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Discussion Starter #1
A friend borrowed my compression meter to check an engine (M52).
The owner complained about low power and backfire in the airfilter. He's not at all mechanical minded so the backfire might as well be puffs in the exhaust :embarrassed
The car runs on LPG (gas).
My mate measured the compression because for us the first suspect would be the valves (car has +200000km).
Results were highest/lowest 225/235PSI.
The difference between both is not that large but its the values that bother me.
There is a Vanos thingy on the inlet camshat. Could malfunction of this be the cause of these high compression values (and the poor performance)?
 

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If you know the compression ration (10 to 1, 11 to 1 etc) you can just multiply it by 20 to give what the pressure should be so a 10 to 1 engine should be 200psi, rule of thumb.
I think the M52 should be either 10.5 or 11 to 1 so up to about 220psi sounds right. Has the head ever been off and skimmed? If not it could have carbon build up on the head and pistons but it doesn't seem a huge amount higher than normal.
Have you pulled the plug on the injectors or otherwise disabled the fuel system while checking? This is especially important that the LPG is disabled while checking, this could really elevate pressures.
One other thing you could check is that the CAT hasn't broken up or melted down and blocked the exhaust. Try bashing it with the palm of you hand to see if it rattles.
Lean running on gas will cause it to pop back and if it has done it much without a blowback arrester it will damage your MAF, that will then screw up the running on petrol too!
 

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Discussion Starter #3
My Haynes manual sais 142 to 156 PSI minimum and I thought the multiplication factor was 14.7 so a CR of 11 would give 162PSI.
Definitely +200 is suspect.
Service history of the car is unknown but seen the state of the airfilter I guess it will rather be a short story.
With this kind of LPG systems the engine automaticaly uses petrol for starting so the gas will not influence the measurement but it makes sense to repeat it with fuel injectors disabled.
Good tip about the CAT. I'll ask him to check that.
 

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I would think an M52 with a minimum as low as Haynes suggest would be very worn indeed and I still would not worry too much at seeing 200psi this thread

http://www.bmwland.co.uk/forums/viewtopic.php?f=5&t=54267

and this on wikapedia

http://en.wikipedia.org/wiki/Compression_ratio


And copied and pasted from http://www.boarddigger.com/t-2UY49MbocTk/BMW-1991-1999-E36-Compression-test-results






I finally got around to taking a compression test on my e36 with the M50tu that took convincing to get running, but now won't idle. So in the Bentley is says compression should be between 145-156 but im reading much ... compression test while it was cold showed (in order), 210-210-200-230-200 ...
In BMW 1991 - 1999 E36


blown Head gasket. So this morning I went and did a compression test. Here are the result on my 1994 325i with ... 4: 210 3: 213 2: 208 1: 206 [front] I did the test when the engine was on the first bar in the cold on the temp gauge. These results look great. So my question is why do I have oil in ...
In BMW 1991 - 1999 E36

today i finally got my self to do my compression test. edgemotorworks reccomended i did so so i did. compression goes as follows: 1. 210 2. 200 3. 210 4. 200 5. 210 6. 220 are they to far apart? i also examined the spart plugs. it has white crust ...

should hopefully put your mind at rest on that score. How does the car run when it is switched over to petrol, does it still blowback then?
 
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