E39 540i low compression

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!

Was there low compression before the replacement? I am not the strongest on engines but perhaps the vanos ???

Originally Posted by 540indiana

Was there low compression before the replacement? I am not the strongest on engines but perhaps the vanos ???

Nothing has been replaced yet and this is a pre-VANOS M62 engine.

I am sure one of the gurus will chime in soon. I am about to change CCV, head gaskets,intake gaskets and anythings else that I am comfortable getting my hand dirty on.

Good luck with yours

That was my plan too until I started discovering all these issues.

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.

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.

How many miles on the car and how many owners?

Originally Posted by 540alex

why would they start leaking and is it possible that they are all leaking almost the same

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).

Originally Posted by franka

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.

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.

Originally Posted by franka

How many miles on the car and how many owners?

I am the third owner, bought the car almost year and a half ago with 69k miles and now it has about 80k.

Originally Posted by bluebee

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?

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.

Originally Posted by bluebee

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).

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.

Originally Posted by 540alex

Bentley says it should be in 174-203 range and I've seen several posts now with numbers roughly around 205

Thanks for the astute update. Much appreciated.

I see yours are universally much lower than those numbers.

I was mostly looking at the difference between cylinders ... I see now why you're looking for the general cause.

Thanks for the great response.

maybe your timing is off a little? maybe it jumped a gear?

ima do this for my 540 once i am done installing the timing chain.. keeping my fingers crossed.

Originally Posted by first540i

maybe your timing is off a little? maybe it jumped a gear?

ima do this for my 540 once i am done installing the timing chain.. keeping my fingers crossed.

Hmmm... I'll look into it, thanks!

Wait a minute, folks! Let's verify the accuracy of the gauge before tearing into the engine!!!

Originally Posted by edjack

Wait a minute, folks! Let's verify the accuracy of the gauge before tearing into the engine!!!

Exactly! Do you know of an easy way to verify it except for buying another one?

^ +1

^ +2

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.

I just got back from AutoZone, got a gauge, will try it tonight and update.

Unfortunately the other gauge confirmed the low numbers, so I'm going to continue with the disassembly tomorrow.

what process are u using to check the compression ratio of ur engine?
Umm OP i always open the throttle all the way when im checking for compression ratio..and the engine temperature is a factor...and the oil...and i always disable the fuel injection system too..

Originally Posted by Mercutio2511

what process are u using to check the compression ratio of ur engine?
Umm OP i always open the throttle all the way when im checking for compression ratio..and the engine temperature is a factor...and the oil...and i always disable the fuel injection system too..

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.

I have always cranked until the needle stops rising.

Originally Posted by pangolin

I have always cranked until the needle stops rising.

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.

Originally Posted by Quackers

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.

Bentley says that the pressure should reach its maximum within 4-6 stokes, which in my case proved to be true. Today I went for a drive and did the compression test again. This time I did over ten strokes, but I noticed that the maximum was reached after 4th or 5th stroke. The fitting is threaded and seals well as far as I can tell.

In the period that I own the car, it is mostly used to commute 2-3 times per week 30 miles one way of spirited driving I always give it a chance to warm up. The owner before me was a lady who used it for short shopping trips and to drive her kids to school.

I disassembled the top of the engine today, here is a video of what the intake looks like:



I think this sludge build up can affect the function of the valves. Any thoughts?