engine vibrations

87 buick park ave 3.8

i bought this car not running. the previous owner had replaced the plugs, wires, egr valve, and maf sensor to try to fix. i replaced the the coils, and ignition module (they were melted together) and got it running. the motor and tranny mounts were bad so i replaced them too. the problem is that the engine has a bad vibration at idle, that gets worse just off idle, but then smooths out as long as you are in the throttle. if you let off the throttle the vibration comes back, but not as bad as it is at idle and just off idle. you can feel the vibration in the steering wheel, floor seats, dash, its bad. at speed under throttle the vibration is gone. i pulled all the plugs, and they look fine. i've checked endlessly for vacuum leaks. the harmonic balancer looks to be in good shape(no rubber cracks or separation) anyone have any ideas?

thanks

My 87 in stock form would alway shave an off idle. Did all the same and went nuts over it. Even weirder, at around 40 - 55*F it ran flawlessly, no vibration, and colder or warmer, junk :dunno:

2 areas of note that made improvement:

1 - CTS & IAT sensors made a difference

2 - There is a vacuulm T that branches for the vacuum canister on the firewall and other towward the transaxle (cruise if I remeber right). Every connection on that whole vacuum line would get pretty flimsy very quicly due to the location (engine/trans/exhaust crassover all being there. Especaill pay note to the 'nipple' on the vacuum canister.

thanks for yhe advise, one other thing it does that may help pinpoint the problem is that when you first crank it up it idles high, like a carb on fast idle, for about 5 seconds before it drops to normal idle. i usuallt don't see this on fuel injected cars.

Oops, I forgot to mention this :sorry:

Check the idle air control (IAC) motor and passages. It's the 4 prong bugger on the left side of the throttle body. You can remove it, clean it (carefully) and check the pintle depth, s/b 1 1'8" base to tip (can you tell I've pulled mine a few times? :lol: )

Anoher possibility is the PCV valve and grommet.

FIX NOTICE

i borrowed a snap-on scan tool and noticed the tcc was engaged. i unplugged it, and 80% of my vibrations are gone.

FIX NOTICE

i borrowed a snap-on scan tool and noticed the tcc was engaged. i unplugged it, and 80% of my vibrations are gone.

Just to throw in my :2cents: I had a vibration at idle with my car as well, and I soon found that the motor mounts needed to be replaced. The vibration was TOTALLY gone after replacing the mounts!

the motor and tranny mounts were bad so i replaced them too.


i already replaced them