95 3.4 DOHC idle surges

[thehood]

I have a 95 Lumina, 3.4 DOHC. The motor starts and runs good, but after it warms up to normal temps the idle surges up and down slightly.
When I drive it down the road and come to a stop sign or stop light the idle surges up to 1500 rpms and back down to 300 rpms repeatedly until it stalls. If I tap the gas pedal it will level out while at a stop. Other than that the car cruises.

Also, I cannot locate the map sensor. The vacuum diagram under the hood does not show one.

[jeffcoslacker]

A lot of things can cause them to hunt for idle speed, you are gonna get a lot of answers for this one. The fact that it works fine until warmed up and in closed loop operation does indicate that the problem is computer feedback related. Oxygen sensors, IAC, MAF, MAP, CTS, TPS are all suspect. Hope someone can narrow it down for you.

[jeffcoslacker]

Does it do it at idle in nuetral? If so, I'd try giving the TPS a little whack while it idles, and see if it straightens out. If so, that's what I'd replace.

[richtazz]

if beating up the tps doesn't work, which I'd try first, try cleaning the IAC valve.

[thehood]

I have cleaned the IAC, but have not tried beating on the TPS yet. I will give it a try later today.

I also found that when I unpluged the vacuum line from the purge valve located on the back side of the motor, that runs to the back of the car to the cannister, the idle levels out. because of that I replaced the purge valve. It didn't help.

The service engine light has not been on at all. could there still be codes stored? Also, to retreive the codes I know you can use a jumper on A and B terminal. This car does not have a B terminal and the book recommends that you not jump them on this year model with the 3.4 DOHC. Even at Auto zone they said they have to have an adapter for this car. Any help would be greatly appreciated. This motor has been very difficult to find info on.

[jeffcoslacker]

Originally Posted by thehood

I have cleaned the IAC, but have not tried beating on the TPS yet. I will give it a try later today.

I also found that when I unpluged the vacuum line from the purge valve located on the back side of the motor, that runs to the back of the car to the cannister, the idle levels out. because of that I replaced the purge valve. It didn't help.

The service engine light has not been on at all. could there still be codes stored? Also, to retreive the codes I know you can use a jumper on A and B terminal. This car does not have a B terminal and the book recommends that you not jump them on this year model with the 3.4 DOHC. Even at Auto zone they said they have to have an adapter for this car. Any help would be greatly appreciated. This motor has been very difficult to find info on.

I think I'll refer you to richtazz for this one, that motor's a mystery to me for the most part. I'd usually run the other way when one came through my shop.