Low RPM engine only vibration
Thread Starter
Instructor
Joined: Apr 2004
Posts: 217
Likes: 0
From: Winnipeg MB
Low RPM engine only vibration
Maybe all my erratic engine idle problems are related....
1985 Vette with an automatic.
When it warms up and the RPM's drop below 1000 you sure can notice an engine vibration. i have been chasing a rough running engine and replaced plugs, cap, rotor, wires, fuel filter, chased vacuum leaks, etc.
but it really runs smooth with no vibration over 1400. The worst is if i hold the throttle between 900 and 1000. You can chatter your teeth it vibrates so much! It is noticeable at 600 as well, but not as bad as 900.
I have removed the serpentine belt to eliminate the driven components. i have dropped it in drive and reverse to check motor mounts and they seem OK. In drive the left mount raises a bit, but doesnt jump out at you.
It seems like a solid enough vibration that I thought it might be the motor jumping around. You feel it just sitting in the seat....you can see the front headlight vibrating alot as well.
Any thoughts on this one?
Thanks
1985 Vette with an automatic.
When it warms up and the RPM's drop below 1000 you sure can notice an engine vibration. i have been chasing a rough running engine and replaced plugs, cap, rotor, wires, fuel filter, chased vacuum leaks, etc.
but it really runs smooth with no vibration over 1400. The worst is if i hold the throttle between 900 and 1000. You can chatter your teeth it vibrates so much! It is noticeable at 600 as well, but not as bad as 900.
I have removed the serpentine belt to eliminate the driven components. i have dropped it in drive and reverse to check motor mounts and they seem OK. In drive the left mount raises a bit, but doesnt jump out at you.
It seems like a solid enough vibration that I thought it might be the motor jumping around. You feel it just sitting in the seat....you can see the front headlight vibrating alot as well.
Any thoughts on this one?
Thanks
Pro
Joined: Dec 2008
Posts: 658
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From: Lubbock for now, headed for Oklahoma
man I hate to say this but if I don't ask.....
Have you followed every one of your plug wires by hand to be sure they are in the right positions? What you are saying is representative of a couple wires being swapped. Been there.
Have you followed every one of your plug wires by hand to be sure they are in the right positions? What you are saying is representative of a couple wires being swapped. Been there.
Pro
Joined: Dec 2008
Posts: 658
Likes: 0
From: Lubbock for now, headed for Oklahoma
Just throwing things out there here.
Drifting
Joined: Mar 2008
Posts: 1,756
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From: Wichita KS
Another odd deal that can happen is the outer ring of the harmonic balancer can slip. So when you try and set the timing by the harmonic balancer mark, the timing is way off.
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
I was going to post "make sure 6&5 weren't crossed". Those two if crossed will sure make an engine run as described.
Another odd deal that can happen is the outer ring of the harmonic balancer can slip. So when you try and set the timing by the harmonic balancer mark, the timing is way off.
Another odd deal that can happen is the outer ring of the harmonic balancer can slip. So when you try and set the timing by the harmonic balancer mark, the timing is way off.
Pro
Joined: Dec 2008
Posts: 658
Likes: 0
From: Lubbock for now, headed for Oklahoma
I was going to post "make sure 6&5 weren't crossed". Those two if crossed will sure make an engine run as described.
Another odd deal that can happen is the outer ring of the harmonic balancer can slip. So when you try and set the timing by the harmonic balancer mark, the timing is way off.
Another odd deal that can happen is the outer ring of the harmonic balancer can slip. So when you try and set the timing by the harmonic balancer mark, the timing is way off.
absolutely
Burning Brakes
Joined: Apr 2004
Posts: 762
Likes: 2
From: Huntington ma
Could be timing, or a "sweet spot" in the air/fuel vs. timing that wants to go weird under no load. Could be Camshaft vs. exhaust..
I've gone 'round and 'round with the Vortec head GM crate motor (base HO version) in My '85 and finally came to the conclusion that the cam simply causes an exhaust "resonance" at about 1300 rpm. Vibrates like it was out of balance, or something. Only does it under very light load, in Any gear, with or without the converter lock-up engaged (That's what the vibration actually Feels like, BTW, a bad converter..). In High gear on the highway in lockup it doesn't do it enough to notice, cruising along in 3rd unlocked at about 30 mph it's Very apparent.
Running true duals (OEM manifolds with some cleaning up inside - no AIR tubes), no cat, no crossover until the rear Y-pipe. That's as good as it gets, still shakes. Stock front Y-pipe or any forward crossover makes it a lot worse..
Like I said, I have come to the belief that only a cam (or cam location) change is going to make it go away. Maybe advancing the cam a couple of degrees would do it, I don't know. Meanwhile I live with it.
I've gone 'round and 'round with the Vortec head GM crate motor (base HO version) in My '85 and finally came to the conclusion that the cam simply causes an exhaust "resonance" at about 1300 rpm. Vibrates like it was out of balance, or something. Only does it under very light load, in Any gear, with or without the converter lock-up engaged (That's what the vibration actually Feels like, BTW, a bad converter..). In High gear on the highway in lockup it doesn't do it enough to notice, cruising along in 3rd unlocked at about 30 mph it's Very apparent.
Running true duals (OEM manifolds with some cleaning up inside - no AIR tubes), no cat, no crossover until the rear Y-pipe. That's as good as it gets, still shakes. Stock front Y-pipe or any forward crossover makes it a lot worse..
Like I said, I have come to the belief that only a cam (or cam location) change is going to make it go away. Maybe advancing the cam a couple of degrees would do it, I don't know. Meanwhile I live with it.
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Un plug MAF. see if it runs better un plugged. If it does, it may be your MAF. My $.02
Thread Starter
Instructor
Joined: Apr 2004
Posts: 217
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From: Winnipeg MB
I would say it is responsive when I hit the gas. Ever so slight delay, but I figure that is due to the MAF system.
I ohmed the injectors in the spring. I may have the numbers written down in the garage. I checked them hot and cold and I remember they were within spec.
I havent disabled the EGR. How do I and is it permanent?
I did gut the CAT thinking I may have had a resriction in the exhaust.
I ohmed the injectors in the spring. I may have the numbers written down in the garage. I checked them hot and cold and I remember they were within spec.
I havent disabled the EGR. How do I and is it permanent?
I did gut the CAT thinking I may have had a resriction in the exhaust.
Pro
Joined: Dec 2008
Posts: 658
Likes: 0
From: Lubbock for now, headed for Oklahoma
I would say it is responsive when I hit the gas. Ever so slight delay, but I figure that is due to the MAF system.
I ohmed the injectors in the spring. I may have the numbers written down in the garage. I checked them hot and cold and I remember they were within spec.
I havent disabled the EGR. How do I and is it permanent?
I did gut the CAT thinking I may have had a resriction in the exhaust.
I ohmed the injectors in the spring. I may have the numbers written down in the garage. I checked them hot and cold and I remember they were within spec.
I havent disabled the EGR. How do I and is it permanent?
I did gut the CAT thinking I may have had a resriction in the exhaust.
well you can disable it by pulling the vacuum line off of it, but that doen't rule out a leak or weak spring. You can just use a cut piece of aluminum can to put under it. punch two holes for the bolts.
Thread Starter
Instructor
Joined: Apr 2004
Posts: 217
Likes: 0
From: Winnipeg MB
Ok, I fired it up and ran it again. It sure starts great and runs on high idle flawlessly!! As soon as the idle dropped to about 900 I held the rpm there. The temp was about 120. I was still shaking pretty good. Smoothed out around 13ish
I unplugged the MAF. It ran like crap. Almost stalled. Wouldnt rev up without popping through the intake.
I will try and plug the EGR tomorrow.
Thanks for the help
I unplugged the MAF. It ran like crap. Almost stalled. Wouldnt rev up without popping through the intake.
I will try and plug the EGR tomorrow.
Thanks for the help
Burning Brakes
Joined: Apr 2004
Posts: 762
Likes: 2
From: Huntington ma
Blu-Ray has posted an incorrect diagram for the EGR location. The '85 isn't an LT1. On the L-98 TPI motors (of which 1985 was the first year) the EGR valve is located under the plenum bolted down to the base manifold, in the center of the motor. Necessary to remove the plenum to reach it for removal, but you can Test it easily enough in place.
Does sound like you Might have a vacuum leak issue - and the EGR is one possibility. You might disconnect and cap all Other vacuum lines and see if you find a problem that way.
Does sound like you Might have a vacuum leak issue - and the EGR is one possibility. You might disconnect and cap all Other vacuum lines and see if you find a problem that way.
Pro
Joined: Dec 2008
Posts: 658
Likes: 0
From: Lubbock for now, headed for Oklahoma
Blu-Ray just doesn't seem to get it that the '85 isn't an LT1. The EGR diagram he's posted is wrong: on the L-98 TPI motors (of which 1985 was the first year) the EGR valve is located under the plenum bolted down to the base manifold, in the center of the motor. Necessary to remove the plenum to reach it for removal, but you can Test it easily enough in place.
Does sound like you Might have a vacuum leak issue - and the EGR is one possibility. You might disconnect and cap all Other vacuum lines and see if you find a problem that way.
Does sound like you Might have a vacuum leak issue - and the EGR is one possibility. You might disconnect and cap all Other vacuum lines and see if you find a problem that way.
