Done compression test.. Wish I hadn't

[Gary71]

Because I now know that I need to rebuild my engine...

I've been trying to nail this partial misfire and general loss of performance for a while and have notice whilst doing the 'pull off the HT lead whilst running' test that doing this on any of the RH bank doesn't change the engine speed particularly. From the numbers I think I can see why:(

Opinions on these numbers please 911 engine gurus. It's a 2.4T, which I believe has standard pistons etc:

LH Bank

1: 128psi
2: 135psi
3. 98psi

RH Bank

4: 98psi
5: 80psi
6: 80psi

Valve clearances are all OK and the test was done cold.

To me this says at best the valves aren't seating properly, at worst.... well sky's the limit really.

Thoughts are appreciated

[Lightweight_911]

The key factor is the variation between the figures - generally a healthy engine shouldn't vary more than about 15% at most between the highest & lowest readings.

In your case, 4 of the cylinders fall significantly below this figure with the worst 2 being only 59% of the highest reading.

A better indication of the state of the engine could be obtained by a leak-down test of each cylinder.

[Gary71]

Thanks Andy,

Unfortunately I get the feeling this is only heading one way...

I still need to use the car as the daily drive for another week until I pick up my eldery BMW, after that I guess it's coming out and apart.

[Barry]

Bummer: as you say, there's very little that could bring that sort of loss back up without getting the engine stripped. If you hadn't said that you'd checked them, I'd have said valve clearances would have been one option.

Just one thing: you did do them with wide open throttles? If not then a variation in butterfly angles can affect (effect?) the readings. They do seem ever so low for a car that isn't beltching blue smoke all of the time. I know that if it's valve problems you won't have the smoking, but even so ....

[Gary71]

Hi Barry, Yup I'm afraid this is it...

Currently costing up the rebuild...

[satan]

Hi Gary,

Did you perform a valve adjustment prior to the test? Are you 100% on the clearances - there are a couple of more precise methods than the rooting-around-with-a-feeler-gauge under the elephant foot one.

Also, is the battery fully charged and providing consistent cranking? Could be an idea to fully charge it and then repeat the test in the reverse order.

Might be worth double-checking these before you go down the rebuild route.

Cheers,

Chris

[Gary71]

I'll repeat the test again tonight just to make absolutely certain before I panic . too much.

Chris, I did the clearances about 500 miles ago, but you are correct I will look again as something may have slipped. Battery wise all is fine, I've two strong batteries so there's no issue there. I cranked until the gauge stopped moving (around 8 seconds per cylinder).

Barry, I'll also tie wrap the throttle butterflies open, I did it before with my foot stuck through the door, so they may not have been 100% WOT.

Fingers crossed (Optimistic head fitted... )

All else fails that $2 to £1 looks good for the bits, especially as I have a friend living over there now to 'help' with postage.

[Barry]

That's true, at least it's timed it well .

Good luck with the repeat test, although if you more or less had the throttle open, it won't make much difference after that . Same with cranking: the conditions (speed) are the same for all cylinders, so they may all come up a bit if cranked faster, but will have the % differences between them.

[Mike911scrs]

HI Gary71 is there any chance you can do a cylinder leakage test befor you remove/strip the engine as it will then narrow it down as to what is required, rings/valves/head joint leaking, pulled studs.

regards mike

[Gary71]

Hi Mike,

This is my plan, I'm going to borrow the kit from our workshop in the new year and have a better look. I praying it's top end... I'm an optimist