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E36 M3 (1992-1999) {Euro - S50 B32 321hp @ 7400 rpm} {U.S. - S52 B32 240 hp @ 6000 rpm}
Total Produced: 71,212 - Years Produced: 1992 to 1999


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Old Tue, Apr-03-2007, 03:32:39 AM   #1
mat3
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Default compression test help

I recently got a compression test at my local shop. I wanted to make sure my 99m3 with 70k mile engine was up for some FI. But I'm a little confused about the results. Here they are:

1: 138
2: 139
3: 142
4: 134
5: 136
6: 137

When I first saw the numbers I thought it looked good because they were pretty close.
Then I started looking at other m3 reads. They were all around 190. It was below freezing when they did the test, could that have factored in? Has anyone else seen similar reads in cold weather?

The engine runs strong. So, I'm not sure if that was an accurate read. Please Help.
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Old Tue, Apr-03-2007, 03:35:03 AM   #2
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The numbers don't vary too much, which is good. as long as they are within somewhere around 5 of each other you are looking good. It was under my impression that compression under 140 was bad though. I am sure someone can clarrify this.
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Old Tue, Apr-03-2007, 03:45:37 AM   #3
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I was told that different measuring devices read differently...similar to dynos, where you can't compare one from the other.

I was told that as long as it reads realtively close to each other then it should be fine. What are the odds that all 6 cylinders are equally bad?
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Old Tue, Apr-03-2007, 03:51:10 AM   #4
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Usually people will say a compression test was good so long as the numbers are above 190 or 195 AND have little variance. Did the shop check each cylinder a few times? Ive seen a test done where 1 cylinder of the 6 came up at 130 the first time around, and then we went back and checked it again it was closer to 200. (this was only 1 of the 6, though and not all 6)

Those #s are low enough that I would be concerned something is wrong with your motor. Im not sure if you can run a leakdown on a motor that is still installed (Im pretty sure you can) but that would be the next step and would tell you if your rings are no longer sealing properly, etc
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Old Tue, Apr-03-2007, 03:52:07 AM   #5
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probably just different units. a pressure gauge wouldn't vary by 30+%; not one that is any good, anyway.

like exacc said, though, your cylinders are close to one another, with a max spread of 5% or so, most falling right in between it looks like. i'd say you're fine, but check with the shop to see what kind of gauge they use (units).
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Old Tue, Apr-03-2007, 03:55:48 AM   #6
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There are several factors:
- strength of your starter motor - remember, if your starter isn't performing optimally, it's not cranking as hard
- accuracy of the gauge - need I say more?
- number of cranks - the difference between 4 cranks and 8 cranks could be a good 80+psi, as they build pressure. You won't build up to 190+ on 4-5 cranks, and in some cases it may take you more. If the shop went to 3, 4, or 5 and then stopped, then that's why your figures are low.
- strength of the battery - battery will crank the starter motor harder
- tightness of the gauge and its fittings I had a reading 30 under from not quite tightening my first one down properly. It's hard to tighten these, as a hose comes out of them, so you're finger-tightening by twisting a hose and hoping for the best (even attachments you then twist in the hose). This is more of the person doing them
- whether all spark plugs are out - Some believe that you should leave the remaining 5 spark plugs in place. This pulls your numbers down, as you're slowing your compression stroke by some other cylinder going through a portion of its (in a 4-cyl, each cylinder will be in a different stage, but in a 6, it can matter). Keeping them all out means that you're testing the compression of one cylinder and removing the other variable.
- throttle body open - We found no change between keeping it open or closed, but it can't hurt.

In general, there's lots of variables there that can affect the actual numbers. You are looking really for consistency, as if done the same on all six, the numbers should be around the same. Yours are.

That being said, lower compression numbers could mean general wear without any particular cylinder being down, but with any reasonable amount of mileage, I wouldn't worry about it. If the motor pulls strongly, then it's just the testing procedure- but since they're all the same, you should be okay.

If I remember correctly, 11 bar, so 1 bar = 14.5037738 pounds per square inch so 160psi. I think 11 was the number I posted up a few days ago. Check that search.

PS: Compression tests is a great DIY. You can almost always find someone willing to dry-start your car for half an hour.



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Old Tue, Apr-03-2007, 02:03:35 PM   #7
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Thanks for all the good info guys. Like Exacc said what are the chances of having all of the cylinders low? Anyone know? Especially one with only 70k on it. I've had friends that had cars with bad compression (1 myself too), but its always a couple cylinders that are low. I've owned a couple bmws so I somewhat know how these cars should drive, so you would think I would know if there was something that wrong...and it wouldnt pull as smooth as it does, right? PHYRE maybe I'll just do it myself when I have time. Thanks
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Old Wed, Apr-04-2007, 03:01:26 AM   #8
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just did a compression test myself. here are the numbers
1:196
2:196
3:199
4:199
5:197
6:198

Looks pretty damn good to me!!
I think the shop must have not cranked it enough.

Thanks Guys!
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Old Wed, Apr-04-2007, 03:36:48 AM   #9
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Quote:
Originally Posted by mat3 View Post
just did a compression test myself. here are the numbers
1:196
2:196
3:199
4:199
5:197
6:198

Looks pretty damn good to me!!
I think the shop must have not cranked it enough.
Yup- that's likely all good. It is an easy and fun DIY as you saw, though you really feel for dry-starting the car. Without the fuel pump fuse, fuel pump relay fuse, and DME fuse, the car just seems sad.

Again, your numbers are consistent. Who knows which of the factors they did differently- the cranking times is very likely, as it's easy to have a 'method'.

That being said, neither is right or wrong- a compression test is to make sure your cylinders will take, build, and hold pressure. How you actually go about testing that doesn't matter, but you can't compare different methods. 5 cranks will give you lower results than 8 (or until it stops going up), but gives you the same information.

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Old Wed, Apr-04-2007, 08:26:18 AM   #10
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Quote:
Originally Posted by mat3 View Post
just did a compression test myself. here are the numbers
1:196
2:196
3:199
4:199
5:197
6:198

Looks pretty damn good to me!!
I think the shop must have not cranked it enough.

Thanks Guys!
Just curious, how many times did you crank to get these numbers compared to the Mid 130's?
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Discussing compression test help in the E36 M3 (1992-1999) Forum - {Euro - S50 B32 321hp @ 7400 rpm} {U.S. - S52 B32 240 hp @ 6000 rpm}
Total Produced: 71,212 - Years Produced: 1992 to 1999 at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)