For my '93 LT1 auto : Rock Auto + $50 = new EGR valve = no more SES light during highway cruise. Old valve had diaphragm leak. Idle is now a bit higher but still a little lumpy with car in drive. Thanks to posters on this forum for making this fix possible.

UPDATE : As some of you may know, a lot of these issues are connected. Fix the bad EGR, then fix the other air leaks that existed long with the bad EGR, THEN use the IAC relearn process to teach the IAC valve it's new position now that those nasty vacuum leaks are gone. My idle is now around 700 rpm and as smooth as it's supposed to be. The only casualty was a broken retaining clip on the IAC harness plug due to 28 year old plastic. I remove and replaced all the associated plugs to reseat them.

 

I have never worked on a LT1 but on my L98 when the EGR valve went bad it was a job to remove all the carbon left behind. I used wire brushes and carburetor cleaner to remove as much of the carbon as possible. If your EGR is too high maybe there is a piece of carbon under it or something.

My system failed due to the EGR Vacuum Solenoid failing first and then the EGR got frozen with carbon. The EGR Vacuum Solenoid controls the EGR with vacuum, this means if you put a vacuum pump on the EGR vacuum hose then you should see it move a bit, at least you can on the L98. When the EGR Valve moves it can affect the idle a bit. Does it smooth out?

Is the EGR system functioning now? Did the roughness come on after you finished replacing the EGR? Did all the hoses and wires get reconnected properly? How is the vacuum signal on your engine? Smooth or bouncy?

Did you re-set the TPS after putting it back (if you removed it all)? I would go over the vacuum lines and the throttle body as a small air leak will bump up your idle. If you accidentally bumped a spark plug wire it might be loose and cause a Miss.
My idle went up when I had a loose vacuum line on my Cruise Control. I snipped off 1/2" and used Cruise control the way home with a smooth idle.

Good Luck!

 

I guess I've still got some work to do, even though the owner's manual actually refers to idle quality I'm just not happy with mine. I believe the solenoid and EGR are working because the SES light does not come on during highway operation like it previously did. I did fuss with a few vacuum lines so I need to recheck them.

 

Happy Independence Day jdcar32!

If I am not mistaken the SES light is triggered by a High temperature read on the EGR exhaust hose. It is the little corrugated tube that takes exhaust to the EGR system and there is a temperature sensor on the L98 engines. I am not sure how they have it attached on the LT1 engines. On the L98 the corrugated tube got so hot it literally started to burn off the silver insulating cover when my EGR was bad. Then I had an emissions test that showed High NOX which is a clincher that I had a bad EGR system. A faulty EGR makes the cylinder combustion temperatures go up and this leads to having to use 93 octane to reduce the detonation or knock from occurring as much. My 1988 C4 uses regular gasoline and has no pinging or detonation. My L98 does not NEED high octane Gasoline, (Thank goodness)!

The next thought is out of the air.... Have you checked your Knock Sensor for operation? My L98 was running poorly and was low on power yet having great compression. I started going through the engine one sensor at a time to verify operation and low and behold the Knock Sensor was bad. With a Knock sensor telling the engine it is knocking al the time the ECM kept pulling timing and no wonder the car was low on power. Your Corvette is much more advanced than mine is but the Knock sensors still go bad. I was really shocked at what I was missing when the car was retarding itself all the time. After a new Knock sensor the car was a Corvette again!

I will catch holy Heck from certain individuals when I do this but I might suggest you try a can of BG 44K Fuel System Cleaner. It is available at Amazon and cost roughly $25 a can. Put the whole can in your gas tank and fill it with fresh gasoline. Drive the Corvette and see if the BG 44K is able to smooth out your idle. I have heard that Liquid Moly makes a version of Fuel Injection Cleaner that works very effectively as well. I have not tested the Liquid Moly but in the past just a simple can of BG 44K will make the idle so smooth afterwards. It cleans out the crap in your injectors and makes them work well again. I have used the BG 44K for decades and it won't hurt your Corvette, I use it in my C3 BB and every vehicle I own. Every vehicle I have used BG44K in idles and runs better afterwards. I suppose if everything was clean you might not see any effect.

Any vacuum leak will make trouble. I hope that you were successful in verifying the Vacuum Hoses. If you are still having issues you might verify that the spark plugs are firing using a proximity tester that lights up if there is a spark.
Try the BG 44K and tell me what you think about it, and no, I don't get commissions..... It is a product that I believe in and I support it.

 

If it's just at idle, have you cleaned the throttle body, Sometimes they get carboned up.

 

I got hooked on BG44k many years ago. A mechanic swore by it. Cleans the injectors as whell as the back of the valves where carbon or whatever builds up.