Low Compression M20 Engine

[QWKE30]

1989 325i 150k+ miles
Manual transmission

After a diagnosis of low compression I had a valve job done and changed the sparkplugs and we got a slight improvement.

Original compression check =========== Compression check after work.
Cylinder 1 110 =========== 1 120 9% gain
Cylinder 2 110 =========== 2 110
Cylinder 3 110 =========== 3 110
Cylinder 4 105 =========== 4 110 5% gain
Cylinder 5 105 =========== 5 110 5% gain
Cylinder 6 105 =========== 6 115 10% gain

What is the proper compression for this motor?

[bmwstephen]

supposedly 150 range but its very subjective. how did you do the compression testing? I assume the low but consitant numbers mean that while you were cranking for the test, you did not apply WOT

[QWKE30]

My mechanic did it, he didn't say how.

[slammin.e28]

Ask.

[QWKE30]

wet

[ForcedFirebird]

Those numbers look fine for an engine with miles. Alldata says a new m20 should have 142-156psi with a maximum of 60% variance (lowest cylinder should be 60% or more of highest cylinder), engine warm, throttle open.

[Jaxx_]

Those numbers look fine for an engine with miles. Alldata says a new m20 should have 142-156psi with a maximum of 60% variance (lowest cylinder should be 60% or more of highest cylinder), engine warm, throttle open.

60%? that seems a bit ridiculous.

If the 2nd number is wet, looks like just a valve train issue -- I would do a valve adjustment and re-test. 110psi is rebuild territory for most motors.
edit: yeah, I just saw that the 2nd number was after a valve adjustment. I would re-test with it wet and see if there is a vast improvement. if there is, rings are worn.


I've never gotten a huge variance from throttle open and throttle closed. the motor is not spinning more than 300rpms anyways. Just to give you an idea -- megasquirt will ready about 97kpa when cranking before the car fires. Not exactly far off from full throttle. cyls fill with air pretty quick.


edit #2: so were all the tests done wet? was the motor warm? I don't even understand the full testing procedure. Seems pretty stupid to just jump to putting oil in the combustion chamber to see what your compression numbers are.

[ForcedFirebird]

60%? that seems a bit ridiculous.

If the 2nd number is wet, looks like just a valve train issue -- I would do a valve adjustment and re-test. 110psi is rebuild territory for most motors.


I've never gotten a huge variance from throttle open and throttle closed. the motor is not spinning more than 300rpms anyways. Just to give you an idea -- megasquirt will ready about 97kpa when cranking before the car fires. Not exactly far off from full throttle. cyls fill with air pretty quick.

He had a valve job done, unless the lash was adjusted extremely wrong, it should be sealing.

I have had several m20's in the shop with 110-120psi and run just fine (street/daily cars).

Totally agree with you on the throttle plate, opening it only seems to make the needle go up faster with similar numbers in the end with it open/closed.

As long as the cylinders are consistent, I don't follow alldata's 60%, I like to see less than 20% variance.

It takes only 80psi cranking to make fuel combust BTW. I have seen an m20 running at 90-100psi on all 6 lol.

[Jaxx_]

He had a valve job done, unless the lash was adjusted extremely wrong, it should be sealing.

I have had several m20's in the shop with 110-120psi and run just fine (street/daily cars).

Totally agree with you on the throttle plate, opening it only seems to make the needle go up faster with similar numbers in the end with it open/closed.

As long as the cylinders are consistent, I don't follow alldata's 60%, I like to see less than 20% variance.

It takes only 80psi cranking to make fuel combust BTW. I have seen an m20 running at 90-100psi on all 6 lol.

I agree with your post, but when the motor should be sitting at like 160+ from the factory (even these stone age motors) I wouldn't want to have 110psi. The good thing is that the motor has worn evenly but the truth is that even if a car runs fine it doesn't mean it's to it's full ability. For getting around town even an oil burning piece of shit will do.

[slammin.e28]

I have an old Willys Jeep 4 banger and it has an even 30psi across all four cylinders.

It ran.

It also puked blue smoke, but it ran.

[ForcedFirebird]

I agree with your post, but when the motor should be sitting at like 160+ from the factory (even these stone age motors) I wouldn't want to have 110psi. The good thing is that the motor has worn evenly but the truth is that even if a car runs fine it doesn't mean it's to it's full ability. For getting around town even an oil burning piece of shit will do.

Factory manual states 156psi. I will stick with experience. These cars are old and have only seen ONE come in the shop that had 125 on 5 and 120 on one. If the OP is satisfied with a decent running car, I wouldn't say that a rebuild is req'd is all I meant.

I have an old Willys Jeep 4 banger and it has an even 30psi across all four cylinders.

It ran.

It also puked blue smoke, but it ran.

lol. I have never seen anything run on that low cranking pressure. Have seen some low numbers with cams too big for the compression ratio, but never that low.

[Jaxx_]

Factory manual states 156psi. I will stick with experience. These cars are old and have only seen ONE come in the shop that had 125 on 5 and 120 on one. If the OP is satisfied with a decent running car, I wouldn't say that a rebuild is req'd is all I meant.

agree. I have ~145 across all 4 of my M10.

[digger]

i had 150psi +/-5 on all my stock B25 cylinders with 110k on the clock

[Galactodactyl]

142-156 PSI is what the Bentley manual says.

Last compression check I did was 164/155/160/128/155/155.

270k on the clock 89 325is.