Compression test questions

[dewme5]

1 = 120
2 = 110
3 = 125
4 = 125
5 = 120
6 = 110
Warm engine. 89 M20b25

I've been guessing that my timing belt is one tooth off. Power seems a little low, and acceleration seems a little slow.

I've replaced the complete fuel system, new injectors and chip, and adjusted valves.

Passed smoke test.

The idle is just a bit rougher then it should be.

I've seen a few other posts with similar compression test numbers, but nothing really resolved. Any suggestions?

[Cabriolet]

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

[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

[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.

[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.

[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]

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.

[jlevie]

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

[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.

[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]

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......

[digger]

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

[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

Sent from my HTC6435LVW using Tapatalk

[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]

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

Sent from my HTC6435LVW using Tapatalk