My 1991 535i BMW (straight 6cyl) has 123,000 miles. It hardly burns any oil and the plugs look fine. the idle stumbles so i went on a crusade to repair leaks, adjust valve clearance, etc. nothing improved. Finally i found my problem, poor compression. now I need to know if 1. will a valve job provide better compression results or 2. I should just try to ignore the poor idle or 3. Be prepared to rebuild the engine (valve job and hone/re-ring). I hope I can get by with just a valve job. Here are the compression results:

dry wet
#1 175psi 175
#2 155psi not done
#3 140psi 170
#4 120psi 145
#5 125psi 145
#6 135psi 160

Rule of thumb (dry test) - all values should be at least 80% of the highest.

Can someone advise me on the interpretation of this dry and wet (one teaspoon motor oil) compression tests?

I spoke with the owner of one and the manager of another machine shop. We reviewed the test data.

They seperately agreed that the engine has a significant engine compression problem. they also felt somewhat confident that a valve job would fix this compression problem.

From what i understand poor compression can be due to valve sealing problems, faulty head gasket or worn rings and cylinder walls. An engine with worn rings/cyl wall will have a good compression when oil is squitrted into the cylinder. If the valves are not sealing properly the oil squirt will bring up the compression less. A valve job takes care of the valves by recutting them so they fit perfectly in their seats. Also the head gasket is replaced and the levelness between the head and the block is assured by milling the head surface. The exhaust and intake valve guides are replaced (as needed) and all the valve stem seals are replaced as well. Sealing compression is tested both before and after the valve job is done with the head off of the engine. Finally a bunch of parts should be replaced during a thoughtfully performed valve job since there is little or no additional labor required for those parts (e.g. timing components, hoses, seals, etc).

Notes:
Vacuum at idle fluctauates rapidly within a small range, 14-15" Hg. Should also check it at higher RPMs.
Should perform a coolant exhaust gas test (to check if head gasket is leaking). No sign of coolant loss or bubbling.
Brake booster check valve (broken T nipple), tempoararily repaired with blind plug, (what is this nipple for?)

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1972 450SL
1982 300D Turbo

Dry test is when you just do the compression test by taking out the plugs and hooking up the tester. Wet test is when you spray say WD-40 or use a teaspoonful of oil in the combustion chamber. What it does is to seal the rings. If your compression goes up significantly, you have a bad set of rings. If it doesn't, the problem lies in the valves. You can also do a leakdown test to see how much leak and more importantly where it leaks at.

IMHO, if it goes up too much with the wet test, it tells me that the rings are the problem. Talk talk to a machine shop and ask them how much is too much.

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01 Ford Excursion Powerstroke
99 E300 Turbodiesel
91 Vette with 383 motor
05 Polaris Sportsman 800 EFI
06 Polaris Sportsman 500 EFI
03 SeaDoo GTX SC Red
03 SeaDoo GTX SC Yellow
04 Tailgator 21 ft Toy Hauler
11 Harley Davidson 883 SuperLow

To finally wrap up this post... i bit the bullet and removed, rebuilt and installed the head. Compression and power restored but poor idle remained.

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1972 450SL
1982 300D Turbo

Then you need to check injector balance.
Most scanners support a cylinder balance test by cutting out any injector with BMW cartridge.
A lean condition will increase idle instability too.

Run a check on the O2 swing....should fluctuate between 0.150 vdc to 0.950 vdc engine warm, at 2000 rpm.

You could try a bottle of Techron.

.

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