Leakdown and compression results... questions

[Skip Newsom]

I posted this in the 911 BBS site and only got a couple of replies, so I though I'd try here as well.

My 11 year old Son and I spent a couple of hours Friday evening testing the condition of my stock 2.4 MFI with 90k on it, the engine has had no major work done that I'm aware of.

It was tested warm, and retested to check accuracy... I did a complete tune up including valve adjustment about 500 miles ago, mfi, timing and every thing else is spot on.
When I pull the #1 plug it's always dark compared to the others.


The results:
#1 18% 80psi
#2 7% 85psi
#3 8% 83psi
#4 7% 100psi
#5 9% 90psi
#6 6% 92psi

I tried to determine on #1 where the air was going during leak down, but it was hard to tell... sounded a little louder under the car so I'm guessing exhaust valve.

It seems odd to me that the left bank would be so much lower on compression than the right. The engine starts and runs fairly well, but it had sat for several years at the PO's garage.
I"ve dropped the motor, cleaned it up, replaced all the easy gaskets and seals already, and I've put 7K on it in the last 18 months of ownership.

Impressions??
Do the compression #s seem low?
Would you rebuild at this point?
Any one have a free 3.2 they want to get rid of?

[Rot 911]

Did you check to make sure your valves were properly adjusted first? If so then you are looking at a rebuild. That engine is tired.

[724doorE]

On mfi cars I belive you should have the throttle butterflys wide open when you do the test to ensure you are getting a good pressure reading in the first place. Tight valves can give the leakdown as can rings or brned valves. Do you have any oil burning or loss issues?

[Skip Newsom]

I've done valve adjustments countless times on VWs and this is my third on this 911 so I'm comfortable with the settings.
The engine burns a little oil, maybe a quart in 1200 miles.

I'd not heard of having the butterflies open... how would that change the pressure in a cylinder that should be sealed at TDC?

[RoninLB]

gotta have good air on the intakes.

and if it was my car and the valve guide clearance was in spec ?
I'd install an MSD.. and maybe run One hotter plug if one was still too dirty.

[Steve@Rennsport]

Skip:

You are right,......valve adjustments do not affect leakdown tests,....

Compression test results on the other hand, are indeed affected by valve adjustment.

Looks to me that some engine work is on the horizon,..

[Wayne 962]

Originally posted by Skip Newsom
I'd not heard of having the butterflies open... how would that change the pressure in a cylinder that should be sealed at TDC?

It won't, but it will affect compression tests. When doing a compression test, make sure your valves are adjusted, and the throttle is WIDE open. This can dramatically affect the performance of the compression test. I would suspect that your left/right variance is caused by your MFI throttle bodies not being balanced left to right. If you redo the test with Wide Open Trottle (WOT), then these left/right variances should disappear. For reference, the variances only appeared on the compression test, which is affected by throttle and/or valve adjustment.

Either way: Leakdown of 18% + dirty plug + compression results showing that as the weakest cylinder = Rebuild Time.

Get a copy of the Engine Rebuild Book...

-Wayne

[Skip Newsom]

Thanks for the replies Gents!
I will have to redo the compression test again with WOT, my son will love it! he can't wait to bury his foot into the gas pedal. I'm probably creating another Pelican head.

Wayne, I already have both the 101 projects, and a signed copy of the engine rebuild book.

Now the fun begins... stock? nah, 2.4S spec? maybe, 2.5 big bore? hoo ha!
Choices, choices.

[snowman]

Leakdown results indicate a good engine, except number 1 may have a small problem. I would expect to see good compression numbers with the throttle plates open (with them closed the engine can't suck any air to compress).

If the compression checks out say 150 or more then redo the leak down and listen for where the leak comes from.