Not so good vibrations

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Sep 22, 2023 | 03:21 PM

greatscott73

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Not so good vibrations

Recently purchased 1989 convertible. Mileage unknown. When the trans, which seems to shift OK, pops into overdrive, a nasty low resonance vibration appears. It is apparent through the driver's seat. Engine seems to run OK, thanks to some new injectors. Found a couple of broken exhaust hangers and replaced, which seemed to help a little. Fluid and filter in trans replaced (just the pan contents), new tires. Dropping the trans into drive seems to make the vibes go away, so its obviously in the driveline. Usually around 38-40 mph and 1100 rpm. I suspect the torque converter is on its last leg, but I'm open to suggestions. I plan on pulling the driveshaft this weekend to check the u joints, but not really sure where else to look. Thanks in advance!

Sep 23, 2023 | 08:58 PM

84 4+3

Le Mans Master

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From: New Jersey

At 1100 rpm in OD... the engine is lugging and you are getting exhaust resonance.... Could there be something wrong? yes... but I'd speculate that is your issue.

Sep 24, 2023 | 07:24 AM

greatscott73

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Thanks for the response. Are you saying the car should not be in OD at such low RPM? It seemed to me that its shifts there pretty quickly, but
i'm new to this platform and trying to figure out how its supposed to work.

Sep 25, 2023 | 06:21 AM

84 4+3

Le Mans Master

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From: New Jersey

I'm not very familiar with the autos on these but... I do think overdrive at that low of an rpm seems off. If I were to shift my car into 4th at that rpm it would chug and buck and sound labored doing about anything.

Sep 26, 2023 | 05:54 AM

dentalfloss

Racer

Joined: Aug 2014

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From: Elizabethtown, NC

Do you have after market exhaust? Most of our C4's emit somewhat of a drowning noise after shifting into overdrive at around 42 mph especially if you have talll gearing like 259 ratio and more so if you have after market exhaust. I would not suspect the converter or U-joints. At part throttle it should shift into overdrive at around 42 mph and after that if you step into it a little it should shift back down. I would do a TV cable adjust.

Sep 26, 2023 | 06:44 AM

greatscott73

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I just did a TV cable reset after removing the plenum to swap fuel injectors, but the issue was there both before and after. The exhaust appears to be stock, even though the rear has a few small rust holes. replacement due soon. The car is a convertible, and since the vibration feels like its coming up right through the seat, I'm wondering if the frame cross brace could be resonating, since it has attachments right under the seat. I have it off today to do some other under car work, so may take it around the block without it and see if anything changes.
I also wondered about a possible metal on metal worn out engine mount, but they sure don't make it easy to see them on the car. I'll bet replacement is fun.

Sep 30, 2023 | 12:19 PM

greatscott73

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While exploring under the car, found that the x-brace looks like it has survived (barely) some off-roading incidents. Lots of bends and dents. This had caused the brace to come in contact with the exhaust close to the spot where the catalytic converter exits. After some heating, bending, swearing and re-aligning, i got the required spacing accomplished. Since I had a large box of thin rubber gaskets left over from some other project, I put one on each of the four attaching points between the brace and the floorboard. Problem solved. Its like a completely different car.