I have a 94 LT-1 6 speed coupe and this weekend I swapped out the old water pump, new intake gasket, and While I had the Throttle body off I gave it a mild port job using a dremmel and fine sandpaper. After I had it all buttoned up I ran it for a little bit and the car exhibited some odd behavior.

* upon first start up it Idled very high for a second 2,500-3,000 RPMs.
* it would drop down to 1,000 RPMs then eventually race up to 1,500 RPMs.
* i notice the EGR is leaking a little causeing whirrling noise under the hood and I believe that is the gasket causing the problem on that one (since I had to reuse the old one).

What could be causing the erratic Idle behavior is it just the computer trying to re-learn everything or is my opti junking out or what.

 

Sounds like a vaccum leak.. It doesn't take much of a leak to screw up your idle.. WW

 

Had the same problem after an intake swap, (on a tpi camaro) doesn't take much of a leak at all for the idle to become erratic. I needed a slightly thicker gasket (special order from Edelbrock of course)...problem fixed.

 

My money's on vacuum leak, too. You can spray a dish detergent/water solution on any hoses and suspect gaskets to see if bubbles form.

Are you a smoker? I've seen some mechanics blow tobacco over the engine while it's running to locate leaks, too.

Speakin' of smokin' that's a nice avatar pic you've got there. Good luck!

 

I am assuming you disconnected the IAC [ Idle Air Control ] electrical connection when you had the Intake off .
I don't have my FSM as I'm out of town but somewhere here is a post where someone besides me posted how to reset your IAC valve .

If you don't have a vacuum leak that might be the culprit.
Best I can remember here it is.
Alright I found the procedure over at Shoebox.com who is the LT 1 garu !
Anyone who works on a LT 1 engine needs this guys site in favorites !
I think all the EFI engines have the electronic IAC but computer resets I don't know how far bact it will work.

Depress accelerator slightly
Start engine, then release accelerator pedal, run engine for 5 seconds
Turn ignition "OFF" for ten seconds
Restart engine and check for proper idle operation
It is NOT recommended to to push or pull on the pintle of an IAC that has been in service. The force required can damage the threads on the worm drive. Also, do not soak the IAC in any liquid cleaner or solvent, as damage may result. When installing a new IAC, you may move the pintle to match the measurement of the old one. The force required to move a NEW valve will not cause damage to it. Use engine oil to lubricate the o-ring. Tighten attaching screws to 27 lb. in.

Note that the 1993 IAC has a square electrical connector and the valve screws into the throttle body, instead of being held on by screws like those of later years.




IAC should be reset . I might have this procedure saved on my other tuning laptop so I will see and correct if I'm off.

 

Sorry to highjack but along the same lines. I just had the throttle body off and IAC unplugged on my '90. Would I have to reset this in the same way as an LTX? I have similar issues, high idle at startup but then goes down to 1000. It will go up and down between 1200 and 1000 at idle.

 

See my amended post above ! Mine did the same thing after replacing the IAC valve and the reset cured the problem.

I would not doubt it will be the same for all computerised EFI Chevys including L98's .

 

Thanks! Should the IAC be plugged in or not for this test?

 

plugged in !
It resets the computer so idle stays in one of two modes instead of hunting !
Normal and A/C idle.

 

I tried it and it didnt really help. I guess I will have to look into how its done on the L98.

 

I think the dishwasher soap thing is used for finding leaks in tires. A vacuum leak will suck the solution in the motor, not cause bubbles to form. Some use carburator cleaner or even propane (carefully) around the manifold to see if the engine speed increases. This would indicate a vacuum leak.