bad compression - Page 2 - PeachParts Mercedes-Benz Forum

Sierra240d · #16 10-10-2006, 02:14 AM

Ok so I have some interesting results from the leak down that mabye you all can help me with because I'm

. I rigged a leak down tester that gives constant pressure that is measurabl and a decrease that is measurable.

After approx 3-4 hours hooked up at 100psi I have only lost about 10psi. Very very slow leak. I'm going to leave it on all night to get a better reading.

What can this mean? FYI the compression was 100 psi lower in the cylinder. the other 3 were all great @ 300psi. I retested just to be sure. It sure dosn;t seem to be leaking out fast, and certainly can't hear it.

This is a long shot but could the piston be not going up all the way? bearing/pin issue... becuase the cylinder seems to be sealed pretty well.

Thanks in advance for the input, I really want to bring this one back from the dead.
Thanks
Steve

barry123400 · #17 10-10-2006, 09:45 AM

Was your inlet valve cam lobe on number one normal? It was not unusual for number one lobe to wear down on the older gas cars. Something seems a little strange here. A bent rod would limit the compression as well but unusual on these. I cannot see your air pressure results being what they are. How about taking the cylinder to a hundred pounds and shutting off the supply and just observing how long it takes to zero. A constant 10 percent loss I could understand. You could do a comparison test with an adjacent cylinder as well.

vstech · #18 10-10-2006, 10:02 AM

check the valve lash again and make sure it's not too loose, and look over the cam lobes pretty well. if all looks normal, you may need to verify lift with a tool for degreeing in a cam.
if you are getting compression, but 100psi lower, I don't see how the PC clogged up could cause it. it's got to be cam/valve problems not letting in enough air per stroke.
John

rrgrassi · #19 10-10-2006, 10:38 AM

Check valve springs as well. They may not be pulling the valves closed very well. Also if you got good leak down numbers at TDC, try the same 100 PSI at BDC and then lock the crank shaft with a breaker bar and do the same 100 PSI about half way between TDC and BDC during either the compression stroke or the power stroke. This will test the rings with out any interference from the upper or lower ridges on the cylinder wall. It is very rare, but a cylinder can get a little out of round in the middle, due to the piston wrist ping binding a bit, causing the piston to not be totally perpindicular to the cylinder wall during compression and power. Piston slap can also cause scoring of the walls as well. Rings that dont' seal well in the center can can cause poor compression teat readings. But give good leak down reading at TDC and BDC.

Sierra240d · #20 10-10-2006, 02:02 PM

Thanks for all the suggestions guys. I'll try them all and report back.

Last night I left the compressor on #1 fixed at 100psi. and let it sit. 11 hours later it had droped to 50psi.... just the latest results before I read this stuff.

Cam lobes look excelent, no scoring they look great. I adjusted the clearance prefect on #1, Springs seemed ok, But I'll have to take them out to be sure.

When trying to start the car as soon as I got it, there was a 'clunk' audible from the front of the engine. I was never sure what it was, but mabye it is the piston clanking against the wall. Wierd thing is that is was not constat, and only when the engine is trying to start not just turning over.

Should I jsut pull the head and the piston and stop this diagnosis stuff?

thanks again
Steve

rrgrassi · #21 10-10-2006, 02:21 PM

That could be the source of your problem. Could be some piston slap. Could be nailing if it is on the compression stroke. Either way, I think you should remove that piston and take a look. Is the clunk consistant, and does it change with engine speed? I'm leaning towards a problem with the wrist pin on the piston and possible out of roundness or scoring of the cylinder wall on the #1 cylinder.

Piston slap
(thing) by scuzzy (2.2 mon) (print) ? Thu Jan 22 2004 at 6:24:42

Piston slap is an issue that's occurring in many GM motors built between 1999 and 2002. It's a loud engine noise produced by pistons as they travel up and down the cylinder bores.
What causes it:
Piston slap is caused by improper fit of the piston in the cylinder bore. Typically the piston is too small or the bore is too large, making the piston wobble back and forth in the cylinder. When the engine is running this slap can become very loud and annoying. It makes a gasoline car engine sound very much like a diesel engine.
The noise that piston slap produces usually clears up as the engine heats up. The pistons expand and fit in the bores more snugly.

Outcome:
The results of piston slap usually lead to increased wear of the piston, cylinder wall, increased oil consumption, failure to pass exhaust emissions, and eventually Catastrophic engine failure.

I would think that the leak down numbers you posted sounds good.

BTW, how much blowby do you get?

Sierra240d · #22 10-10-2006, 02:43 PM

Not sure on the blowby... could never get it to run for more than an minute. I did take the oil cap off during one of those quick runs and oil did not come spraying out like my old 220d.

The clunk was never completely consistant, but did speed up with engine speed. It would come and go but was presant most of the time.

I did pull the pan and found some metal bits... very fine but a good deal mabye 1-2mm on the covering bottom of the pan in metal/sludge. mabye bearing bits? Hopefully not cylinder bits. Also some larger bits, but very few all over the top of the head. I could not visually see anything wrong with the crank, nor could I move it in any way.

rrgrassi · #23 10-10-2006, 03:04 PM

Quote:

Originally Posted by Sierra240d

I did pull the pan and found some metal bits... very fine but a good deal mabye 1-2mm on the covering bottom of the pan in metal/sludge. mabye bearing bits? Hopefully not cylinder bits. Also some larger bits, but very few all over the top of the head. I could not visually see anything wrong with the crank, nor could I move it in any way.

That is quite a bit of metal. Could be bearing, piston, cylinder sleeve, rings, or a mixture of it all. That was circulating in the engine, when is did run. Sounds like it time for a tear down to see what it is, and how much it will cost for repair. A rod bearing that is worn out, or missing, can give lower compression as the stroke is shortened by the missing thickness of the bearing. Given that the noise does changed with RPM and the amount of metal found, looks like the possibility of some major component failure.

Sierra240d · #24 10-10-2006, 03:14 PM

I'll thke her apart and see what everything looks like. Hopefully it is somehting simple... if not I don;t have the money to deal with this right now... how much does a 74 240d parts car go for

bgkast · #25 10-10-2006, 03:20 PM

If I were you I would just keep my eye out for a good used engine to drop in.

rrgrassi · #26 10-10-2006, 03:28 PM

If the car is in good shape, just look for a used engine. Classics like that should not be relagted to part status just because of an engine. If no money, just store it in the garage, under a cover. A car over 24 y/o is considered a classic or antique here in TX.

t walgamuth · #27 10-10-2006, 11:02 PM

about five years ago i bought a 76 300d with a sound engine for about 600. probably would cost a bit more now.
i would expect a 240 would be about the same.

tom w