depami

Now repeat after a stroke or two with a couple ounces of heavy oil or STP and compare leak down times. If times increase greatly rings are leaking. If times don't change, valves are leaking.

Ducman82

so add a spoon of oil to each cylinder, crank that SOB over a few time with the starter (fuel off)

i have some 75/90 i can use...... :-P

docmirror

Where is the air GOING by your ear? It should be obvious.

I've never done a time constant leakdown. I've used the kit with a calibrated orifice and measured the differential pressure on the return gauge. 80PSI in, and measure the leakdown amount across a 0.040" opening in the tool. This will give a reliable deal, and you can hear where it's going easily.

The plugs look like ****.

Ducman82

Ya plugs suck do to running crap timing. Never changed them when fixed it.

Air is going to crank case. Nowhere else. Even busted out the stethoscope.

docmirror

Rings are toast. Something is wrong down there. Either an assembly defect or sizing, or something.

Lizard928

I wouldn't be so quick to rush to that conclusion doc.

#1, how can that much oil get up past the rings, ESP when one cylinder was near top?

I'm thinking there is a crack from incorrectly changing the valve guides on cyl 3/8

docmirror

You may be right Colin. Hard to believe that much air is getting through anywhere but if he's hearing it from the crankcase, and not from the intake or exhaust it's going through the rings. Still want to get the valve wobble test done but that's more work.

Dave928S

Those times correlate with what I got on healthy bores ... about 8 seconds. The higher readings are similar to what I got when I did a wet (with 10ml of oil) test.

With a broken compression ring I got leakdown in a couple of seconds, and it didn't improve much with oil, so it doesn't look to me as if there is a problem with the rings or valves. You can still hear air leaking past the rings into the crankcase when rings are OK, but when a ring is gone the leakdown is so fast that it comes out in massive volume through the oil filler, and it's unmistakable that there's a problem. On my 82 with a broken compression ring it came out in such volume that it spun the filler screen basket like a top. Edit: Just checked figures and I had 1.3 seconds with a broken top ring only, and no cylinder scoring.

I'm thinking big time valve guide/cracked head problem too.

Note: With a traditional leak down tester you're comparing the leakage of the cylinder to a reference hole, and the result will be expressed as a percentage.

In doing the test that I describe, you're not comparing the leakage to a set hole size, but measuring the time to leak down from 100 to 10 psi. I've performed the test on a few 928's, and other cars, so I know that if compression and everything else is OK it should take about 8 to 10 seconds to leak down on a 928; if something is wrong, it takes WAY less. It doesn't give you a result as a percentage of a hole size, like a standard leak down tester, but it still gives you a result that means effectively the same, and gives you the same information about what is wrong/right.

Ducman82

looks like time to take her out and do some more checks..... :-(

i think i will add that .040 hole, and another gauge to get "known numbers" for the shop that had the heads done. (numbers they know etc)

UGH

i wish i had a heated shop.

LT Texan

Did you ever do a compression check on each cylinder? What were the readings?

My money is on a poor head work.

Ducman82

Sure did. But I might do it again just for giggles

depami

What were the numbers? Dry and wet.



I agree with Taz Dave about the rings being ok. I would still do the wet test. A compression test might be worth while at this point. Once you take it apart you no longer have that option.

If you can get satisfaction from the shop that did the head work then pursue that first. If not, I would be inclined to try new valve guide seals before pulling heads or engine.

I say that not having firsthand knowledge of the level of difficulty of that task on a 928.

Time investment aside, I do know that changing valve seals will be way less $$$ than pulling heads or engine.

If the seals don’t fix it you wasted a couple weeks and a couple hundred bucks at most.

If you pull the engine it will be at least a couple thousand bucks by the time you are done.

LT Texan

I bet the values weren't bad.

Ducman82

here are my compression numbers from last time

1-174
2-174
3-165
4-170
5-165
6-165
7-165
8-170

LT Texan

i vote for shoddy head work