Engine Vibration 2500-3000 RPM [Archive] - Team Camaro Tech

: Engine Vibration 2500-3000 RPM

May 20th, 10, 08:01 AM
I have been chasing this vibration down for almost a year now. I have done countless searches and done everything suggested in those posts but cannot seem to find this vibration. A little background, the engine is the 290hp GM Performance Parts 350ci and it is showing this vibration between 2500 and 3000 RPM in park, so that eliminates the trans and remaining driveline. So far, I have replaced all pulleys, bought and installed a new ATI balancer, checked the runout on the crank snout, pulled plugs while engine was at vibration rpm and all plugs caused a drop in rpm. This vibration happens while accelerating and decelerating, so I'm hoping this can eliminate timing/carburetor issues. So I have now moved to the rear of the engine and removed the transmission and clutch and am still getting the vibration. Does anyone have any suggestions on what I could do next? The flywheel is stamped GM and 3973456N which looks like a neutral balanced flywheel (per Google) and there are no weights on it. Can a flywheel go out of balance?

May 20th, 10, 08:29 AM
Yes, an unbalanced flywheel will cause a slight disturbing vibration. Nothing exaggerating though, kind of like it would make any loose change (coins) in the ashtray chatter a bit. That is what I had found in my RX-7 back in the mid 80's. The flywheel had no weights drilled in it and it made my engine vibrate. Very annoying. You have checked everything else. Engine would shake if the crank was not balanced.

May 20th, 10, 09:15 AM
Yeah, thats the vibration I'm getting. Its not jarring, but enough to be really annoying. I'm also worried that a vibration is going to cause some sort of damage to the engine. I'll see if I can get it balanced to see if that fixes the problem. The last quote I got was $110 which seemed a little high considering its a neutral balanced flywheel...

May 20th, 10, 10:17 AM
All things considered, that price doesn't seem too high. Hopefully someone else can comment on this thread. I don't believe a bad front balancer damper would cause slight vibrations. It is definitely a weight distribution issue. Take the flywheel off and see or feel ?? :cool: The vibration will indeed cause more problems in the future. Good luck

May 20th, 10, 10:24 AM
Does your engine have a 1pc rear main seal? If so, you need the weighted flywheel since there are no weights on the crank end.

May 20th, 10, 11:01 AM
Does your engine have a 1pc rear main seal? If so, you need the weighted flywheel since there are no weights on the crank end.

The 350/290 HP engine is internally balanced and requires no balance weights on either the flywheel/flexplate or
the harmonic balancer. ( Copied from GM website, specs on 350/290 crate engine)

May 20th, 10, 11:09 AM
Its a two-piece rear main seal

May 20th, 10, 11:13 AM
ok, ok, ok ....DON'T TAKE THE FLYWHEEL OFF!!!!. I gaffed like Joe Biden there a bit. I forgot he did mention the engine type he has which is the one piece seal.

May 20th, 10, 11:23 AM
It would be kind of hard to start it without the flywheel. Maybe some twine around the balancer and I'll pull really hard....:)

May 20th, 10, 11:35 AM
two piece? I thought the GMPP 350/290 hp engines were center bolt valve cover kind with the 1 piece rear main seal, oh well....swap a flexplate, Tranny shops sometimes give them away or sell them for almost nothing.

May 20th, 10, 01:03 PM
What type of motor mounts? If they are poly or solid it may be normal engine harmonics.

May 20th, 10, 03:59 PM
I'm pretty sure they're stock but they could be poly. It rattles right through the shifter handle. Definitely not something I would consider normal.

May 20th, 10, 04:14 PM
Don't discount body, tranny, and engine mounts. They are designed to absorb harmonics. Some vibration throught the shifter is always present. New car engineers spend countless hours with vibration software. That's part of the reason new cars feel so numb, they have to be to please the masses.

May 20th, 10, 04:23 PM
I can definitely rule out the tranny mount because the transmission isn't even in the car now. I'm hoping it's the flywheel. I'm in the process of trying to find a used flex plate to swap in to confirm its the flywheel before I spend the money on the flywheel

May 20th, 10, 04:58 PM
Any decent machine shop can balance a flywheel.

May 20th, 10, 06:49 PM

I could certainly be wrong, but I am betting that it is not a "vibration" caused by something being mechanically out of balance. If the flywheel (or anything else) was not balanced, the "vibration" would get worse as the rpms increased above the 3000 point. Sort of like a tire that is out of balance, it gets worse, not better as you go faster. Sounds like yours gets better above that rpm point.

I am betting that it is a "harmonic" of some type related to the particular camshaft that is in the engine combined with a possible lean spot in the carb or just a bad spot in the entire power curve.

The 355 I had in my 67 drag car would rattle all of the sheet metal in the interior coming down the return road at about 2500 rpm, yet would spin up to 8000 on the track as smooth as silk, it was just a "bad spot" in the camshaft combination.

Try retarding the timing 10 degrees and try it and see if the vibration is still there at the same rpm. If it is truly an out of balance condition, retarding the timing should not make any difference.

May 20th, 10, 06:51 PM
The last quote I got was $110 which seemed a little high considering its a neutral balanced flywheel...

Boy that is a little steep for a job that usually takes no more than 15 minutes :( I generally get $50

May 21st, 10, 01:45 AM
you changed pulleys but did you try it with no belts eliminating power steering pump and water pump.

May 21st, 10, 02:04 PM
Dropped the flywheel off to get balanced. Hopefully this will be resolved tomorrow afternoon. My fingers are crossed!

May 22nd, 10, 05:27 AM
I had a similar issue with my 68 442.. The engine was rebuilt and the guy before me did not break the cam in correctly... it got worse over time and I had to install a new cam shaft..

May 23rd, 10, 07:49 PM
So I got the flywheel back from the machine shop and they said it was only out 1/2 ounce. Got the flywheel back in the car and the vibration was still there. Looking at the water pump pulley wobble, and considering I have replaced everything else including a water pump) I pulled the alternator/water pump belt off and ran the car. The vibration was gone... gingerbreadman1977 was right. Looking into it more, the alternator pulley was somehow messed up. The alternator shaft wasn't bent, but the pulley seemed to wobble. Because the person before me put in a 1-wire alternator, I did a little research and found that I could upgrade from a 10si to a 12si 3-wire. $45 later, I had the new alternator in and it's charging well and more importantly not wobbling.

Thank you to everyone for the help. I have been chasing this for almost 6 months...


May 23rd, 10, 11:05 PM
I had the same problem about a month ago. Really annoying vibration would slightly shake the car and you can see the engine shaking a little too. It happened between 2500 and 3500 RPM no fail every time. Went bonkers looking for it. Even changed out the harmonic balancer in an attempt with no luck. I installed the engine myself after it went to a race shop to get machined. I thought they did something wrong... and was about to lay blame until I saw a Horsepower episode on Spike TV's Powerblock. They made just a 3 second mention about a flywheel alignment that was REALLY important on chevys and a light (more like an atom bomb) went off right above my head. I called a buddy to help me drop the tranny and saw I had goofed! The flywheel IS balanced. If you look at it there are holes all over the place... some large and some about 3/4 inch in diameter. Most of those 3/4 inch holes are open except for maybe 1. It is on the outer edge for balancing and is filled in with metal (almost looks like they tried to punch a hole and forgot to punch out the knock out. Then ALSO if you look at the inner ring where the flywheel bolts to the crank, have you noticed that not all the bolt holes align? There is an extra hole in the flywheel. Interestingly enough the crank mating surface also has a hole in it. When all the bolts are in, the "extra" flywheel hole should align with the hole in the crank. That will be balanced. I was following a chevy engine rebuild book with videos included and they NEVER made mention of it. Turns out that right about that 3000 rpm range if the flywheel is out of balance like that, it will send a harmonic through the crank and will cause the shake. Don't know what it would ultimately cause but I wouldn't run it like that. Check again bro. It fixed my issue 100% and my engine runs strong with no vibes now.

Good luck!

OOOPS! didn't see the part where you said it was fixed. Good find!

Aug 16th, 10, 04:38 PM
Hi Guys,
I am having the same issue between 2500-3500 rpm I am too getting a strong vibration in all 4 gears. It even does it in neutral so I can eliminate the driveshaft / tranny/clutch. I have a 383 stroker and spent over 11 grand to build this thing. I will start it without all the pulleys but my gut is telling me it's my flywheel. Any other suggestions please weigh in.


Aug 17th, 10, 04:37 AM
Paul, my vibration was a pulley issue but other things I checked were:
Lean issue
Made sure the crank snout is true
also make sure you have the correct flywheel/balanced for your setup.

Hope this helps!