and that does not seem to be the issue. It seems to be related to the IAC sensor and/or circuit.

Any ideas what would cause this to happen? Other then the troubleshooting that I have already done, I am at a loss where to look next.

[tkoforpresident]

Re: "Return to Idle" Issue

check the IAC/TPS voltage, that will tell you if the sensors are still good.

any codes?

[nascar24jd]

Re: "Return to Idle" Issue

No codes, and 2 sets of sensors are doing the same thing. I am really thinking of buying a used PCM and swapping it out to see if it helps.

[nascar24jd]

Re: "Return to Idle" Issue

Can anyone explain the relationship between the vehicle speed and the IAC function? The reason that I ask is that I have narrowed this problem down a little bit more. When the car is in motion (25-30 mph) and I put it into neutral it idles at around 1600 rpm. When the car comes to a complete stop it takes about 3 seconds for it to return to a normal idle of about 950 rpm in neutral.

As a test, I drove the car with the IAC unplugged and this problem did not occur. When I put it in neutral at about 30 mph the idle dropped straight to 950 rpm.

It seems to me that must be some relationship between the vehicle speed sensor and the IAC.

I know that I should not keep running this with the IAC disconnected, so I really hope that someone can tell me where to look next. Just to clarify, no codes at this time...

Could it be a bad VSS???

[hockeyman]

Re: "Return to Idle" Issue

http://forum.camarov6.com/showthread.php?t=94101

Not sure where the IAC is on the 3.4's, but you should start your diagnosis there.

[nascar24jd]

Re: "Return to Idle" Issue

I sure do appreciate the help, but I have already done that. It is clean.

[auto666]

Re: "Return to Idle" Issue

is it a 5 speed
if so its supposed to do that for 2 reasons emmisions and drivability

[nascar24jd]

Re: "Return to Idle" Issue

No, it is an A4.

[Barbadian Camaro]

Re: "Return to Idle" Issue

Hmmm I read through this and what I find in the end is why would you think it is your VSS? when you say you unplug the IAC and the problem stop,well from my experience I when you replace the IAC and the problem seem to go but the next morning it's back I thought frist off that the IAC valve ain't seating properly or the actual motor which causes in the in and out movement of that valve is sticking,than in the Chilton Manual there is a procedure On when replacing the IAC valve what should be done for the computer to give a more stable Idle.

[nascar24jd]

Re: "Return to Idle" Issue

Hmmm I read through this and what I find in the end is why would you think it is your VSS? when you say you unplug the IAC and the problem stop,well from my experience I when you replace the IAC and the problem seem to go but the next morning it's back I thought frist off that the IAC valve ain't seating properly or the actual motor which causes in the in and out movement of that valve is sticking,than in the Chilton Manual there is a procedure On when replacing the IAC valve what should be done for the computer to give a more stable Idle.

Interesting. I do have both the Haynes manual and the Chiltons manual. I will read into that when I get home today. Thanks for the help. The reason that I am thinking it could be related to the VSS is because it only idles high while in motion. If I rev it up a bit in Park it drops back down to a normal idle right away. The problem only occurs while the vehicle is in motion.

I think after work today I am going to re-clean the IAC and port that it sits in. I will use some deep creep and a brush in order to clean it out really well. Maybe that will help a bit if the IAC is the issue.

[Barbadian Camaro]

Re: "Return to Idle" Issue

Just keep us inform :cool:

[nascar24jd]

Re: "Return to Idle" Issue

I wish I had good news, but I do not. It is as clean as possible and the problem remains. I did test the IAC with my ohm meter and it checks out fine. I am starting to think PCM???

[Barbadian Camaro]

Re: "Return to Idle" Issue

This may not really help,but something to try in the chilton manual read in Emission Control Pg 4-35 to Pg 4-37 dealing with the Idle Air control valve and the procedure I was talking about is on Pg4-31 Idle Learn Procedure

[nascar24jd]

Re: "Return to Idle" Issue

This may not really help,but something to try in the chilton manual read in Emission Control Pg 4-35 to Pg 4-37 dealing with the Idle Air control valve and the procedure I was talking about is on Pg4-31 Idle Learn Procedure

I did go back and read the book, but it says to do it exactly the way that I had already done it. I am going to disconnect the battery tonight just as a long shot and let the whole system relearn everything. Maybe by some slim chance it will help with this issue.

If not, I have been looking on ebay for a PCM. They have several on there, I just need to check mine to make sure that I buy the same exact one. I have heard that the service code and programming code must match the one that you are replacing or it will not function properly.

[tkoforpresident]

Re: "Return to Idle" Issue

I did go back and read the book, but it says to do it exactly the way that I had already done it. I am going to disconnect the battery tonight just as a long shot and let the whole system relearn everything. Maybe by some slim chance it will help with this issue.

If not, I have been looking on ebay for a PCM. They have several on there, I just need to check mine to make sure that I buy the same exact one. I have heard that the service code and programming code must match the one that you are replacing or it will not function properly.

yeah you need the exact same service # and belive me, ones for the 93 are hard to find.