Other reasons for engine vibrations?

Hey guys, I replaced my motor mounts a couple months ago, still have a slight engine vibration (enough to shake the passenger seat.) I asked my mechanic to check out the transmission mount but he said it looks fine. Ordered a set of those transmission mount inserts, gunna give them a try anyways. Any other things that could be causing the vibration? I've also noticed after driving on the highway and coming to a stop on the off ramp, sometimes the engine will stutter a bit at idle and I'll see the voltage meter fluctuate. Anything I should be worried about? Thanks, Chris '02 Targa, 80k miles

It could be the Flywheel (DMF). It's job is to dampen the vibrations. I have one going in soon, and some of the same issues.

Cheap thing you can do is clean your throttle body. Remove air box and intake tube. Spray some carb cleaner on a lint-free rag and wipe the metal inside the throttle body and metal flap. Also push flap to clean other side. This always smooths out my idle. Don't know about vibrations.

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Spark plugs, air filter, correct grade of gas?

Just had the spark plugs replaced, will check the air filter, been happening for more than one tank of gas. It's a weird issue, idles just fine most of the time, only notice the rough idle for a couple seconds when I come off the highway and come to a stop.

-Spark plugs -flywheel -cam timing deviations. Try turning the AC off when the engine is doing this, if the vibration stops with the AC off, then the AC compressor is dying and is the source of the issue. I see lots of bad AC compressors when doing my extensive post-procedure road testing and the symptoms are always like you state.

Now that you mention it, I think I have noticed it may be related to running the AC. Will do some more testing.

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:-) A can of A/C system lubricant may solve it.

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That could be an early sign of AOS trouble. The DME has adapted to the air flow while running at highway speed then when you leave the highway and the engine idles the AOS air leak causes the DME to have to relearn. In the time it takes to do this the engine runs a bit rough. I had something similar happen with my Boxster after running all day at freeway speeds. The difference was the momentary roughness was accompanied by a CEL with the error codes pointing to a mixture problem than had me suspecting a bad MAF. This proved to not be the case for a few days later at my final destination the engine upon a cold start emitted a billowing cloud of oil smoke that lasted for nearly the minute I let the engine idle. Another possible explanation is the oil filler tube cap is leaking. At higher engine RPMs the air leak does not contribute a significant amount of air to the total amount of air the engine is using. But at idle this changes and until the DME adapts the engine idles a bit rough. You can artificially reproduce this behavior in a healthy engine by at hot idle removing the oil filler tube cap. The engine will react, idle a bit rough, but in just a few moments the DME will adapt and the idle will smooth out to near the same smoothness it had before the cap was removed. When you replace the cap the engine will react once again -- though not as dramatically -- and DME will then go through another adaptation period and the engine idle will again recover.

Is the AC lubricant something I could do easily myself? Have a link to a product that would be compatible with our cars? Thanks! Chris

Not likely. A/C compressor pump oil "top up" is usually done by putting the A/C system under low pressure for a time (IIRC for 1 hour but this may be different for different Porsche models) and removing the compressor oil along with the refrigerant and concurrently checking the system for any leaks. (This low pressure condition also removes any moisture from the system though if the system is still pressure/fluid tight there should be no moisture in the system.) In the case of the pump oil the amount removed is noted and the proper amount to bring the total amount up to the called for total amount is added. The system doesn't contain much compressor oil and you shouldn't just tip a can of it into the system even if you could open the system up without compromising the system. If the A/C system has no leaks, cools properly, the odds are the A/C system has the proper amount of compressor oil (and refrigerant). Just to be sure, since you are suspecting the A/C compressor having a problem and accounting for the rough idle you can have the A/C system checked for leaks and the amount of refrigerant and compressor oil checked and brought back to within spec if out of spec.

I have had 5ccs of oil solve this issue and prolong compressor life for 2 more years in the past.

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How do you add the oil to the A/C system?

You can purchase PAG oil with a small amount of R134a in a pressurized can. You add it like a regular charge top-off.