Hey Everyone, I just recently bought an '85 C4 corvette and am trying to get it to pass emissions, but the idle is too high. Thought it might be a vacuum/air leak, so I took it in for a smoke test and it doesn't. Mechanic says something probably electrical. Has anybody else had this issue and/or know how to solve it?

I'm engine dumb, I just recently got into cars and the C4 is my first kinda project, any help would be greatly appreciated

 

A scanner would help, need to know what the idle rpm is now, and what the TPS voltage reads at that idle and the IAC counts.

Assuming there are no SES lights on in the cluster pointing to a problem.

 

Have you cleaned the throttle plates, made sure the plates and linkages are free moving. If all this is good check IAC make sure it's working and not frozen up or has some junk not allowing it to seat.(minimum idle check)

 

Look @ the boss on intake plenum @ the throttle stop, on the back side of the boss you will see a very small cup plug that covers the throttle stop screw adjustment. To remove the plug use an awl, ice pick or small drill bit to put a hole in the cup plug then a sheet metal screw into it a few turns then plyers to pull the plug out = access to adjust base idle.

Or:

Bend the throttle stop tab.

Base idle should be set with the IAC disconnected.

 

The idle is programed into the chip , so changing the screw won't fix anything.
As soon as the car is started again the idle will be right back where it was...Not only that but turning the idle screw also changes the setting of the TPS, then that would have to be reset.....WW.

 

Google minimum idle 1985 Corvette and make sure all cylinders are firing. Tip do not let the plug wires touch steel.

 

There is a procedure in the shop manual to set base idle = access to the throttle stop may be required.

 

Check the cruise control cable is not restricting the throttle from returning fully, also check the IAC stepper motor has not come loose.
The square plug near passenger side lower part of throttle body, check the IAC valve is tight.

Another problem area is the EGR valve in middle under plenum they can stick open affect idle and cause stalls.
You can connect a hose and apply vacuum to test it moves up and down freely.

You really need to check the basics the throttle position sensor is it set to 0.54 volts dc?
ignition timing .... dont trust the ballancer the outer ring can slip.
The IAC stepper motor could be bad, it should adjust the idle to 600rpm after the initial 1200 rpm cold start.
Does the idle speed vary at all? either the IAC valve is bad or the throttle body stop screw has been adjusted too far for the IAC cannot compensate.
Check the fuel pressure regulator, make sure the diaphragm is not ruptured gas can get sucked thru vacuum hoses can also fill sump with gas...

First check for any check engine light codes ..

Dont forget simply resetting the ecm via plug near battery or disconnect battery 10 seconds or so and reconnect it.
Sometimes the ecm can get itself in knots trying to compensate for a perceived problem.

The base idle procedure needs the timing to be set, TPS set to 0.54vdc.
Anything not right will affect your ability to set the base idle, be best if someone can help you check a few things beforehand.

 

Thanks everyone for the tips, I checked the TPS and it was fine, I did use the throttle stop screw adjustment to change it just to pass emissions so I don't have to worry about rushing finding the true problem. I haven't gotten to check the IAC yet, but I did find a cruise control vacuum hose that is not connected (but capped off) from what I've read that could also be the issue. I'll post a pic of it (probably in a new thread about cruise control) so someone can help me locate where it goes cuz honestly I'm lost and can't see anything that looks like it would connect to.

 

Hi had the same issue.
the TPS was stuck a bit.

 

If both ends are capped off, it won't matter. Not sure how the throttle screw helps unless it was way out of spec the first time. Still, a scanner is a good suggestion to go further.

 

If you were able to adjust the idle to a lower RPM with the throttle stop screw then you have a problem with the IAC system.

The throttle stop should only adjust base idle with the IAC closed and unpowered.

 

I had a similar issue with my 92 LT1, a vacuum hose came off and caused the engine to idle too high, found the problem and reattached the vac hose and it dropped down from about 1200 rpm to 800 rpm.

 

That or you have pushed it out of the range the ECM is able to compensate for.

Not really. I adjusted it with the motor running.