1967 250s compression - PeachParts Mercedes-Benz Forum

1967250s · #1 09-25-2002, 10:28 PM

I have the MBCD with all the specs.
Compression is mesured in ATM? (atmosphere?).
Standard engine 10-11 minimum 8.5.
Low compression engine 8.5-9.5 minimum 7.0.

I have tested with a normal tester that gos up to 300 and result was ranging from 75-85 and the last one was 55.
Now I know (think) that apart from the 55, there might be something wrong with those numbers cus i ear other guys here getting 150-180 on there gas engine!? As my chain jumped?

Any comment please.

Thanks

DAN

Mark DiSilvestro · #2 09-26-2002, 12:33 PM

Did you try testing again, after squirting oil in the cylinders? Is the engine crankng too slow? Is the compression guage accurate? (Try it on another car.) How badly is your 250S running?

Happy Motoring, Mark

1967250s · #3 09-26-2002, 06:04 PM

Quote:

Originally posted by Mark DiSilvestro
Did you try testing again, after squirting oil in the cylinders? Is the engine crankng too slow? Is the compression guage accurate? (Try it on another car.) How badly is your 250S running?

Happy Motoring, Mark

you try testing again, after squirting oil in the cylinders? no.. the engine crankng too slow? Is the compression guage accurate? don't know...
How badly is your 250S running? The engine seems to be running fine apart from some smoke at start-up then OK after running for a bit (valve seal/guides?). I drove it from Montreal to Ottawa (120 milles) at 70mph no smoke lots of power.

Is it possible that the chain jumped and that the timing was advanced or retarded to match the jump? (the cover was taken off because there is silicone on the gasket!) This would certanly affect the pressure testing! or am i completely off on this one?
In any case a compression of 80psi can't be wright...

If any of you know the compression in PSI not ATM please post.

Thanks

DAN

psfred · #4 09-26-2002, 07:15 PM

If the stock compression ratio is 9:1, the cranking pressure should be about 150 or so. I'd check the valve timing (it could be late), but that won't affect compression at cranking speed all that much. 85 psi is about 6:1 compression ratio.

Valve clearance will make a huge difference, however -- check the valve clearances and correct. If the valves are too tight, compression will be way down.

If this is the case, you won't have SLOW cranking -- it will spin like crazy, since there is nothing to slow it down! It will run fine, but tend to get hot easily and to use extra fuel.

Peter

Mark DiSilvestro · #5 09-26-2002, 07:25 PM

My old Chilton manual gives 147 to 162 PSI with a minimum of 125. Having the valve cover off shouldn't affect compression readings. There's a basic check for camshaft timing at the thrust washer, behind the camshaft gear. With top dead center marks lined up on the crankshaft, the notch on the camshaft thrust washer should line up with the mark on the cam bearing. Some oil smoke after starting is probably not a compression problem. Given your trip experience, it sounds like the engine is OK for its age. Maybe the guage needs work.

Happy Motoring, Mark

Arthur Dalton · #6 09-26-2002, 08:08 PM

You may be new to testing procedures..
A few tips on comp testing:

Take all plugs out before any test. This allows for good. even cranking speed and does not drain battery [ resulting in lower crank/readings] as you get to last one

Crank over 4-5 compresson strokes before removing gauge..
Try to do the same on each reading..

Open throtttle plates for air intake..[ this will give highest reading]

Do test on warm engine...

Your car came with standard compression and low comp heads..
[ low on some Euros] That is why the 2 specs....
Look at ratio marked on head just above the gasket line , above # 2 spark plug...
ATM conversion is 14.7 psi

If you find low compression, than the oil test will help the rings seal for re-testing, but a leak-down test will tell more by listening
for air leakage at the different places [ tail-pipe,intake, etc...]
Lastly, verify you gauge...

1967250s · #7 09-26-2002, 08:19 PM

Quote:

Originally posted by Mark DiSilvestro
Having the valve cover off shouldn't affect compression readings.
Happy Motoring, Mark

My comment on the valve cover (in parentèse) was to express the fact that someone did or check something in there...
What would affect the reading was the jumped chain + timing!

Quote:

psfred : Valve clearance will make a huge difference, however -- check the valve clearances and correct. If the valves are too tight, compression will be way down.

Sounds good but some idiot would have tightened all of them!?
That wouldn't be a first i guess, if someone an idiot, it's an idiot all tha way

Do i realy need the special tool that is shown on the CD to do it or can i use a simple wrench?

Thanks guys

DAN

1967250s · #8 09-26-2002, 08:29 PM

Quote:

Originally posted by Arthur Dalton
You may be new to testing procedures..
A few tips on comp testing:

You are right, I am new to testing procedures..
I looked at the CD after having done the test

and it's practicaly exactely what you said.

The numbers i gave was on a warm engine on a second test.
first test was lower 68/72 exept for the 55 wich was the same.
Both test was with closed trottel and plugs IN....

I'll keep you posted if and when....

Thanks

DAN

psfred · #9 09-26-2002, 10:21 PM

The valve adjusters are friction fit, no lock nut, and are by nature VERY tight -- and stay that way when turned. The special wrench is necessary because you should really check the torque for each adjustment -- if they get looser than the spec, they will "readjust" themselves.

Don't use a normal crow-foot -- it will simply bend, and round off the nut. A very high quality, tight fitting wrench is a requirement.

I've been advised to go too small, then back to spec, then rotate engine a couple times and re-check due to the nature of the rocker arms -- they can get slightly cocked, and stay that way with pressure from the feeler gauge, causing you to leave them too loose. Generally described as a pain in the sitting part.

Repeat the compression check, I'll bet you are close to stock, and that you have a tight valve on the low one.

Peter

1967250s · #10 09-27-2002, 10:42 PM

Thanks to all for your help.
This is what the web was ment to be...

DAN