1985 corvette idle issue

[Adam B.]

i have a 85 that often times when idleing does not idle rite, under load or not. i have trued tune-up like things (plugs,wires,cap,rotor,coil,pcv value) the throttle position sensor etc... i cleaned the throttle body. after putting plugs in it the idle stays decent for a while but shortly after it idles rough. it doesnt miss fire until it fouls one of the plugs out and its a different cyclinder each time. the idle will change for about 800 rpm to 600 rpm to 900 rpm and as low as 400rpm almost stahling. when i unplug the throttle positon sensor the idle goes up to 1200 rpm and remains steady. Not sure exactly what the promblem is is it an ecm? an ideas or thought would be awesome! thanks everyone!

[Tom H.]

Re: 1985 corvette idle issue

Hi Adam ! How many miles are on the car ? Original injectors ???

[John D.]

Re: 1985 corvette idle issue

Sounds like an idle air control valve issue....

[George C.]

Re: 1985 corvette idle issue

Sounds like an idle air control valve issue....

Also make sure you have the correct 195 themostat.

[Jim T.]

Re: 1985 corvette idle issue

I concur with John Daly that your 85's IAC could be the problem. It is a easy change. The IAC can also be cleaned from what I have read, but very carefully. With your foot off the gas pedal the IAC takes over and regulates the idle speed with command information from the ECM.

Interesting that different sparkplugs are fouling out.

[Peter M.]

Re: 1985 corvette idle issue

Sounds like an idle air control valve issue....

You need to remove and thoroughly clean the throttle body. When it is cleaned up (use carb or TB cleaner & a brush) make sure that you oil the butterfly shafts going into the TB. Then install a new IAC valve ($35) & properly torque it in place. Next, use the instructions below to properly set your throttle position sensor. If you do not have a TPS tester wire harness, you can get one from Mid-America for a few dollars. They are well worh it.

Following these steps should solve your roaming idle problems.

Let us know how you make out.

http://www.parts123.com/parts123/yb....Z5Z5Z50000050F#

[Edward J.]

Re: 1985 corvette idle issue

i have a 85 that often times when idleing does not idle rite, under load or not. i have trued tune-up like things (plugs,wires,cap,rotor,coil,pcv value) the throttle position sensor etc... i cleaned the throttle body. after putting plugs in it the idle stays decent for a while but shortly after it idles rough. it doesnt miss fire until it fouls one of the plugs out and its a different cyclinder each time. the idle will change for about 800 rpm to 600 rpm to 900 rpm and as low as 400rpm almost stahling. when i unplug the throttle positon sensor the idle goes up to 1200 rpm and remains steady. Not sure exactly what the promblem is is it an ecm? an ideas or thought would be awesome! thanks everyone!

Adam, If you replaced the throttle position sensor, They do require a adjustment, generally very easy with scan tool, .50 volts is about where you what to be, if the voltage is to low the base idle speed will be to low. The idle air control motors are somrtimes a problem as they age the motors do slow and connot re-act to the changes fast enough. but would not replace until you set the throttle sensor, Also have you checked for computer trouble codes? Sometiimes they lead you to a problem.

For fouling out plugs,You need to see the plug tips to see whats really going on, this is a health check so to speak of whats happening with the engines running. As Tom H. points out about the injecters, In 85/86 they were really a problem with corvettes TPI engines. and now with the fuel we run through the engines its more that ever a problem. rough idles, hesitations and surging engines, engine skips, Are just a few of the drivablilty problems.

[Joe C.]

Re: 1985 corvette idle issue

i have a 85 that often times when idleing does not idle rite, under load or not. i have trued tune-up like things (plugs,wires,cap,rotor,coil,pcv value) the throttle position sensor etc... i cleaned the throttle body. after putting plugs in it the idle stays decent for a while but shortly after it idles rough. it doesnt miss fire until it fouls one of the plugs out and its a different cyclinder each time. the idle will change for about 800 rpm to 600 rpm to 900 rpm and as low as 400rpm almost stahling. when i unplug the throttle positon sensor the idle goes up to 1200 rpm and remains steady. Not sure exactly what the promblem is is it an ecm? an ideas or thought would be awesome! thanks everyone!

You and everybody else is on a wild goose chase until you first check for any ECM codes by grounding the ALCL connector.

[Edward J.]

Re: 1985 corvette idle issue

Joe, Read my first paragraph.

[Adam B.]

Re: 1985 corvette idle issue

thank you everyone for your help. the car has 65,000 miles on it. i do not know if the injectors have every been changed i have not done them however. the check engine light was not on last summer and also i dont have an obd 1 scan tool. i have cleaned the iac also that did not change anything. thank you everyone!

[Paul O.]

Re: 1985 corvette idle issue

Adam if I remember there were a few service bulletins on the 85's there was one on the manual trans and the auto. There was a PROM service replacement low idle speed and stalled launches. Plus erratic idle speed were the problems.

Then the was one to change out the coil and change the spark plug gap to .035 from the original gap.

Then there was one where you resets the cars base idle for idle. By get the engine with no computer distributor advance then remove the metal plug at the idle stop screw and adjust it to I believe 500 RPM.

Here are a few of the bulletins

http://www.corvetteactioncenter.com/kb/questions/203/1985:+Service+Bulletin:+Low+Idle+Speed+and+Launch+Stalls

http://www.corvetteactioncenter.com/kb/questions/280/1985%3A++Service+Bulletin%3A+Minimum+Idle+Speed+Revision

http://www.corvetteactioncenter.com/kb/questions/237/1985%3A+Service+Bulletin%3A+High+Output+Ignition+Coil+Replacement

Paul 18046

[Joe C.]

Re: 1985 corvette idle issue

thank you everyone for your help. the car has 65,000 miles on it. i do not know if the injectors have every been changed i have not done them however. the check engine light was not on last summer and also i dont have an obd 1 scan tool. i have cleaned the iac also that did not change anything. thank you everyone!

You don't need an OBD tool to diagnose the ECM on an '85 Corvette.

Paul O has correctly indicated that there was a recall of poorly designed PROM chips back in early 1986. The production PROMs were probably programmed extremely lean in order to boost fleet fuel economy (CAFE). My car was involved, in fact, I began complaining even before the recall campaign was issued. After about 3 months of erratic idle (oscillating, or "hunting" with amplitude growing until stall), the recall campaign began. After replacement, I noted a faster idle speed, better drivability and low speed power, as well as higher shift points and higher converter lockup speed.

Interestingly (), I passed a PV with the car, and got the McLellan Award. Nope.......I'm not giving it back! Any top-flight judge should have been able to discern that the transmission shift points were a few MPH higher.

Your Corvette may have "fallen through the cracks", but I doubt it, so check to see that the PROM was changed out. I still have my original PROM, as the dealer and I had a cordial working relationship, and the SM let me keep it at my request.

[Peter M.]

Re: 1985 corvette idle issue

You don't need an OBD tool to diagnose the ECM on an '85 Corvette.

Paul O has correctly indicated that there was a recall of poorly designed PROM chips back in early 1986.

You can download WinALDL or TunerPro software free on the Internet. Install the software on your computer, add a USB ALDL connector for $60 andf you are in business. You can pull your codes yourself and do your own diagnotic work, assuming you have a GM Service Manual for your car.

The GM ECM prom that you need to go back into your Corvette is #16198259. #16198259 is the GM replacement for the original #1227165.

[Peter M.]

Re: 1985 corvette idle issue

16198259 / 1227165 is a PN for a complete ECM. I haven't researched this for a long time, but IIRC, WinALDL and Tunerpro do not support the 85 ECM.

The problem is the 85 does not use the "165" ECM. The 85 is a one-year-only wonder that AFAIK is no longer supported. The 'fix' for a bum 85 ECM is to re-pin the connectors, change the burn-off controls and MAF, and install the 86-up 165 ECM.

Correct me if I'm wrong, please. But you're correct. WinALDL and Tunerpro are great tools when used with supported ECM's. Unfortunately, the 85 which is the subject of this thread is not one of them.

Dave - you are only partially right here. Yes, the #1226870 ECM was used one year only in '85 and will supply data only @ 1600 BAUD. TunerPro does offer files for use with the '85 ($1F) although I believe WinALDL is the application of choice for most '85 TPI tuners. Both applications will work.What’s more, you are also correct in that '85 L98s can be re-pinned to use the later #16198259 ECM which offers better support and tuning capabilities. This is most likely the cheaper way to go in the long run because you don't need the rare & expensive '85 only MAF burn-off module under the dash; you just need some $12 relays.
Finally, here is a how to write-up on converting the '85 L98 ECM unit to the #16198259 ECM.

http://www.eecis.udel.edu/~davis/z28/ecm_swap/