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.

[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

[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?