My 85 TPI has a real inconsistent idle. Usually it's very rough. Occasionally, it's very smooth. Different brands of gas sometimes make a difference, but not always. Sometimes it's better when its raining. Any ideas?

 

Dave

Could be a sticking EGR valve, dirty IAC valve, vacuum leak. Have you changed your gas filter? Could there be water in the gas tank? These are all things I have done to get my idle even.

Walt

 

could also be your TPS voltage. I set mine at .58 and it made a real big difference in idle quality. I also reccomend taking your IAC apart and thoroughly cleaning it.

 

Look at your vacuum lines. Make sure you are getting 20# of vacuum. I also have an '85 and just replaced all vacuum hoses and boots and it made alot of difference. I would look next at the throttle body and IAC. Clean both if it hasn't been done in awhile. In the "Tech Tips" section of this site there is a post on setting TPS and IAC. I believe it also tells you how to clean the throttle body.
Good Luck!

 

The EGR is new, and I don't think it's a vacuum leak. I've looked for a vacuum leak before by pinching off all the hoses and spraying carb cleaner all around the intake. Didn't find anything. I've never cleaned the IAC or throttle body. Might try this next. Might also try setting the TPS at .58. Can't hurt, can it?

 

check your codes and see if one comes up, i'm voting for bad IAC or fuel filter, but MAF's can cause similar problems when they go bad and they usually throw a code. Is it missing at all?


[Modified by vader86, 7:35 PM 1/4/2002]

 

It's not missing, and it has a new fuel filter and fuel pump. Fuel pressure is OK. There aren't any codes, either. I get the feeling that its dirty fuel injectors/throttle body/IAC, etc. Still haven't totally ruled out a vacuum leak, even though I can't find a leak.

 

Here's the troubleshooting chart from Alldata:

Rough Idle, Stalling

*Check Ignition timing and ensure ignition system components are satisfactory.
*Check for binding or sticking TPS and throttle linkage.
*Check IAC valve and control system.
*Ensure charging system output is within specifications.
*Check park/neutral switch circuit.
*Perform injector balance test.
*Check operation of EEC system and canister purge control.
*Check power steering pressure switch circuit.
*Check EGR system for condition causing valve to remain open or partly open at all times.
*Inspect oxygen sensor for contamination. Contaminated sensor will cause ECM to reduce amount of fuel delivery.
*Check Ignition timing and ensure ignition system components are satisfactory.
*Check for fuel in pressure regulator signal hose. If present, replace regulator.
*Inspect duct between MAF and throttle body and repair as needed.
*Disconnect MAF sensor electrical connector. If condition is corrected, replace sensor.
*If problem exists with A/C on, check A/C request, A/C signal and IAC circuits with SCAN tool.



[Modified by Anguish, 1:13 PM 1/4/2002]

 

Dave, I think you are well on the way to solve this. I would pull the TB if it's never been done, they get coked up pretty good. We have had varied experience with cleaning the IAC on this board, some get results and some don't. The IAC is not expensive and while you have everything clean why not replace the darn thing?

I also like to buy the gasket set for the TB. Note that the torx screws on the cover with the coolant in it may be very hard to get out. If you do this, soak them for a long time and work them out slowly - I had to buy new bolts for my 89 as they were shot.

Do read up on the tech tip that Lars submitted on setting minumum idle when you re-install.

One thing to check if you pull the TB. The shaft that holds the butterfly valves is steel and pivots through the aluminum TB case. There is not bushing surface. Slop develops at these points that allows the valve shaft "****" and it will return to various amounts of "open" . I had to replace my TB for this reason after not being able to get the idle to my likeing. The new TB had 0 movement of the shaft within the TB, while the old one had a lot of movement. Check for this if you pull it, it's a little difficult with the spring pressure, but you can tell. Could be a carb shop could put bushings for you, but I couldn't find anybody around here that wanted to mess with it.

Finally the TB bolts need to be re-torqued after a few hot / cool cycles, they will work their way loose!

Good Luck,

Pete

 

Thanks for posting the tips.

I had a similiar problem this past summer...to the extent the car would occasionally stall. The vette shop guy couldn't find anything wrong but he did clean the IAC.

Now, the alternator just went. So I suspect it may not have been putting out the required voltage of late.

According to this list, improper output could be the cause...in the end that may have been my problem.

Thanks.

Jeff

 

Great tip Pete. I'm glad I read that. I rebuilt mine last month and haven't retorqued them yet. :smash: