Infrequently used LT1 vibrating at idle. No codes.

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84k LT1 in 1995 Buick vibrates at idle in gear. It sits quite a bit. However it doesn't run rough under any acceleration when it does get used. Really doesn't affect the way it drives so I don't think it's a traditional misfire. No codes have been stored at all since it started. When the AC is on the vibration is more intense and then goes away when the car moves, yet the baseline vibration is still noticeable when the engine is back at idle speed, AC on or off. Am I looking at some kind of a harmonic imbalance? Maybe a pulley out of round?
 
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That could literally be anything. Don't part swap. You'll be going down a long rabbit hole. Are the readyness monitors online? How many drive cycles have been completed? Most codes will be 2-trip logic. Are there pending codes? You need to look at fuel trims at idle vs 3k rpm, verify IAT vs ECT, 02 waveforms, then go from there with a compression test/leakdown if needed.
 
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Like he said it could be many things. Given the age and an otherwise well running vehicle it could be as simple as an engine mount.
 
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If you don't have access to a scan tool, I'd probably recommend pulling the belt and firing it up to see if there is any difference. Don't run too long you won't have a water pump or alternator.
 
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I had the EXACT same thing in my car. Thought it was the engine mounts or the harmonic balancer or a misfire. Turned out the exhaust mounts had perished causing the entire exhaust to shake at idle (worse when stationary in gear, unbearable with A/C on) was fine when accelerating and up at speed. Cost me 10 minutes and $12 to fix and she is smooth as silk again now
 
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Carnac the Magnificent bets it's your EGR valve. Reach behind your intake manifold and unplug your EGR valve. If it all of a sudden runs smoothly, you know what part to replace. Costs nothing in terms of time or money to check.
 
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Originally Posted By: DoubleWasp
Carnac the Magnificent bets it's your EGR valve. Reach behind your intake manifold and unplug your EGR valve. If it all of a sudden runs smoothly, you know what part to replace. Costs nothing in terms of time or money to check.
in my experience gms will throw an egr flow/commanded mismatch code if the pintle is stuck
 
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Originally Posted By: Gasbuggy
95 is obd1 or 2?
oh I missed that it's a 95. could be either one.
 

ls1mike

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I have owned 3 LT1 cars, based off your age and mileage seems the car sits a bit. If you have not replaced the opti spark, I bet it has some moisture in it. Have the car scanned. My 95 Z28 was an OBD II car. Some optis will cause a vibration but will be fine under load. That is usually the start of their demise. Scanner first see if you have any weird timing issues or misfires.
 
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Originally Posted By: ls1mike
I have owned 3 LT1 cars, based off your age and mileage seems the car sits a bit. If you have not replaced the opti spark, I bet it has some moisture in it. Have the car scanned. My 95 Z28 was an OBD II car. Some optis will cause a vibration but will be fine under load. That is usually the start of their demise. Scanner first see if you have any weird timing issues or misfires.
As the owner of an LT1 Corvette I agree with Mike ^^^^
 
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