compression test results ??? what is good?

[george graves]

"dry" compression test results: (wot, fresh battery with booster)

1. 175psi
2. 150psi
3. 150psi
4. 150psi
5. 153psi
6. 155psi

The motor is said to have a blown head gasket - with about 200k miles on it.

What are accaptiable results for a compression check? Or do they vary so much it's hard to tell?

What is a ball park psi measurement you should get? 180? 200? 220?

Thanks!

George

[Tiger]

Don't know what engine you have... but those numbers look good to me... a bit worn but still a healthy engine. I think #1 figure is probably wrong due to an extra cycle pump. BMW engine tends to have higher number... What made you think you have blown head gasket? Losing alot of coolant? Unusual cooling problem?

[george graves]

Sorry - I am looking to buy a m30 from a e34 - this motor came up at a yard.

I've been told that the motor is boiling over, and that coolant is comming out of the overflow tank.

finding a good m30 motor is next to imposiable!

[mholbrook]

Those compression numbers looked ok to me. I ditto Tiger on motor condition. I did my compression test last week and got 175 with one cyl at 155 (6). I agree that you should definately try to get this going to confirm blown head gasket. A leak down test will tell with certainty.

[Bill R.]

10 to 11 bars minimum which is 142 to 156 psi minimum with a .5 bar maximum variation between cylinders which is 7 psi difference between cylinders. The normal rule of thumb that most mechanics go by is 10% maximum variation for most cars... meaning if you have 175 then the lowest could be 157.5 , if it were me I'd keep looking for another motor.
Especially if you go by bmw's maximum alllowance for variation.







"dry" compression test results: (wot, fresh battery with booster)

1. 175psi
2. 150psi
3. 150psi
4. 150psi
5. 153psi
6. 155psi

The motor is said to have a blown head gasket - with about 200k miles on it.

What are accaptiable results for a compression check? Or do they vary so much it's hard to tell?

What is a ball park psi measurement you should get? 180? 200? 220?

Thanks!

George

[winfred]

if you are anywear close to louisiana i have atleast a half dozen good m30's that have run in the last couple months $600


Sorry - I am looking to buy a m30 from a e34 - this motor came up at a yard.

I've been told that the motor is boiling over, and that coolant is comming out of the overflow tank.

finding a good m30 motor is next to imposiable!

[winfred]

leak down on a cold motor will suck bad, worse if the motor's sat for a while
the way i look for a leaking cylinder is to fill the cooling system to the top and pressurze the cylinder with the leak down tester or a compression tester with the valve removed, and watch for the water to rise, generally the fail point on a m30 is the rear exhaust side of #6


Those compression numbers looked ok to me. I ditto Tiger on motor condition. I did my compression test last week and got 175 with one cyl at 155 (6). I agree that you should definately try to get this going to confirm blown head gasket. A leak down test will tell with certainty.