[snova]

I have a 1992 Coupe, 6 MT with 115,000 miles.   Since I have owned it have had persistent issues which I think are due to engine vibration.

I have a known slow leak around the balance shaft, I have switched to M1 15w50 and there is no leak in the colder months, but it does start to leak in the summer. Not a lot, maybe an 1/4 quart over the summer.   When I first start the engine it gets a pretty hard vibration during the starting process which last only about 1-2 seconds when the engine is cold. Not much vibration happens when I start it and the engine is warmed up.  After the engine is warmed up,  I don't feel any vibration until I get into high rev, around 4500 rpm and higher.  The vibration is felt the gear shift knob and it's not very extreme.    

 

Here are the issues which I have been seeing which I think might be root caused by this vibration.

The bolts in the original a/c compressor holding it to the engine bracket would constantly vibrate out.  Both the front and the rear bolt. Finally, I said F it and replaced the a/c compressor.  They stopped vibrating out with the new a/c compressor.  Went I replaced the alternator with a remanufactured out, the bolts holding the alternative housing together vibrated out.  Using blue thread locker didn't help.  So I finally resorted to red thread locker and they no longer vibrate out.   However, later I had an issue again with the rear bolt of the a/c compressor that attaches it to the engine bracket.  It didn't vibrate out this time, it just sheared internally mid way into the engine bracket. I also notice power steering fluid seeping out of the power steering assembly.  Even though I recently rebuild the p/s pump, the seeping continues and seems to be coming between the two halves of the pump. I have also been going through voltage regulators on the back of the alternator. They work for a while and then eventually they started to fail when the engine gets hot. It works again then the engine cools down. when I removed last voltage regulator, the top D+ brush just cracked away.   I feel like all this is related to vibration.  Thoughts? 

I'm thinking bad motor mount or balance shaft bearing?    What would you recommend?

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[ds968]

I would start with looking at the motor mounts, but the balance belt skipping a tooth could also cause engine vibration at startups / low rpms.   Not idea about the shaft bearing though. 

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[Rap]

Harmonic balance

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[MCL968]

Check and then double check the alignment of the balance shafts. Mine have been off by a tooth and the vibration change is noticeable. I didn't notice much of a change in vibration when I had the old motor mounts replaced, even though they were ancient and totally shot, but the balance shaft being off by a single tooth made a huge difference.

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[tejon007]

These motors do vibrate. High compression, high performance, making 236 HP from 3 liters NA...



However, oil leaking on balance shaft belt, is a clue that the belt might be deteriorated, make even lost a tooth or two. Like Matt, my first thoughts were the timing of the balance shafts.

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[snova]

thanks Gentlemen. I'll focus on the balance shaft.

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[vliegwielolaf1234]

And since you have leaks , an should take things appart to fix those , have a close look if the sprockets are in the right position on the balance shafts , i have redone a few over the years wich where installed incorrect .

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[ds968]

By the way , IF it turns out to be the balance belt skipping a tooth , remember that although it’s commonly known balance belts have to be re-tensioned at 2 - 2.5 K mi. after installation, there are some on this forum who insist the balance belt must also be re-tensioned again at every 10K miles intervals afterwards .  

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[oppositelock]

Since the vibration is also associated with cold start/hot start, also check your ignition system, especially the spark plugs. If you don't have maintenance history, these are worth checking/replacing. If you don't have a maintenance history, have a mechanic who knows transaxle Porsches well check it out. You may need to chip away at belts, ignition, and engine mounts. These will run forever given love, but will cause pain if ignored.

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[snova]

Since the vibration is also associated with cold start/hot start, also check your ignition system, especially the spark plugs. If you don't have maintenance history, these are worth checking/replacing. If you don't have a maintenance history, have a mechanic who knows transaxle Porsches well check it out. You may need to chip away at belts, ignition, and engine mounts. These will run forever given love, but will cause pain if ignored.

who do you use up North?  I haven't found anyone I trust down here in PDX.

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[Waylander]

Motor mounts are on the pretty difficult list



But they can be measured in situ

You also have to consider the engine is massively over fuelled for the first few seconds while the MAF measures air temp and flow rates, a few seconds to get its self sorted out

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