Howdy,

I would appreciate your opinion on the results of some engine testing I did.

This is a 1986 951 with Autothority chips and an ARC-2, which in the past was set to provide a little bit more fuel than stock. Test pipe and K26-based "cheater" turbo.

Spark plugs: 1-3 are slightly sooty (dry). #4 is very sooty. No electrode deformation etc.

Compression (1-4): 145, 140, 120, 100. Compression was tested cold, because some dodo (me) apparently had cross-threaded the #4 spark plug several years ago. I could not get the plug out with warm engine.

Leak down: I only performed a leak down on 1 and 4, because I was not sure how to reliably find compression TDC on 2 and 3. Again tested cold. I did not record leak down %, because it was obvious that there were problems: on #1, air escaped through the #2 spark plug hole. Some air through oil dipstick tube, but did not seem excessive. No coolant bubbles. On #4, air through #3 spark plug hole. There seemed to be no air through dipstick, but some air through throttle body.

So... I am thinking head gasket. I am willing to disregard the ring blow-by at this time, because the engine is cold. Thoughts?

The head needs to come off anyway to fix the #4 spark plug threads, but is there something else I ought to check now while the head is still on?

EDIT: I just realized that when #1 is at TDC compression, #4 is at TDC exhaust, correct? Does it matter? The valves should be closed either way?

Thanks!

Gary

 

no the valves will not be closed..you have to be on compression stroke as the cam turns 1/2 as much as the crank.

Best way is to find TDC on compression stroke on #1, and run through the firing order setting the crank 90 degrees further each time.

Probably still have a HG problem but I would make sure you are on the right crank position first

 

On the 951, with correct cam timing, the valves are completely closed @ TDC. Check 1 & 4, rotate 180 degrees & check 2 & 3. I use a wooden dowel in the spark plug hole (a screwdriver will work) to find TDC but you need to be careful not to wedge it in the cylinder by going to quickly.

 

Thanks for the responses. I re-did the test for all cylinders (at TDC compression, just in case), and here is what I got:

1: air through #2 plug; some air at oil filler and TB
3: air through #4 plug; some air at oil filler and TB
4: air through #2 plug; some air at oil filler and TB
2: some air at oil filler and TB

Note that the results for #4 are different now. Can't explain why.

Also, why would #2 be fine and not leak through the other spark plug holes, when #1 and #4 leak through the #2 hole?

These observations really do not make much sense to me. Any thoughts?

Thanks!

Gary

 

Bueller?