Rough Engine, No Fault Codes
Thread Starter
Heel & Toe
Joined: Oct 2018
Posts: 15
Likes: 0
From: Goodyear, Arizona
Rough Engine, No Fault Codes
This week I bought a 1992 LT-1 Corvette with 100,000 miles on it. The previous owner said a shop told him that the timing chain was bad, but I'm not so sure. It starts right up, but sounds rough. The condition persist at other RPM's leading me to think it could be a valve problem or broken spring. However, at idle, it acts like it could be a vacuum leak, as the speed walks up and down. Interestingly, even with the rough condition, no fault codes are displayed! I don't mind digging into a rebuild, but want to make sure I'm not missing something.
Last edited by Chuck_Roberts; Oct 4, 2018 at 03:59 PM. Reason: Spelling
Le Mans Master
Joined: Sep 2013
Posts: 6,192
Likes: 750
From: near the thumb in the mitten
If the problem at idle while driving at a steady speed or both? Vacuum leak near check valve under right injector cover is very common. FPR leaking fuel into vacuum hose is another common concern.
Thread Starter
Heel & Toe
Joined: Oct 2018
Posts: 15
Likes: 0
From: Goodyear, Arizona
Thank you for taking the time to help!
The rough condition is worst at idle, under some load (stopped, in drive), but the condition is noticeable at higher RPM's and in Park as well. It has never died, but the variations in idle speed make me think your ideas are promising.
I did notice the short 90 degrees bend vacuum line on the right front, under the cover was cracked. I'll change that. Do you have any tip to find other potential sources?
Where should I look for the fuel leak in a vacuum line?
I alao wondered if the ECM could be bad, as I'm not getting any fault codes on the car, which surprises me.
The rough condition is worst at idle, under some load (stopped, in drive), but the condition is noticeable at higher RPM's and in Park as well. It has never died, but the variations in idle speed make me think your ideas are promising.
I did notice the short 90 degrees bend vacuum line on the right front, under the cover was cracked. I'll change that. Do you have any tip to find other potential sources?
Where should I look for the fuel leak in a vacuum line?
I alao wondered if the ECM could be bad, as I'm not getting any fault codes on the car, which surprises me.
Thread Starter
Heel & Toe
Joined: Oct 2018
Posts: 15
Likes: 0
From: Goodyear, Arizona
It runs rough at all speeds, but most noticeably under some load (idle, stopped, in drive). It has never died, and starts quickly every time. I'll take a look at the Fuel Pressure Regulator. I did find one vacuum line that was degraded. I'll check for more.
I just don't understand why it's not generating any fault codes. I wonder if the ECM could be bad. The engine RPM cycles by about 300 rpm (higher and lower at idle), like its searching. Still, if the throttle position switch, fuel pressure regulator, or some emissions component was seeing or sending bad signals, I would expect to see a fault code as the engine is definitely missing at all times.
The previous owner said he only drove it 5 miles every 2 months of so, and it stayed in the garage. Since he only drove it 8,000 miles in 14 years, I believe the car has sat a lot recently. I wondered about old fuel in the tank, and have been trying to burn through it with SeaFoam cleaner, mixed as specified on the can. In over 100 miles driven, I can't notice any difference yet, and I'm leary to drive it too long in this condition.
Le Mans Master
Joined: Sep 2013
Posts: 6,192
Likes: 750
From: near the thumb in the mitten
OBD doesn't have parameters to set codes as OBD2. There was a recall related to the Optispark effecting the 92s. A scope test could indicate open wire or fouled plugs. A Tune Up maybe where to start. Plugs, wires, filter..etc. Cap and rotor unfortunately water pump has to be removed to replace them.
Le Mans Master
Joined: Jul 2004
Posts: 6,227
Likes: 697
From: Clifton Park, NY ............Clearwater, FL ... 85 Original Owner
This week I bought a 1992 LT-1 Corvette with 100,000 miles on it. The previous owner said a shop told him that the timing chain was bad, but I'm not so sure. It starts right up, but sounds rough. The condition persist at other RPM's leading me to think it could be a valve problem or broken spring. However, at idle, it acts like it could be a vacuum leak, as the speed walks up and down. Interestingly, even with the rough condition, no fault codes are displayed! I don't mind digging into a rebuild, but want to make sure I'm not missing something.
Thread Starter
Heel & Toe
Joined: Oct 2018
Posts: 15
Likes: 0
From: Goodyear, Arizona
This Forum is outstanding,! Thank you for sharing your knowledge and experience.
Today I fixed a vacuum leak, my wife prayed, and maybe the SeaFoam worked to help clean up an injector or two, but the car smoothed out significantly, and easily passed emmissions. Wow. I didn't expect that.
My To-Do List includes that tune-up, and some electrical work. Thanks again!
Today I fixed a vacuum leak, my wife prayed, and maybe the SeaFoam worked to help clean up an injector or two, but the car smoothed out significantly, and easily passed emmissions. Wow. I didn't expect that.
My To-Do List includes that tune-up, and some electrical work. Thanks again!
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Thread Starter
Heel & Toe
Joined: Oct 2018
Posts: 15
Likes: 0
From: Goodyear, Arizona
Just to follow-up, the car spent a lot of time sitting over the past 14 years with the previous owner, and the engine had a consistent miss. I fixed a vacuum leak near the right front injector, as suggested by Kevova, and completed a tune-up to include new spark plugs (one was definitely bad), spark plug wires, fuel injectors with Bosh 3, and the fuel filter as suggested by Cruisinfanatic and 383vett. Now the engine runs strong for a stock 27 year-old car with 101K miles on it. Thanks for the suggestions!
Race Director
Joined: Apr 2003
Posts: 17,699
Likes: 1,667
From: moraga ca
[QUOTE=Chuck_Roberts;1598979078]Just to follow-up, the car spent a lot of time sitting over the past 14 years with the previous owner, and the engine had a consistent miss. I fixed a vacuum leak near the right front injector, as suggested by Kevova, and completed a tune-up to include new spark plugs (one was definitely bad), spark plug wires, fuel injectors with Bosh 3, and the fuel filter as suggested by Cruisinfanatic and 383vett. Now the engine runs strong for a stock 27 year-old car with 101K miles on it. Thanks for the suggestions
Great, congratulations. Enjoy the ride.
Le Mans Master
Joined: Oct 2004
Posts: 6,314
Likes: 508
From: South-central Missouri
You mentioned replacing the injectors. Just about any new injectors are going to be alcohol tolerant. But, just my luck, my car went into a GM shop (in 2007) for new injectors too. However, they installed more of the same alcohol susceptible (NOS) injectors. So, in a year or so not only did my symptoms sound very much like yours, but I also ended up with a burnt exhaust valve to boot!
Point being...just wanna be sure the injectors were NOT manufactrued pre-93!
Just sayin....
Point being...just wanna be sure the injectors were NOT manufactrued pre-93!
Just sayin....
Racer
Joined: May 2014
Posts: 416
Likes: 29
From: diamond bar ca
now that you have fixed possible problems, make sure all of your vacuum lines and elbows are good and not cracked, and just for the heck of it pull the vac line off the egr in back of engine, you will get a rough idle,and drive it a short distance you should get a code if you dont then you may have a problem with computer , again replace all rubber v lines, they will get hard an crack.
Last edited by bob arrowsmith; Mar 4, 2019 at 10:03 AM.