How to correctly compression test with electronic throttle?

[BLINGMW]

I did a compression test on the bimmer the other day and it occurred to me. You're supposed to hold open the throttle when doing a compression test.... how do you ensure that the throttle is open when there's no mechanical linkage anymore? Does the ECU actually open the throttle when you floor the pedal? I would be surprised if it did.

Any thoughts? Is ensuring the throttle is open actually that important? In our case, compression test went fine, I just don't really know what the throttle plate was doing. Obviously I could have pulled off the intake tract and checked for myself or jammed something in there, but that's kind of a pain.

[HAULN-SS]

why does that matter when the valves are closed? I have never done a test, so maybe I'm missing something

[Dave]

That's actually the one thing that I always forget to do when I do compression tests...
Personally, I don't think it'd have THAT much of an effect on it. Somebody should do an experiment on their car...

[.RJ]

It makes a difference - readings will be lower and a little more inconsistent with the throttle shut.

I'd figure some way to prop the the throttle plate open if you did the test again.

[terdcivic]

i was always told that when doing a compression test it didn't matter if the ecu plug was pulled or the throttle body was open, because each cylinder would be affected the same, and the comp test is only used to verify consistent compression in each cylinder. so as long as you test each cylinder the same way and they're within about 10psi of each other, you're fine.

[.RJ]

because each cylinder would be affected the same

This is true if you're just trying to make a general assessment of a street car as "not busted".

I suppose if you're trying to get a more accurate picture you'd be doing a leakdown test instead.

[terdcivic]

because each cylinder would be affected the same

This is true if you're just trying to make a general assessment of a street car as "not busted".

I suppose if you're trying to get a more accurate picture you'd be doing a leakdown test instead.

Yea, a leakdown test is much more accurate. Comp tests are only for seeing if it isn't broken. Leakdown tells you exactly what % each cylinder is leaking at tdc (with all valves closed).

[BLINGMW]

This is true if you're just trying to make a general assessment of a street car as "not busted".

Yeah, that was the goal this time. So the consensus then is that the compression test shouldn't be affected too greatly by not knowing the throttle position. At least for a quick, "non-busted" test, not "testing my race engine" test.

Maybe next time I get around to it (likely never, I will forget), I'll try to verify by finding the difference between open and closed throttle.

FWIW, the bimmer was 190-210 on all 6. Which was a relief, because I did it at the dealer and had trouble getting repeatable #'s out of cyls 1 & 3, sometimes they were scary low. Like, 60. hock: Maybe the fuel system cleaner actually got rid of some carbon deposits, maybe just a long drive helped, and I definitely didn't warm it up fully, AND didn't disable the fuel system. I chalked it up to me doing something wrong and went with my gut. Whatever it was, it's ok now. Yay! :dunno: