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E39 540i low compression

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104K views 416 replies 31 participants last post by  JimLev  
#1 ·
Hi guys,

I'm in the middle of what I thought it would be a valve cover gasket replacement and a few other relatively simple maintenance jobs, but the deeper I go the worse it becomes. First I noticed leaks from the head gaskets (yet to be confirmed). Yesterday I measured cylinder compression and it turned out evenly low in all cylinders at about 130psi as opposed to 170-203psi per spec (I've seen a post from one guy who measured his to be at 205 in average!). It was late at night and I didn't get a chance to do a wet test. I left the battery charge overnight just in case and will retest it again today followed by the wet test.

I read that if the wet test significantly and evenly improves compression it means worn piston rings, is that true in most cases?

My car is 1998, but only has 80k miles on it, is it a bit early for the piston rings to go? In the time that I owned it (a little over a year) I added roughly 10k miles on it and it never overheated.

Unless I'm missing it, I don't see a procedure for piston ring replacement in Bentley (if it comes to it). I haven't checked TIS yet, not sure if it has it. Where can I find the procedure explained in detail?

Any thoughts or suggestions are always greatly appreciated! :)

Here's a video of one of the cylinder tests, does it sound normal?


Thank you, guys, for any input!
 
T
#99 ·
Hmmmmmmmmmmmm Aaaaaaa you wanna take a vaction to the good ole natural state "Arkansas" ???? Could use your dirty hands on my 540 aswell. :bigpimp:
 
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#6 ·
I did the dry and wet tests today with the following results:

1. 135 (142 wet)
2. 135 (142 wet)
3. 135
4. 135 (142 wet)
5. 140
6. 125
7. 130
8. 135

Since the change was not dramatic, this points to the valve leakage, correct? If that's true, why would they start leaking and is it possible that they are all leaking almost the same?

Regarding the information that's missing from Bentley, I've found that it all covered in TIS.
 
#9 · (Edited)
540alex said:
why would they start leaking and is it possible that they are all leaking almost the same
Click to expand...
Those numbers don't look too badly off. I don't know what percentage is considered abnormal - maybe someone has a spec for what the percentage off between cylinders for the BMW so you can judge against the spec?

As for what could cause loss of compression ... just one idea is a DISA valve ... which there are a few threads on ... where parts get sucked into the intake manifold, and from there into the cylinders where they are destroyed and sucked out the exhaust manifold.

There are pictures here:
- Example of how a DISA valve can ruin your engine compression (1)

But, I would think there is a well-defined test sequence for what causes the loss of compression below spec (I don't have it though ... so if/when you find it, send it my way and I'll add it to the best links thread for others to benefit from).

Image
 
F
#7 ·
Just skimming the posts above try some Techron in the gas tank. It does work and part of what it does is clean the valves of carbon allowing them to seat properly.
 
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#10 ·
franka said:
Just skimming the posts above try some Techron in the gas tank. It does work and part of what it does is clean the valves o f carbon allowing them to seat properly.
Click to expand...
Thanks for the suggestion, but I'm taking the top apart for gasket replacement already, and I think I should pull the heads too and then deal with whatever I find.

franka said:
How many miles on the car and how many owners?
Click to expand...
I am the third owner, bought the car almost year and a half ago with 69k miles and now it has about 80k.

bluebee said:
Those numbers don't look too badly off. I don't know what percentage is considered abnormal - maybe someone has a spec for what the percentage off between cylinders for the BMW so you can judge against the spec?
Click to expand...
Bentley says it should be in 174-203 range and I've seen several posts now with numbers roughly around 205, some even higher, so while my numbers may not be horrible, I think they are definitely low.

bluebee said:
But, I would think there is a well-defined test sequence for what causes the loss of compression below spec (I don't have it though ... so if/when you find it, send it my way and I'll add it to the best links thread for others to benefit from).
Click to expand...
It's all in Bentley. It suggests that if the wet test does not improve compression significantly, the leaky valves are the likely cause.

One last thing I want to do before I take the heads off is to verify my gauge, it's brand new, but you never know, somehow I feel suspicious of it.
 
E
#18 · (Edited)
You need to find a known accurate gauge. Auto repair shop may accomodate you.

When I want to check a pressure gauge, I compare it to several I have in my tool chest. These all measure very close to the same value, so I assume that they are statistically accurate enough to identify a bad gauge.

Let me know if you want to drive down to San Jose, and we can examine your gauge. Or, buy a general-purpose pressure gauge from Harbor Freight, and compare.
 
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#22 ·
That's what I do too:

- Let the engine run until the operating temperature is reached;
- Remove DME relay;
- Remove all ignition coils/spark plugs;
- Attach the gauge;
- Crank the engine 6 times with the throttle open all the way;

Wet test is done the same way except a teaspoon of oil is added to the cylinder before the gauge is screwed in.
 
#24 · (Edited)
Me too, definitely more than 6 cranks!
What kind of fitting is used? Is it threaded? Does it seal well?

Also, has the car only been used on very short journeys? 80k is no mileage for worn piston rings, but premature wear can be caused by constant cold starts and cold running.
If the car is consistently used on very light throttle settings the top end can coke up quite badly (carbon deposits) which can affect valve seating. Maybe that's a possibility, but a long shot,perhaps.
 
#28 · (Edited)
If the lady owner drove the car like that for some years it is very possible that carbon deposits have built up on the piston crowns and, in particular, around the exhaust valves. This could account for the uniformly reduced compression readings.
Lots of people seem to think that low mileage equates to less oil changes. I would say that the opposite is true - more oil changes are required in these circumstances as the oil is degraded by cold running, as I understand it. This could account for low mileage/dirty engines.
 
#30 ·
I wonder how effective is Techron in a sense that is it worth going through the trouble of removing the heads or just leave them be and add Techron later? In my case I'm taking apart many things anyway so the heads wouldn't be much extra, the only "scary" thing for me at the moment are the timing chains, but then Bentley and TIS have it all well explained so I should be fine in the end. Just wonder if it's worth it, what do you guys think?
 
#31 · (Edited)
If it is valve seating that is causing the low compression, it seems likely to me that in view of how the car may have been driven and assuming ! that this condition has existed for some time, it seems probable to me that the actual valves and seats could be pitted and would need lapping in (re-seating). It is undoubtedly a major step (not to mention the added expense) to remove the heads. Unfortunately, I cannot say how efficient Techron is likely to be in these circumstances.
But, if you decide to remove the heads, you will at least be able to see if there is any noticeable bore wear. You could also replace the rear water hoses, ccv and valley pan gasket more easily :)
See if others can offer any advice regarding Techron's abilities.
 
F
#32 ·
If the seats are possibly pitted then Techron would not cure that. Techtron would have been a good solution to try before you removed anything.

But at this point, AND assuming you are comfortable with your skills and have a Bentley, I would proceed to remove the heads. Then you will know everything. Even the cyl wear as Quackers noted. Then you can have the valves lapped too and they will be like new. Maybe even go to new valve guides while right there (in the head with the valves).
 
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J
#34 ·
Out of curiosity, why do you think something is wrong? Low compression numbers are not, in and of themselves, a problem. Especially when they are so even, and they don't change so dramatically when you add a little oil.

Listen, I'd be the first one to try to raise my compression numbers as well. My point is simply that low compression numbers can lead you to go to extreme lengths to solve something that isn't an actual problem.
 
#36 ·
In my eyes low compression certainly means that there is a problem. And the fact that oil does not change the numbers much likely points to a problem with valves.

Now that I took the intake off I can see buildup inside the intake ports. All of this and the fact that I'm taking the top of the engine apart anyway for oil/coolant gasket replacement I'm only a step away from taking the heads off and getting to the root of the problem.
 
#35 ·
I've decided to go ahead with the heads, unfortunately today I didn't have enough time to actually do it, but I did manage to clean the top part of the engine and some other parts and removed the valve covers. Here's what it looks like now:


And here are the valve covers getting ready to be shipped for cleaning/powder coating:


I thought that it actually looks pretty good on the inside. Do you guys see anything worth attention?
 
J
#37 · (Edited)
Forgive me if I've missed something, but your next test should certainly be a leakdown test to determine where the pressure is going and to what extent.

For example: if you've got pretty even low pressure across all cylinders but little leakage (i.e. a standard amount, such as 10-20% loss), then it's likely your timing is off.

By all means, get the valves clean and so forth. Heck, i encourage a regular "italian tune-up" :)

My point is simply that people can get really freaked out over low compression numbers with nothing actually wrong, go through all sorts of steps to improve the results and get very little without actually ensuring there is a problem somewhere (i.e. DIYers at altitude who do compression tests)

Edited to correct how I worded one phrase
 
#38 · (Edited)
540alex, I have spoken to my brother (a BMW trained mechanic who is the knower of all things BMW, in my world :) ) and he tells me that with readings of 130ish across all cylinders, he would not be at all concerned. If one or two cylinders were considerably higher or lower, or all the readings were below 100, he would be thinking about taking a look. He tells me that compression test results can be skewed by things like temperature and the efficiency of the starter motor, amongst other things. He does not suspect any kind of valve deterioration or seating problems. A build-up of carbon could be a possible cause and would recommend a product like Fortes (probably similar to Techron in the States) to be added to the fuel. Let the additive get mixed up in the tank and running through the system, then stick it in second gear and hold it there at high revs for a while :) You may see some smoke emitted as carbon deposits are burnt off. It seems that Techron and the like, is quite efficient at burning away carbon deposits from the top end of engines if the revs are high enough.
BTW, nice cleaning job :)

Sorry for the conflicting advice, but it is an option which could be considered. It's a lot less work too!
 
F
#39 ·
I think that is good advice. What number is high revs, and hold it there for how long?

Leakdown is also good advice.

Another variable is oil viscosity.
 
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