Hey guys,

I recently did a spark plug and coil change on my 01 tt (just it 80k miles), the shop that help me did the job also help me to did a cold & hot compression test at the same time.
While the shop get 17x psi across all cylinders on cold compression test, they said they only got 16x psi on the warm compression test.

The car runs fine on regular basis, and doesn't seems to have any issue nor does it have any engine light pop up in the last year.

So my question is, what could cause this result? I know the engine is a bit cold at 80k, and 17x psi is not exactly high, but doesn't warm compression test result a lower number seems odd even if the engine is running crappy?

Now, I don't blame the shop if they did the test incorrectly, they are not a porsche specialist by any mean, but they did very good jobs on my previous cars and is an honest shop, so if there are any special steps I should be aware of when doing a compression test on a 996 turbo, any input would be greatly appreciate.

And back to the result. Should I be concern with this issue? Should I put aside some saving and ready for a rebuild in the near future? I've done some searching in this forum and seems like people are getting somewhere from 180psi to 200psi on the 996turbo engine with lower milages, but I understand different car will result in different numbers, so that can't be a rule of thumbs for anyone.

Any input would be helpful, thank you very much guys!

Oh and, HAPPY NEW YEAR!

 

i guess on cold engine the vario cam has a different timing for a short time, that could be the reason for a different compression at cold or hot engine condition

 

It is my understanding that it's not the number from the compression test but the difference between cylinder results that is the indicator of a failing engine. That is (if i recall correctly) if there is a 10% difference between your lowest cylinder result and your highest cylinder result, there may be need for concern.

I have seen cylinder tests conducted in many ways (cold engine, hot engine, placing a few drops of oil in the cylinder being tested) all resulting in different results. Just my 2 cents.

 

As a rule of thumb, 170 are good numbers for a cold engine not sure if they are good for a cold 996TT motor however maybe somebody who has done a cold engine before can chime in. As stated 10% variance is a good starting point. If your cylinders are 170cold 160warm I'd suggest you did something wrong on the test or there is some other factor at play.

Typically I find people (who are not automotive experts) overly concern themselves with tests who then can't interpret the results and worry more, thus conducting more tests. Eventually you google the symptoms and find out you have brain cancer and 1 week to live.

Just drive the car and enjoy. If something feels, sounds or drives different, have a specialist check it out then.

 

I sounds like everything thing is good,, you are still pretty much with in acceptable range across the board.
Some gauges very from manufacturers, so I wouldn't concern my self with other stuff numbers.
They must work fast to get all the plugs out on a hot motor and still get do a test on all..

 

THIS…………..ive done numerous compression tests with boat motors for buys and sales…….the difference between cylinder while hot should be better (less) then when cold also….

 

There are a few factors to consider on that topic.... Cylinder wall condition, oil weight and bleed down rates on the lifter.