Surges at Partial Throttle
 

1986GT, mods listed below. 167k on the motor.

If I try to hold the motor at 1200-1300 rpm, the car surges between 800-1600 rpm, when it's cold it does it quite severly, when it's warm, it still does it, but to a lesser extent. The car starts and runs great. Any ideas?

TPS voltage is .95v, IAC is disconnected, fuel filter was replaced about 5k miles ago. The car idles at 750-800, smoothly, no surging. Also, if I rev and hold the motor at 1700-1800rpm or more the car doesn't do anything out of the ordinary.

BTW--The with the IAC plugged in, the car bucks and idles too high most of the time. I've looked for vacuum leaks and found none. I have yet to try to pull codes.

Thanks for the help.

gotta be a vacuum leak! beacause that is what the iac motor does make a vacuum leak that is controled/check again maybe use a spray at the intake, could aslo be internal leak(intake)also the brake booster, listen under the dash push on brake pedal.

let us know what codes you get. i would really check out your EGR functioning......

hook a vac gauge to the vac tree and see what gives - lookin for a leak can be hard on a 5.0 - there are too many hoses out of sight (under plenum, etc). good luck.

I checked the vacuum about a month ago and got a reading of 18inches of vacuum, the needle held steady. The brake booster works well, without any noise. I've sprayed TB cleaner in certain areas to check for vacuum leaks--no luck. All the vacuum hoses appear to be in good condition--I thoroughly inspected every one of them I could find.

Thanks for the replies, keep them coming. I'll try to get codes pulled tomorrow.

The EGR is vacuum operated, correct?

I just got done doing a KOEO test with a test light of sorts. It gave me a code 11 which is a pass, it also gave me a code 32 in the continious memory. It seems I have an intermediate EGR/EVP problem. Do I need to replace the EGR valve/EVP sensor?

Thanks again for the help.

the EGR puts gasses from the exhaust into the motor to be recombusted. this cleans up emissions and cools the chambers.

quick test: pull a vac line off of something unimportant (i.e. dont use the line to the FPR). put it on the EGR with the motor running at idle. the idle should degrade once you apply the vac line. the valve is ok if the idle degrades. that leaves the solenoid, vac line to the EGR, etc.
if the idle does not change, that suggests the valve is bad or the ports are clogged. (the ports being clogged is highly unlikely).

Thanks, I give that a shot today.

I know for a fact that the EGR passage are clean, I cleaned it up when I did the TB swap about 2 months ago.

You should also disconnect and plug the vacuum line going to the EGR, and see if the problem goes away.

Just out of curiosity, why haven't you replaced the IAC?

I plugged the EGR valve into manifold vacuum and it darn near killed the car.

I also plugged my IAC back it and it seems to be working correctly. I think the symptoms I was experiencing with the IAC wasn't the IAC going south, I believe it was the fact that the base timing was set at 0*BTC. I haven't plugged the IAC in since I set the timing right....until this morning. I drive 60 miles a day to and from work so I'll find out tomorrow whether or not its working properly. It was cleaned thoroughly when I did the TB swap.

Well, according to your ECM, you still have an EGR issue, but you might want to reset the ECM and see what happens. Whenever you clean or replace the IAC, you must reset the ECM so that it can learn the new/cleaned IAC's tendancies. Otherwise, it controls it based on what worked before. So, since you should do that anyway, you would also erase the codes, giving you a chance to see if there are any fresh codes. Reset the ECM, and take it for a 10 minute drive around the block, and then check for codes.

Will resetting the computer eliminate all codes?

It will erase any stored codes.

I just got done resetting the computer and took the car for a test drive. It still pulled a code 32. It looks like I'll have to replace the EGR valve position sensor later this week.

BTW--The IAC is still plugged in and seems to be working properly.

Thanks again for everyones help.