Compression test questions

[ForcedFirebird]

...Do a leak down test to confirm...

Well, it may not be the head. Have a shop run a leak down test and you should then know if it is the head or the rings.

Leak down will reveal all.

[dewme5]

The one part that bugs me, prior to any other testing, is all the numbers being uniformly low.

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[jlevie]

Well, it may not be the head. Have a shop run a leak down test and you should then know if it is the head or the rings.

[dewme5]

I know pulling the head isn't difficult, just time consuming. Work has me moving around alot, so I don't have a garage, don't know any reputable machine shops, and can't have the car down for long. I don't have a leak down tester out here, and lack a compressor as well.

Just finished putting on the timing belt. Everything went smoothly, and power seems to be what it was before. I've run most every option possible prior to a rebuild or replacement. I've found a clean head for sale just a few hours away, so i might go get that. I'd really like to get a leak down first, but I'm headed towards the head tomorrow morning anyway, if he gets back with me, I'll just plan on picking up and swapping the head

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[digger]

can you not do a leakdown test before you assemble back together.its not hard to pull the head

[dewme5]

9 times out of 10, the valves were burnt on m20's I have opened up with low compression numbers. The exhaust valves tend to get pitted. Do a leak down test to confirm, but chances are you need to rebuild the head. Typically after a head rebuild I will see 175-185 cranking compression with a stock cam, and 160-170 with a 272 (more overlap decreases cranking compression).

Good info. and Damn it! I don't want to rip the head off......

[ForcedFirebird]

9 times out of 10, the valves were burnt on m20's I have opened up with low compression numbers. The exhaust valves tend to get pitted. Do a leak down test to confirm, but chances are you need to rebuild the head. Typically after a head rebuild I will see 175-185 cranking compression with a stock cam, and 160-170 with a 272 (more overlap decreases cranking compression).

[dewme5]

Well, it looks like it wasn't the timing belt. Good thing I got in there, the belt was cracked, and didn't have long left on it. However, the marks line up correctly.

Yesterday, while picking up a few things at the parts store, I figured I would try a bottle of "guranteed to pass smog" stuff from stp. Poured that in, and topped off the tank. Drive home was short. 3 miles ish, and parked for the night. When I got in it this morning, the idle was smoother, and felt like just a bit snappier performance. Nothing amazing, but just a subtle difference. I'll run another compression test after everything gets reassembled. Have to wait 3 more hours for the waterpump.

[jlevie]

In that case, change the water pump and fan clutch along with the timing belt & tensioner

[dewme5]

1 tooth off, is off enough to cause all the issues I am experiencing. On top of that, I have no idea when it was changed previously. I bought this car from auction, so everything I haven't changed is questionable.

[digger]

you dont need to redo the timing belt unless it is way off you should be able to see/check that without actually changing it...?

[dewme5]

I think there are just to many clues to a mis-adjusted timing belt. I'm going to get a new one, redo the timing, and will post results.

[jlevie]

Numbers are low, but pretty uniform. That could be mistimed cam, worn/misadjusted valves, or worn rings. Assuming the test was properly conducted with the throttle locked wide open and fuel injection disabled. A leak down test will tell more.

[JinormusJ]

Numbers are definitely low

Old e30 had 160 average
The iS has 150 average
M3 has 2 cylinders under 110, but it has known head gasket issues

[Cabriolet]

that's pretty low. anything bellow 135 would have me worried. normal is 165-185