Running a compression test - a few questions.

Hey Guys,

Planning to do a compression test this weekend, just for kicks. Car is going on 120k and I'd just like to know for my own peace of mind.

I have a tester and have done compression tests on other cars. Just wondering how to kill the fuel on the e46? IE, which fuse do I pull?

While I'm at it, any other tips for running a compression test on the M54? I haven't even looked up the acceptable ranges....

Pull the fuel pump fuse.

It's more important that your numbers are consistent between cylinders, but compression psi should be 200+

Yes, the fuel pump fuse. I doubt the numbers will be that high, shouldn't it be more along 150-160psi?

Yeah, I know consistency is what I'm looking for.

Which fuse is the fuel pump fuse? I don't have a Bentley handy at the moment.

Fuse #54 in the glovebox

Originally Posted by vaio76109

Yes, the fuel pump fuse. I doubt the numbers will be that high, shouldn't it be more along 150-160psi?

I just looked at some pics of the last comp. test I did, and the numbers were ~210psi. My gauge could be inaccurate though

Sc'ed?

Thanks

Originally Posted by wrecklessfool

Fuse #54 in the glovebox

Originally Posted by vaio76109

Sc'ed?

If that question was for me, yes

yeah look right above his signature pic.

activ, how much hp you putting down?

I think he's just wondering if the blower makes a difference in the compression. Greg knows I'm not SC'd

Originally Posted by Activ3

If that question was for me, yes

I don't think the SC makes a difference in compression. When the motor is turning over to start, I don't believe it's creating enough pressure to affect the compression reading.. although I could be wrong.

Broach, my last dyno was 321whp

okay so my car has a 176k miles. should be doing my comp check later. to get to the plugs, do you take the engine cover off and you will see them? or are they in the distributer? noob question i know but im young and i have honestly never messed with my cars engine other than install CAI. thanks, broach

Originally Posted by broach328

okay so my car has a 176k miles. should be doing my comp check later. to get to the plugs, do you take the engine cover off and you will see them? or are they in the distributer? noob question i know but im young and i have honestly never messed with my cars engine other than install CAI. thanks, broach

once you take the plastic cover off you will see the coilpacks, spark plugs are beneath the coilpacks

seems easy enough. i will try it right now. so remove fuse number 54 and take all plugs out? should i expect around 170 consistently?

Bentley says,

*Minimum
M52 TU, M54, M56 ............. 10-11 bar (142-156 psi)
*Maximum difference between cylinders .................. 0.5 bar (7 psi)

Figured I'd throw some more numbers in.

that makes me feel better. i had like 152-155 consistently.

I did mine about a month ago. Numbers were all very good and consistent, except for #4 which was something like 8 psi low.

Anyway,

What I learned is that not all compression testers are equal. If you get your local Pep Boys brand you will also get an 8" extension. The extension screws into the spark plug bore, the tester screws into the top of the extension.

The compression tester has a one-way check valve but the extension does not. The extension will add significantly to your chamber volume and give you much lower readings. So, you can either accept the lower compression numbers or screw the tester directly into the sparkplug hole. It's easily do-able, I did it that at way.

so you mean i was probably more like 160 psi? and i use snap-on tools

Originally Posted by aggieE46

Yeah, I know consistency is what I'm looking for.

Which fuse is the fuel pump fuse? I don't have a Bentley handy at the moment.

You know there's a little cheat sheet right on top of the fuse tray that tilts down, right?