How to conduct a compression test...

[e34.535i.sport]

Hey guys, I'm picking up a compression gauge tomorrow to put my mind at rest once and for all about the ol' m30! I've recently gathered information from various sources on how to perform a compression test, however I would really appreciate it if someone could outline the procedure for me in detail then it's in one place here for people to search for in future too.

Some of the main points that are unclear after searching around are:

#If you need the car at operating temp - is it safe to remove spark plugs then?
#What do you need to disable (i.e. fuel/ignition/???) and how to do it?
#Acceptable results - m30: 175psi from factory... What's acceptable now?!

I'll post results as soon as I do the test too, hopefully they'll come up good.

TIA...

[winfred]

warm engine (doesn't need to be hot) pull all of the plugs out so you don't have the drag of the other cylinders, you can jump 11 to 14 in the diagnostic plug to run the starter without the key even being on so no fuel/spark, the raw # is not that important just fairly even across the engine with in 10% of one another or so and above id say 145 to be fairly healthy

[e34.535i.sport]

Thanks Winfred... So would you say run the car until the temp guage is at the top of the blue marker then?

When you say 'Jump' 11 and 14 in the diagnostic port - could you elaborate? I am not electronically minded at all but I assume you mean connect the two pins with something metal... Is there an easy way to do this that you have found?

Oh and how will I tell which is 11 and which is 14 or are they numbered?

Also, how long do I jump it for on each one? Some places have said three or four jumps on the compression needle and take the highest one.

Thanks!

[Dave M]

In case winfred doesn't chime in and you need to do it ASAP,

Pull the fuel pump relay or the fuel pump fuse and get someone to turn the key.

T urn it over five times. Record the maximum comp and how many cranks it takes to get there. If you notice a low one, drop a teaspoon of oil in the cylinder and repeat the test. This is called a 'wet test' and can help determine whether its valve or ring related.

Dave

[winfred]

run it maybe 5 minutes (you could do it cold or flaming hot if you want), i like to take 5 hits per cylinder but you can leg it out till it no longer rises any more which may take 10 or more revolutions, the pins in the plug are numbered and are a direct link to the starter so you don't power up computer and inject fuel or spark, only add oil if theres something wrong as it can get a little messy and make the results abnormally high, if you do add oil turn the engine over a few times before screwing in the gauge to spread the oil around and eject any excess to avoid damage if you put in too much

[e34.535i.sport]

Ok thanks so far I think i'm getting there... However I checked the spark plugs earlier and found they had oil on them above the thread. Not much, only like a small drop on each one. I suspected a leak on the cylinder head cover gasket a while back when a while small amount of oil was seeping past one of the nuts on the cylinder head. I tightened the nut ever so slightly and that solved the problem. At the time I bought a gasket with the intention of doing it, but I just never got around to it, it's still in the boot! Could this contribute to oil on the sparks? Would it be wiser to do the gasket before the compression test then???

So how does this sound for a plan:
1. Run it for five minutes from cold
2. Remove fuel pump fuse & run till it stalls
3. Remove all ignition wires
4. Scew compression guage inplace of one spark plug & crank it 5 times - note the highest reading
5. Replace spark plug & move on to the next & repeat?

I'm too worried about causing damage to just get out there and do it!

[Dave M]

Changing the head cover gasket won't make a difference in comp results.

You don't need to run it untill it stalls (unless you plan on changing your fuel filter ;) )

You can also remove all the plugs as leaving them in will work your starter motor harder. If you get funky results in two adjacent cylinders (both significantly low), leaving a plug in while the other is tested can help identify a bad head gasket etc.

Go for it and let us know what you get,

Dave

[e34.535i.sport]

Thanks Dave...

What I'm wondering is when I crank it there's still going to be sparks going to the leads, which could be dangerous?!

Could I do it by using the CODE function on the OBC - as this allows it to crank but not start?! Does the CODE function stop the sparks/fuel or something else?

I worry too much I'm sure.

[e34.535i.sport]

Hey thanks for the swift response! Yeah we thought of that the Captain and I - I think I'll pull that and give it a go. I assume its ok to pull it at the cap end?

[e34.535i.sport]

This might help. They are located under the black cover just rear of the pass. side strut mount.

http://img.photobucket.com/albums/v11/dave_macisaac/Bimmer/Fuel%20Pump/DSC00502.jpg

Brilliant! Many thanks for going to that effort for me Dave... So pull the white one, the blue fuse thing and the Lead going from the distributer to the cap and bang the compression guage in! Will do tomorrow hopefully and report back with some big numbers hopefully!

[Dave M]

Thanks Dave...

What I'm wondering is when I crank it there's still going to be sparks going to the leads, which could be dangerous?!

Could I do it by using the CODE function on the OBC - as this allows it to crank but not start?! Does the CODE function stop the sparks/fuel or something else?

I worry too much I'm sure.

Pull your fuel and DME relays.

Dave

[Dave M]

This might help. They are located under the black cover just rear of the pass. side strut mount.

http://img.photobucket.com/albums/v11/dave_macisaac/Bimmer/Fuel%20Pump/DSC00502.jpg

[Dave M]

Brilliant! Many thanks for going to that effort for me Dave... So pull the white one, the blue fuse thing and the Lead going from the distributer to the cap and bang the compression guage in! Will do tomorrow hopefully and report back with some big numbers hopefully!

The pic is from a fuel pump test. You ill not find the blue fuse thing . You will, however, find the fuel pump relay in that spot, which you will remove for the comp. test.

Best of luck,

Dave

[leicesterboy15]

Have you done it? How did you get on?

[e34.535i.sport]

Engine may make a sort of 'bloop bloop' sort of noise. Dont worry - its only blowing air out of the other plug holes:)

Thanks Whiskey... I'll be prepared for that then. I hope this is one of them things I get nervous over and then find out its a peice of piss. I suppose I should be worrying more over the results than the test itself!

[e34.535i.sport]

No been in work but heading home now... Lets see what happens!

[e34.535i.sport]

Just so I've got this straight I'll do it with no sparks in - should I expect air to come out of the other cylinders or anything else that may seem unnerving?!

[whiskychaser]

Just so I've got this straight I'll do it with no sparks in - should I expect air to come out of the other cylinders or anything else that may seem unnerving?!
Engine may make a sort of 'bloop bloop' sort of noise. Dont worry - its only blowing air out of the other plug holes:)

[Dr. evil]

you should try a differential compression test

[e34.535i.sport]

Hey guys results are as follows... Not sure whether I should have been measuring the highest point the needle reached whilst in motion or the point where the needle settled after the seven cranks I did - so here's both figures:

HIGHEST POINT REACHED (psi)
#1 - 180
#2 - 180
#3 - 190
#4 - 190
#5 - 186
#6 - 186

SETTLED VALUE AFTER SEVEN CRANKS (psi)
#1 - 173
#2 - 173
#3 - 181
#4 - 186
#5 - 180
#6 - 182

I don't know which is the correct set of values but by my reckoning there is only a 6.99% error on the settled value and only slightly higher on the other which is good I hope?!

One thing though, as I conducted the test, and removed the key after each set of crankings the words OIL PRESS SENSOR came up on the dash... It didn't come on after I removed the guage but then when I done another set of readings it would chime again... Please tell me this is normal or nothing to stress over so I can stop worrying about it. :(

Overall I feel a bit more secure over the engine health as the values seem ok as far as I know (which isn't normally much) and that I didn't damage the HG when it overheated a couple of weeks ago...

[whiskychaser]

Readings look pretty damn good to me. The oil pressure warning is spurious. Car probably doesnt know the difference between cranking and running. I doubt VERY much you will see it when you put your plugs back in a fire it up. Good job. Even better results.

[e34.535i.sport]

Readings look pretty damn good to me. The oil pressure warning is spurious. Car probably doesnt know the difference between cranking and running. I doubt VERY much you will see it when you put your plugs back in a fire it up. Good job. Even better results.

Hey thanks again for the continued advice (to all you guys, don't know what I'd do without you!!)- glad the results look good! Just took my baby for a drive and it drove beautifully! Felt really good, like it had been 'cleared out'... For some reason the car felt more refined kind of like driving the 540i. I know its probably just a muppet placebo - but at least I'm a happy muppet!!!! :D Oh and no warning for the oil press sensor which is good... Was worried about that. I did get the washer fluid low message though which gonged during the journey (crapped myself incase it was something else!).

*I would've replied quicker but I had to look up what spurious meant...

[e34.535i.sport]

Readings look pretty damn good to me. The oil pressure warning is spurious. Car probably doesnt know the difference between cranking and running. I doubt VERY much you will see it when you put your plugs back in a fire it up. Good job. Even better results.

Hey thanks again for the continued advice (to all you guys, don't know what I'd do without you!!)- glad the results look good! Just took my baby for a drive and it drove beautifully! Felt really good, like it had been 'cleared out'... For some reason the car felt more refined kind of like driving the 540i. I know its probably just a muppet placebo - but at least I'm a happy muppet!!!! :D Oh and no warning for the oil press sensor which is good... Was worried about that. I did get the washer fluid low message though which gonged during the journey (crapped myself incase it was something else!).

*I would've replied quicker but I had to look up what spurious meant...

[Dave M]

There ya go. As already stated, results look fine.

You've just learned a valuable skill, now go and drive :D

Dave

[e34.535i.sport]

There ya go. As already stated, results look fine.

You've just learned a valuable skill, now go and drive :D

Dave

Cheers Dave - It puts your mind at rest at least that there's nothing bad going on downstairs... (Car-wise of course!)

Still a bit worried about the small amount of oil I found on the sparks though...

Got pictures will have to get them on soon.