LT4 Rough Idle
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,526
Likes: 104
From: Stafford Va
LT4 Rough Idle
I have had a LT4 w/a Hot Cam in my 64 GS that I have been building for a few yrs, start it up once a week, warm it up to 210 and let it run a while changing the revs. All has been well to include a very smooth idle. Last week while the car was warming up, the car began to idle rough and then stopped. I thought maybe the fuel cell was reading incorrectly and added 2 more gallons of fuel. Still it brfly starts and has a very bad idle, almost a back fire. I have 40psi at the fuel rails, all the wires are snug and nothing appears to be loose or burned. Extremely frustrated since I am just an interior and bleed the brakes from getting this on the road.
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
I have had a LT4 w/a Hot Cam in my 64 GS that I have been building for a few yrs, start it up once a week, warm it up to 210 and let it run a while changing the revs. All has been well to include a very smooth idle. Last week while the car was warming up, the car began to idle rough and then stopped. I thought maybe the fuel cell was reading incorrectly and added 2 more gallons of fuel. Still it brfly starts and has a very bad idle, almost a back fire. I have 40psi at the fuel rails, all the wires are snug and nothing appears to be loose or burned. Extremely frustrated since I am just an interior and bleed the brakes from getting this on the road.
I would unplug the IAC and then try to start it. It should start and run with a very high idle, but it should be smooth. If it does, the IAC is your culprit, clean it up and then have the PCM relearn it. If it still doesn't want to run, or runs rough, you're back to the basics, fuel and spark. It could be anything from a bad coil to junk in your injectors.
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,526
Likes: 104
From: Stafford Va
fair enough thanks, will not get to it until the weekend. The hotcam and motor have been in the car running like this for 2 yrs. I let it warm up then bring the revs up and down.
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
What I was curious about was if the engine was fresh from the crate or if it came from another running car? If it came from a car there could be a buildup of gunk on the IAC, the opti could be worn, rusted, etc, etc. If it was brand new in a crate and has been garage kept, I would doubt either of these would be a problem.
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Unless you actually took the IAC off of the throttle body and cleaned it, it may be gummed up. Mine acted fine for years and then suddenly started giving me problems. I guess it finally got just the right amount of gunk in there.
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,526
Likes: 104
From: Stafford Va
Ok new plugs and cleaned the IAC but still will not idle and will back fire a bit as well. As I mentioned in the opening thread, the car although sitting in the garage would start right up, idle smooth and when warmed I would run the revs up to 2-4k and even blipped it once or twice to 5500k. Well it was idling smooth then just sputtered then stopped. Thought the fuel was low so I added 2 more gallons but I am still **** out of luck. All wires are on solid.
:will y:
:will y:
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
2 things The ICM has been know to work OK till warm then crap out what temp is it cutting out at? Regardless of the mileage opti's can go bad at any time. Start checking for loss of spark clean all connectors to ignition parts. Are you able to pull the codes from the PCM?
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,526
Likes: 104
From: Stafford Va
I will try to pull the codes, its a PCM for Less ECM so its an OBD I with a OBD II connection. I am going to drain the fuel cell, maybe since I keep the fuel low all the time condensation got in it and I have some H2O in the cell. Long.... shot but. For it to be idling fine then just crap out I am hoping its not the OPTI. I will also check the resistance of the fuel injectors and they too could be gummy. Last it does not idle at all. Still hoping to roll it out this March
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,526
Likes: 104
From: Stafford Va
I drained the fuel and added a new batch thinking I may have some bad fuel. It will not even light off, still good fuel pressure at the rails, close to 50 and when it was turning and sputtering it was moving around idicating the injectors were firing. Every now and then during the past 2 yrs just warming the car up and moving the revs around I would get a rough idle. Ok sl I crawled under the car again and the water pump has a leak in the front, ok it looks like it is leaning twds the opti. How can I test the opti, and is the MSD opti better? I will check the codes next week, need to borrow a scanner. Ign Coil is another option as well. If its the opti I will finish the interior etc then dive into the engine. At this point I am almost ready to convert to a carb and distributor but not yet. Thanks,
Le Mans Master
Joined: Dec 2004
Posts: 5,789
Likes: 8
From: Myrtle Beach SC
St. Jude Donor '05-'06
I drained the fuel and added a new batch thinking I may have some bad fuel. It will not even light off, still good fuel pressure at the rails, close to 50 and when it was turning and sputtering it was moving around idicating the injectors were firing. Every now and then during the past 2 yrs just warming the car up and moving the revs around I would get a rough idle. Ok sl I crawled under the car again and the water pump has a leak in the front, ok it looks like it is leaning twds the opti. How can I test the opti, and is the MSD opti better? I will check the codes next week, need to borrow a scanner. Ign Coil is another option as well. If its the opti I will finish the interior etc then dive into the engine. At this point I am almost ready to convert to a carb and distributor but not yet. Thanks,
The only thing that will show are codes 16&36 if the electronics are bad in the opti. Sometimes no codes if the cap or rotor is the issue. Personally for your engine I do not think the extra cost of the MSD is warranted. You have the improved design Opti and seal the cap and around the plug with permatex and you should be good. Carb and Dizzy are just more money EFI & Opti work great when they are working much better than Carbs.
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,526
Likes: 104
From: Stafford Va
Thanks ,I think I will disconnect the MAF sensor and see if that may be it. Also I am seeing if the engine enters a clear flood and does not reset that could also be an issue.
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,526
Likes: 104
From: Stafford Va
I wanted to close the loop on this. I went out to start tearing into the car to replace the opti and H2o pump and tried one more time to start it. It started right up and ran like a champ. There is fluid under the h2o pump and yes ithe opti has time to dry so I will replace the pump, and the rotor and cap with the MSD one.
I did not have any codes when I ran the test
I did not have any codes when I ran the test
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,526
Likes: 104
From: Stafford Va
however, now when the car is hot it does not want to idle well and will cut off and if I un hook the MAF it smooths out and runs well through out all RPM's. I have this issue on another thread, but if you have the answetr I would be happy to hear it. I have tested for codes and none pop up.
Melting Slicks
Joined: Jun 1999
Posts: 2,949
Likes: 23
From: Bowling Green KY
however, now when the car is hot it does not want to idle well and will cut off and if I un hook the MAF it smooths out and runs well through out all RPM's. I have this issue on another thread, but if you have the answetr I would be happy to hear it. I have tested for codes and none pop up.