DeWolf

Just wondering what sort of tester you guys use. If one with the flex hose but wonder how do you get it tight into the spark plug hole. I can't find one with a long solid tube as such.

Adamant1971

Just screw it in. Usually hand tight is sufficient on a compression tester or leak down tester. As long the o-ring is sealing well your golden.

davek9

the auto stores sell ones that fit different cars, and come w/ different fittings w/ 'o' rings, you screw them in hand tight.
Pull all the plugs, pull the Fuel pump fuse and EZK/EZF (ignition) fuse and crank until pressure no longer increases.

Kevin in Atlanta

Also, I think you are supposed to do it on a warm engine.

GlenL

The tube is fine. Tighten snugly and do each hole the same way. Crank for a few secods and do each cylinder consistently. Repeat the first hole last to see if grinding the battery down is lowering pressures. Re-test any low holes.

Analyzing the results is best done looking for consistent values. The rings and valves don't degrade evenly. The actual pressures will depend on the engine's CR, the cams and the gauge.

DeWolf

Cheers guys.

dr bob

Add "Throttle Blocked Wide Open" to the list of test parameters.

DeWolf

It's an 86 so has the large intake and the plenum will be off to get to the plugs so in effect it will be wide open throttle.

ramcram

Yes! Definitely don't crank it with ignition with the plenums off.

I like leak down testers best. Tell you more generally, no cranking by starter. Listen to intake, to the exhaust and in the oil filler with a stethoscope, and get a good idea of what's leaking and where, plus the pressure drop give you a percentage of wear per cylinder.

DeWolf

Ok, test done. Results not so good. 5-8 175-180psi, 1-4 145-150psi.

FredR

If one bank is down then your methodology is the initial suspect I would politely suggest - not impossible but not really plausible. I cannot think of anything that could/would cause such to happen BUT...?

Please confirm you had the throttle wide open for each test and the motor was warmish for starters.

I consistently got 185 psig when I last did mine [90 S4 motor]. I am advised a fresh build should give a bit higher number but 26 year old rings etc....

If you cannot resolve this discrepancy then go to a leak down test. The two tests together give a more comprehensive picture of what is [or is not] going on in your motor but at the moment I would tend to suspect methodology until you can prove otherwise. Very easy to be overly concerned.

Rgds

Fred

hacker-pschorr

When the two banks don't match that could mean your cam timing is off.

Bill Ball

I recently encountered one 928 with this kind of discrepancy and the timing belt was installed one tooth off on the lower side.

DeWolf

Hi Fred,

Motor warm and both plenums off so essentially wide open throttle. Doesn't get any more wide open than straight through....lol. I then went back and did two more on the 5-8 bank and got the same results, 180psi. Then went and did the 1-4 bank again and still only got 145-150psi.

FredR

OK- then as Bill/Hacker have intimated next thing would be to check the cam timing.

Perhaps you can give a bit more history here-
1. Has the timing belt been off during your ownership
2. Have you checked the cam timing at all
3. What prompted you to do the compression check in the first place [a perceived power loss?]
4. Not too relevant but please confirm model [86.5 32V ?]. A bit strange down under yes?

If you have 4 cylinders consistently down then the usual concerns for lack of compression pressure logically do not apply and as intimated, this throws suspicion on cam timing. Bill's comment is very pertinent as small amounts of cam timing should not cause the kind of differential you indicate but a full tooth on one bank is a distinctly possible error albeit someone had to be "very naughty" not to spot such when they worked the motor. This type of error is quite easy to do but to not spot it during cross checks is unforgivable especially if you or a previous owner was paying for the work to be done.

Rgds

Fred