C4 Corvette Rough Idle Normal?
Drifting
Joined: Jun 2020
Posts: 1,355
Likes: 396
From: Northwest Ohio
Absolutely. Been chasing a low rpm (~1200) shake for a while. Opti is new, needed as it was throwing codes. Wires plugs last year. Lets just say all sensors as well.
Shake started a few months ago, but not seemingly related or in correlation with any work performed.
Higher rpm feels good. 94 6spd. Mileage could be better, around 17-18 city. Daily driver.
I did the injector test, disconnecting one at a time, and #2 shows no significant change in idle when disconnected. That was going to be my next potential source, but haven’t gotten around to that yet.
Been running Mystery Oil through it, and that improved mileage substantially. At that time, it was at 15-16 city.
Shake started a few months ago, but not seemingly related or in correlation with any work performed.
Higher rpm feels good. 94 6spd. Mileage could be better, around 17-18 city. Daily driver.
I did the injector test, disconnecting one at a time, and #2 shows no significant change in idle when disconnected. That was going to be my next potential source, but haven’t gotten around to that yet.
Been running Mystery Oil through it, and that improved mileage substantially. At that time, it was at 15-16 city.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,549
Likes: 4,024
From: Cape Coral, Florida
Yes, starting a NEW thread is much better and not hijack this one...OP, can you start a new thread and it's a good idea to fill out your user profile...model year, engine, trans, mods, tune etc...
Advanced
Joined: May 2022
Posts: 69
Likes: 4
From: Central Florida
I don't do a lot of OBD1 Corvettes down here in the Sunshine State but from what you're saying it sounds like a vacuum leak..you said it runs better at higher RPM's and that is what you'll see with a vacuum leak...you have less manifold vacuum at higher RPM and a leak has less of an affect...shaky at idle but runs better higher RPM...you can either spray some Brake Kleen around any vacuum lines/intake runners and see if the RPM changes....just be careful with that stuff since it's pretty flammable....propane is what I use now...good luck !!
Also, Im in central FL, and love visiting Naples, so perhaps a future visit is in order.
Peace!
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,549
Likes: 4,024
From: Cape Coral, Florida
Thanks. I’ve checked vacuum lines by hand but not with spray. Will try that. Fuel pressure is good and steady at idle and under throttle, so that ruled out (for me) FPR. Perhaps the spray will reveal. Keep you posted.
Also, Im in central FL, and love visiting Naples, so perhaps a future visit is in order.
Peace!
Also, Im in central FL, and love visiting Naples, so perhaps a future visit is in order.
Peace!
Advanced
Joined: May 2022
Posts: 69
Likes: 4
From: Central Florida
Update profile info, done.
I was surprised to see some surge with brake cleaner, but not surprised that it came from the #2 injector area. I think I can assume bad seal or possibly the injector itself. This one has little effect when disconnected. I did the intake manifold gasket last year, maybe the o-ring didn’t properly seat.
I also found the #1 wire arching on the bracket. Must’ve shaken off recently. Didn’t help much but now at least it’s secured again.
Also some surge from spraying the evap canister valve. I’ll probably just replace it, and hoses running into it, ‘cause I’m that guy.
Thanks for all great advice!
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,549
Likes: 4,024
From: Cape Coral, Florida
OK, last post for this thread, I’ll give it back to OP
Update profile info, done.
I was surprised to see some surge with brake cleaner, but not surprised that it came from the #2 injector area. I think I can assume bad seal or possibly the injector itself. This one has little effect when disconnected. I did the intake manifold gasket last year, maybe the o-ring didn’t properly seat.
I also found the #1 wire arching on the bracket. Must’ve shaken off recently. Didn’t help much but now at least it’s secured again.
Also some surge from spraying the evap canister valve. I’ll probably just replace it, and hoses running into it, ‘cause I’m that guy.
Thanks for all great advice!
Update profile info, done.
I was surprised to see some surge with brake cleaner, but not surprised that it came from the #2 injector area. I think I can assume bad seal or possibly the injector itself. This one has little effect when disconnected. I did the intake manifold gasket last year, maybe the o-ring didn’t properly seat.
I also found the #1 wire arching on the bracket. Must’ve shaken off recently. Didn’t help much but now at least it’s secured again.
Also some surge from spraying the evap canister valve. I’ll probably just replace it, and hoses running into it, ‘cause I’m that guy.
Thanks for all great advice!
Most likely a leaking injector O ring !!…the canister purge valve is located in the engine compartment…EVAP vent valve is located underneath the back of the car.
Advanced
Joined: May 2022
Posts: 69
Likes: 4
From: Central Florida
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Advanced
Joined: May 2022
Posts: 69
Likes: 4
From: Central Florida
Purge valve tubing was cracked, did a delete. Hit the injector base with wd40 and worked it around, no more injector leak. No more vacuum issues found, but still have vibration @1200 rpm. Car has plenty of power. The search continues.
2nd Gear
Joined: May 2026
Posts: 2
Likes: 1
From: West Palm Beach , Florida
Hey man , I see you have a shop that can work on troubleshooting the idle issues... Just got a 92 Corvette and been trying to troubleshoot all the issues ...