Compression difference 1 cylinder on M62tu

[rusteee 52]

Hi Guys

Just got the pre-purchase inspection report (well lack of, as they failed to send it through tut tut) for a 2000 540i Ive been looking at. I was talking to the person who done the inspection, came back pretty good minus a few faults which I had spotted anyway. The only thing I was worried about is, 7 cylinders had Roughly 170psi even, Number 4 cylinder has around 148psi. I know its not too much difference, but seen as all the rest are basically even, is this a sign of worn valve or something? Engine is smooth as silk, and he couldn't fault it otherwise (no power loss, oil usage, rough idle, smoking etc), but didn't want to say for sure what he thought it could be without fully investigating. Any ideas of wether it would still be worth purchasing, or steer clear of it?

Cheers

[yng_750 247]

Would be worth getting compression properly tested if it hasnt already as aa often use relative compression where they measure the current draw to the starter and measure if its easier to compress any cylinder in comparison.to others. It doesnt tell you which cylinder is which either

Cant imagine they spent an hour removing covers amd coil packs to do it properly.

[rusteee 52]

Yes, the compression test was done properly as requested, removing spark plugs etc, done by euro car services. So it must have been pretty accurate, seen as he got all the readings. I just wish he sent the sheet like they were supposed to, so I could see exactly what they were. But thats what he told me over the phone, roughly 170 on 7, and Number 4 cylinder was down to about 148.

Edited February 14, 2014 by rusteee

[BM WORLD 1265]

double check that cylinder may have not sealed properly , needs to be at full throttle too

[*Glenn* 854]

When doing a compression test on any twin cam (hemi type) head engine, you should loosen the plugs and run the engine first to get rid of any carbon the falls from around the plug. This often gets trapped under the valve seats and creates a slight loss of compression on any particular cylinder at cranking speed. Common practice should be to do this on any engine. The test should also be done with WOT.

BTW: Jason.. Dynamic compression tests are not measured in the manor you have described. It is done with the engine running. We can also do it with our scanner by measuring cylinder smooth running in live data

Edited February 14, 2014 by *Glenn*

[*Glenn* 854]

A 5% variation is acceptible when testing at cranking speed... some people might accept more. If a BMW has low compression on any cylinder it will not idle smoothly as plenum chamber vaccum gets upset and that effects airmass readings.

[rusteee 52]

Cheers for that. I'm not too sure exactly how they had done the test. He did suggest because the engine is still running very smooth with no sign of oil burning and what not, it could be carbon, but didn't want to say for sure. I only worried, as the other 7 were pretty similar, was only that 1 cylinder that was 20 odd pounds difference. I'm only assuming it was done correctly given the time they spent, and the cost of the inspection. Seemed pretty thorough on everything else.

[rusteee 52]

A 5% variation is acceptible when testing at cranking speed... some people might accept more. If a BMW has low compression on any cylinder it will not idle smoothly as plenum chamber vaccum gets upset and that effects airmass readings.

Hmmm, yea well its a bit over 5%. TBH I wasn't even going to get the compression test done as both hot and cold the car started and idled perfect, no hesitation or stutter, and power through the range was very good.

[rusteee 52]

Got the full test,

Cyl. 1: 175 psi; Cyl. 2: 170 psi; Cyl.3: 160 psi; Cyl. 4:145 psi Cyl. 5: 165 psi; Cyl. 6: 170psi; Cyl. 7:170 psi; Cyl.8 170 psi.

It does vary a bit, but seeing that, doesn't look too bad, what do you rekon?

[*Glenn* 854]

I would be expecting higher readings up around 200psi if it was done at WOT if the test gauge was in good condition. Someone else also suggested it wasn't done like that. It's also best to use a screw in type gauge that has a seal as well, rather than a hand held type. The battery may not be that strong and No4 cylinder would be one of the last cylinders to be tested. The only way to be reassured is to get it rechecked or do a cylinder leak test if necessary. What did they suggest ?

Did the test also include a full scan and report on any faults in any of the control units ??

Edited February 14, 2014 by *Glenn*

[yng_750 247]

Huh never would have guessed it could be done running. Amazing technology

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[rusteee 52]

I would be expecting higher readings up around 200psi if it was done at WOT if the test gauge was in good condition. Someone else also suggested it wasn't done like that. It's also best to use a screw in type gauge that has a seal as well, rather than a hand held type. The battery may not be that strong and No4 cylinder would be one of the last cylinders to be tested. The only way to be reassured is to get it rechecked or do a cylinder leak test if necessary. What did they suggest ?

Did the test also include a full scan and report on any faults in any of the control units ??

Thanks Glenn, yeah I was expecting higher readings and closer together. He didn't say whether it was warm or not, but was certain that if they were to redo the test it would not change. The battery definetly isn't strong, as per the test he put "battery low and replacement recommended". He said as its runs, idles etc so smooth, he would almost say its a bit of carbon on the exhaust valve so its not seating correctly, but cannot say without doing a cylinder leak down test, which they do not have the facility to do.

He sent me a scan report and there a couple of faults, would you mind if I sent you a copy to have a look at Glenn?

Edited February 14, 2014 by rusteee

[rusteee 52]

Thought I'd add closure. Must have been a mistake. Had the cylinder leak down test and another compression test done today somewhere else. Was perfect across all 8. Compression test came in at 175-180psi between them all. So have now purchased the vehicle and eagerly awaiting to take ownership =)

Wheres the best place to get a battery from? It has an after market one thats 10 years old and showed its age on the test by the sounds of it.

[yng_750 247]

I got a din88 from.supercheap jad to order in from.century. Got a three year warrenty and cist about 360 that was the ideal size for a 740. You can easily gey away with the stocked din65 from sca for about 260. But i went for the biggest one

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[rusteee 52]

Cheers for that Jason, might just do that. Last thing I want is a dead battery in Waerenga =)