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Discussion Starter #1
I just got done doing a compression test on my 2001 330xi with 135,xxx Miles.
The results were:
Cylinder #1:115
Cylinder #2:120
Cylinder #3:120
Cylinder #4:120
Cylinder #5:116
Cylinder #6:115

I thought that good compression was between 142-156psi for a m54, and mine is nowhere near that. So Is the engine good or bad? I had planned on supercharging my Xi, But after these results im not too sure. What would be my next best thing to do, Rebuild engine, engine swap or Sell car?
 

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Those are pretty low...

Did you perform the test or did you have someone else do it? Let's not start talking engine swap until we dissect everything first. A poor compression test would usually show one or two cylinders with drastically less compression than the others. That leads me to believe that something was done wrong because of how consistent the numbers are.

Also, before supercharging make sure EVERYTHING is up to spec on your car. This includes your tranny, diffs, drive shafts, axles, cooling, alternator, battery, spark plugs, coils, etc. EVERYTHING needs to be nearly perfect before you dump 100hp more on it.
 

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Discussion Starter #3
Those are pretty low...

Did you perform the test or did you have someone else do it? Let's not start talking engine swap until we dissect everything first. A poor compression test would usually show one or two cylinders with drastically less compression than the others. That leads me to believe that something was done wrong because of how consistent the numbers are.

Also, before supercharging make sure EVERYTHING is up to spec on your car. This includes your tranny, diffs, drive shafts, axles, cooling, alternator, battery, spark plugs, coils, etc. EVERYTHING needs to be nearly perfect before you dump 100hp more on it.
I did the test myself, I First pull the DME relay & fuel pump fuse.
Next, I pulled all spark plugs out. then while one person cranks with the throttle wide open the second person watched the gauge. I Cranked engine 10 times per test. I have also done all the maintenance and pre-maintenance that the har has needed up to this point. Do you think I just did something Wrong?
 

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How about a wet compression test. Spray some light oil like WD40 into the cylinders to get the slightly wet then test. This method helps to determine whether it is your rings or valves.
 

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Discussion Starter #5
How about a wet compression test. Spray some light oil like WD40 into the cylinders to get the slightly wet then test. This method helps to determine whether it is your rings or valves.
Im going to try that later today
 

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Discussion Starter #9
If readings went up means the piston rings are worn. You can perform cylinder leak down test to find out where else it's leaking.
Although if the rings were worn, wouldn't I be burning oil or have white smoke coming out of the exhaust?
 

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Although if the rings were worn, wouldn't I be burning oil or have white smoke coming out of the exhaust?
also could be some carbon build up inside the cylinders and valves. maybe seafoam it? and try again?
 

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How many miles do you have?
 

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Regardless of if the engine was warm or not, you should be getting at least 130 - 145PSI on all cylinders. If anything, the rings would have less tension when warm and the tolerance between the piston and cylinder would be tighter when cold (higher compression).

I agree that a leakdown test would be a good idea at this point. Isn't 135K miles pretty low to be needing a rebuild?
 

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Regardless of if the engine was warm or not, you should be getting at least 130 - 145PSI on all cylinders.
Agree. Mine was done cold and I showed 180-175 across the board. When warm, it was 10-15 psi higher in all cylinders.


If anything, the rings would have less tension when warm and the tolerance between the piston and cylinder would be tighter when cold (higher compression).
No, not at all. Metals expand when hot and the warm, more viscous oil creates a better seal.

I'd be curious to know how long OP owned this car and if he had any info on what oil was used and what the OCI's were.
 

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Regardless of if the engine was warm or not, you should be getting at least 130 - 145PSI on all cylinders. If anything, the rings would have less tension when warm and the tolerance between the piston and cylinder would be tighter when cold (higher compression).

I agree that a leakdown test would be a good idea at this point. Isn't 135K miles pretty low to be needing a rebuild?
Tolerances will be tighter when engine is at operating temp since heat makes the metal expand, which equals less bybass and higher compression when hot. Thats why ring end gap is also important
 

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Discussion Starter #17
How many miles do you have?
Around 135,000


Regardless of if the engine was warm or not, you should be getting at least 130 - 145PSI on all cylinders. If anything, the rings would have less tension when warm and the tolerance between the piston and cylinder would be tighter when cold (higher compression).

I agree that a leakdown test would be a good idea at this point. Isn't 135K miles pretty low to be needing a rebuild?
Im going to go rent a leakdown tester today. Ill post results when im done.

Agree. Mine was done cold and I showed 180-175 across the board. When warm, it was 10-15 psi higher in all cylinders.




No, not at all. Metals expand when hot and the warm, more viscous oil creates a better seal.

I'd be curious to know how long OP owned this car and if he had any info on what oil was used and what the OCI's were.
I have had this car for a little less then a year. I don't know any of the cars past information, but since I've had it I have only used BMW 5-30w.
 

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No, not at all. Metals expand when hot and the warm, more viscous oil creates a better seal.

I'd be curious to know how long OP owned this car and if he had any info on what oil was used and what the OCI's were.
I agree that the metal expands. I've seen a lot of smaller single cylinder engines lose compression once warm, especially 2-stroke, so its surprising to see opposite results on these cars. In my training I was always taught to perform the compression test while the engine is cold.
There's many more factors that affect the compression. Even a flat camshaft can cause pumping losses that will show as lower PSI on a compression test.
Displacing air with oil/WD40 will also raise your compression which would show higher on the test....
 

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OK, so this might or might not be a silly question for Mormon; what type of compression gauge did you use, one that screwed into the head or one that just had a conical rubber seal and was held in place? If a screw in type, did it have a good o-ring to seal it? Did you lubricate the o-ring each time with a small amount of oil or light grease? And lastly, are you sure that the compression gauge was good and not faulty, i.e. that it was still calibrated (tools borrowed from places like Auto Zone leave lots to be desired as they are often abused by those that borrow them and have no idea how to correctly use them).
In a warm engine, the block/cylinders expand, as do the pistons and the piston rings. The rings have the job of keeping the seal between the cylinders and the pistons. Unless the engine was greatly abused, the compression should be within spec at 130,xxx miles, this is really not high mileage for these engines. I have one with 250,000 miles on it and the compression is within a few pounds of spec.
Something else that can cause low compression readings is burnt valves, i.e. the sealing edge of the valves are worn/burnt away and thus not sealing completely. Again though, at this mileage it shouldn't be an issue unless the engine was greatly abused and neglected.
In most cases, either bad rings and/or burnt valves, there will be a drivability issue, lack of power and usually oil consumption (some smoke out the exhaust but not always depending on how much oil is burnt during combustion, but definitely oil usage between oil changes.
Though your numbers do seem low, there was only 5 psi difference from highest to lowest, within reasonable allowances and if the results adding a little oil to each cylinder prior to pumping it were still within 5 psi between highest and lowest readings, you may well be worried about nothing.
Perform your leak down test and write down the results (make sure that the leak down tester is in good shape and reliable). Post your results.
 
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