Compression test on a v12 750il

[1999750il]

Hi all,

I've been wanting to perform a compression and possibly a leak down test on my 1999 750il.
I believe the correct procedure is to remove all 12 spark plugs in order to get the most accurate readings from the gauge.
My car has 117k miles on it and shows no indication of compression loss but I'm curious to see just healthy it is.

Since I've owned it, I have replaced the entire suspension and kept up with the ignition, cooling and drive train components.
I want to get a sense of just how much life is still left in the engine before I spend even more money.
To make my life easier....would it make sense to remove only one plug at a time as I test an individual cylinder or is it absolutely necessary to remove all 12 at once?

I've researched relative compression tests but they seem more prone to errors and less accurate.
I'm curious to see your thoughts

Thanks
Rick

[White94RX]

You've got no reason to do a compression test. Unless you're a glutton for punishment, don't waste your time. I can tell you that BMW engines are rock solid. What is not rock solid is everything attached to the engine. From injectors, to coils, to rubber parts, to sensors, etc. If you were driving a rotary engine with 90k miles, yes, you need a compression test. But a stock BMW engine at 117k? No way!


Sent from my iPhone using Tapatalk

[1999750il]

Hi White
Thanks for your words of wisdom. I thought it might be a waste of time to do a compression test at this point.
I did replace the caps/rotors, wires and coils, both MAFS, all 4 02 sensors about 5000 miles ago.

I recently replaced the fuel filter and pressure regulator. I will soon replace the intake gaskets ($400 just in parts). I was thinking of replacing the injectors while I was there ($86 each injector) but i wanted to do a fuel pressure drop test first to see if the pump or injectors might be leaking. Again...I have no indication of issues with the intake or injectors.....but i know the valve cover gaskets are leaking slightly so I figured I might as well replace the other components while i had everything apart.

[asianvenom]

To answer your question. Pull all 12 spark plugs, disable/pull fuel pump fuse. I have 210k miles on my v12 and I don't even doubt that it can run another 200k miles if I keep replacing everything else around it.

[carnut1947]

I did replace the caps/rotors, wires and coils, both MAFS, all 4 02 sensors about 5000 miles ago.

I recently replaced the fuel filter and pressure regulator. i know the valve cover gaskets are leaking slightly so I figured I might as well replace the other components while i had everything apart.[/QUOTE]
I'm confused?? This car has no wires or cap. Why 12 coils? Why both MAFS? Why all 4 OXY. sensors?

[asianvenom]

The last time I check it has 14 wires, including 12 spark plugs and 2 coil wires, and 2 distributor caps. 4 O2 sensors are preventative maintenance.

[motion]

To answer your question. Pull all 12 spark plugs, disable/pull fuel pump fuse. I have 210k miles on my v12 and I don't even doubt that it can run another 200k miles if I keep replacing everything else around it.

+1 As always do as you feel is best. Try to have the engine as warm as possible. Results may/may not lead to further testing. Make sure to record readings, date and mileage. As others have said it may not be necessary however I believe it's good practice to establish a base line. If you're changing the plugs it's a good time to perform this test, insure the boots are clean and in good condition, I like cleaning them with denatured alcohol and use silicone spray inside the boot before reinstalling. Prior to pulling the plugs blow out any debris that may be down in there. I prefer to chase the threads from the inside out and I know everyone has their preference but I use anti-seize on the plug threads on my and haven't had a problem (BMW's 37 years).

- - - Updated - - -

To answer your question. Pull all 12 spark plugs, disable/pull fuel pump fuse. I have 210k miles on my v12 and I don't even doubt that it can run another 200k miles if I keep replacing everything else around it.

+1 As always do as you feel is best. Try to have the engine as warm as possible. Results may/may not lead to further testing. Make sure to record readings, date and mileage. As others have said it may not be necessary however I believe it's good practice to establish a base line. If you're changing the plugs it's a good time to perform this test, insure the boots are clean and in good condition, I like cleaning them with denatured alcohol and use silicone spray inside the boot before reinstalling. Prior to pulling the plugs blow out any debris that may be down in there. I prefer to chase the threads from the inside out and I know everyone has their preference but I use anti-seize on the plug threads on my and haven't had a problem (BMW's 37 years).