I have a 1985 L98 corvette. The throttle position sensor is correctly set, the car will not idle above 500rpm in gear. I have made a number of minor modification to the engine to gain hp which is now at 256hp at the rear wheels. The car is an automatic. At 500 rpm the car idles very rough, however the car runs great, very strong. I cannot find anyone that still has software to support the ecm. If I try to raise the idle using the idle stop screw the ecm readjusts back to 500rpm. I would like an 850 rpm idle in gear, please advise. I thank you for your time and advise.

 

Setting base idle:
-Jumper ALDL pins A&B (two upper right),
-Turn On the key, wait >30sec
-Unplug the IAC (under RH side of TB)
-Remove ALDL jumpers and start the engine.
-Use the TB screw to set base idle to 450rpm.
-Shut off engine and install IAC cable to plug.

When you restart, warm idle should be 700-800rpm.

 

with 65Z01.

 

You should scan it to get Targeted Idle versus Actual but it's pretty clear that it knows to hit the Target within specs because you moved the screw and it stayed the same. Those specs are an Actual Idle of 50 +/- RPMS from Targeted in Gear. In Park or Neutral, Actual should be 100 +/- RPMS from Targeted.

Early '85's did have a Prom re-write because the IAC motor, which controls Idle, was re-learning itself to zero and causing them to stall. You could pull the PROM to see what it has - REMAN on the case probably means it's got the right one.

At some point, you need to get the Minimum Idle back to Factory as you moved the screw and that can cause other issues - but I don't think your Idle issue has anything to do with it. Rather, I think there's a load that it's unaware of and it's not compensating for it. I'd start with the Park/Neutral signal. You should be able to check that with your Scan or if the Idle is ok in Park or Neutral, I'd say it doesn't know it's in gear, hence the rough idle.

As to raising idle to 850 RPMS, try the Scan and Tune Section. That's quite a high number for an AUTO though and it's going to effect braking and create heat/wear issues. You might want to pare that number back a bit - 750 RPMS is about as high as I'd go.

 

Classic symptom of a vacuum leak.

 

First thing I checked was all visible vacuum lines, they are good

 

thank you for your direction. I will give it a try this weekend.

 

visable vac lines? that covers about half of them...

EGR been tested for fully closed? Thats a vac test.

Leaky injectors? wet fuel causes rough idle and even stalling.

 

The correct way to check for a vacuum leak is to disconnect the lines one by one from the plenum, throttle body and intake manifold. Plug the source and see how it runs.

There are several non obvious sources for vacuum leaks. As Lee mentioned, the EGR valve is one. The brake booster is also commonly overlooked. I had a leak in the PCV valve vacuum line where it attaches to the intake manifold.

Did you change to higher flow injectors? You could be running rich if the ECM can't decrease the pulse width low enough at idle. On my car the minimum pulse width is 1.6 ms. If the injectors are too big they'll flow too much fuel at idle.

 

TECH TIPS has an excellent How-To for setting Base isle. Setting must be done correctly or the engine will run like crapola.