Engine Compression Test Questions - PeachParts Mercedes-Benz Forum

#1 10-10-2003, 10:40 AM

I'd like to do an engine compression test on a 123.190 (1985 300 TDT) this weekend. I've searched the forum for compression test methods, but my question is a little different. I bought a diesel engine compression tester made by Rimac. I also bought the Rimac glow plug adapter as well as the fuel injector adapter. Most people talk about removing the fuel injectors, the fuel lines etc. Since I have the glow plug adapter, I think it's easier to just remove those and test compression from there. It's less work than removing the fuel lines, injectors, heat shields etc. I'll be replacing the glow plugs on this car anyway.

My plan is to: make sure the engine is hot; remove fuse to fuel pump (if the fuse doesn't control the pump-I can't remember between Volvo diesels and MB-then kill the fuel supply to the pump); remove glow plugs; screw-in adapter; attach gauge; crank engine between six-to-ten different times and record the readings.

What should the range be on each piston? 315? 425? Does this method sound feasible? Help and thanks!

autozen · #2 10-10-2003, 11:29 AM

Tod,

I almost always use the glow plug method for the reason you stated. It's easier. The engine should be at operating temp, but I wouldn't worry about disabling fuel. The MB instructions don't mention it. My biggest concern is the quality of the testing equipment you bought. I have never heard or Rimac. If it is a cheap product made offshore, I would dump the shraeder valve and get a known good quality valve from a source like Snap-On. The valve sealing part needs to be made from VITON rubber or it will waffle from the heat built in the chamber during cranking and give you a false low reading. If you want the compression specs from MB, I don't have my books here now. I'll have to get back to you later.

Peter

#3 10-10-2003, 12:15 PM

Thanks for the help. I bought the Rimac a few years ago but never used it. You're right I should probably invest in a Snap-on or something equivalent. I think Rimac is made in the U.S., but very low-end.

autozen · #4 10-10-2003, 01:43 PM

Tod,
The fittings are fine. Just get a new valve and possibly a gauge. I have experienced people using a cheap gauge that didn't seem to read right and then borrowing my professional equipment and getting different readings. There are some things you can get by with cheap and somethings where quality counts. I buy cheaper versions of products sometime if the outcome isn't critical.

Peter

#5 10-10-2003, 02:11 PM

True. Those compression values would help whenever you (or anyone) can get them. Thanks again.

Wes Bender · #6 10-10-2003, 07:24 PM

The engine manual for my 300D Turbos suggests that you press on the engine stop lever in order to not inject fuel while checking compression.

Cheers,
Wes

wgreen · #7 10-10-2003, 09:09 PM

i would remove all of the glow plugs if i were checking , it would also be more easy for the engine to turn over , that way it cannot start , no compression........

#8 10-11-2003, 12:34 PM

Quote:

Originally posted by Wes Bender
The engine manual for my 300D Turbos suggests that you press on the engine stop lever in order to not inject fuel while checking compression.

It could also be done by using a MityVac on the vacuum fuel shut off diaphram. Get a GM vacuum sphere from a junk yard and it should hold vacuum long enough for a good test.

P.E.Haiges · #9 10-12-2003, 10:32 AM

Tod,

The actual values are not as important as all cylinders reading the same. So, measure the compression in the cylinders in order and then measure them again.

If the readings are repeatable, the guage you are using is OK. If not, you need a better compression guage.

BTW, why are you replacing all the GP?

P E H

autozen · #10 10-12-2003, 05:05 PM

P E H,

I think you had a senior moment or what is better known as a brain f*rt when you answered this one, because I know you know better. I think you were thinking gas engine where the spark plug ignites the mixture. As you well know the piston in a diesel needs to compress those little air molecules onto a space so tight that all their elbow rubbing causes the temp to rise to around 900* F so combustion can take place when the fuel is injected. If the temp isn't high enough, that puppy won't fire.

Tod,
I finally got a chance to look in my factory OM617.5 book.New engine pressure is 24-30 (atu). Minimum pressure is 15(atu). Permissible is 3(atu). That translates to any cyl below 231 lbs is in trouble. By the book calls for pressing the stop button for each cyl test. I've just never messed with it.

Good luck.
Peter

P.E.Haiges · #11 10-12-2003, 10:50 PM

Peter,

Not sure what you are talking about. The only comment I made was about repeatability of the compression readings with the Rimac guage. Possibly the brain f*rt was yours. LOL.

I do know that a Diesel engine needs compression to raise the temperature enough to fire the fuel and I also know that a gasoline engine also needs compression of the fuel air mixture for the spark to ignite the mixture.

P E H

autozen · #12 10-12-2003, 11:04 PM

[QUOTE]Originally posted by P.E.Haiges
[B]Tod,

The actual values are not as important as all cylinders reading the same.

PEH,
Maybe I misunderstood this line.

Peter

PC Dave · #13 10-12-2003, 11:20 PM

Autozen or PEH: I had my local indie run a compression check on my 300SD a few days ago, values were 25-16-25-25-22 (all in bars). The number two cylinder is on the low end of acceptable/marginal from everything I've read. Aside from some impact on cold starts, how concerned should I be about the difference between the #2 and the other cylinders - does that disparity imply future deterioration?

BTW, my indie said he could get it rebuilt for me; alternatively, if I can live with the cold start issues, I could just change the oil and run it for years.

autozen · #14 10-12-2003, 11:50 PM

PC Dave,

Must be nice to live in the high rent district. I can only afford to ski on the other side of the Wasatch range.

I have a hard time understanding why that cyl is so much lower than the others. Have you adjusted the valves on that cyl a couple of times? Also you might tap those valves with a hammer in case there is something on the seat. You might have a ring not seating properly. I have freed them with brake fluid. That injector may be injecting streams instead of atomizing which can cause a cyl wash and lower compression. Bottom line could be a burnt valve which could be cured with a valve job. A cyl leakage test could be useful here. If you end up with a valve job, make sure the machinist follows specs, because if he cuts those seats too deep, you can have real starting problems.

Peter

P.E.Haiges · #15 10-13-2003, 12:16 AM

PCDave,

It all depends how much of a perfectionist you are, how much you want to spend on your engine and what is wrong with it.

You might try to determine what is the cause of the low compression 16 bar (~240 PSI) cylinder. A wet and dry compression test might determine whether it is a leaking valve or leaking piston rings. Valve job is easier and less expensive to repair than piston rings. I don't know if you can get the oil pan off your engine without removing the engine to put in new rings.

As long as you can get it started, which shouldn't be too much trouble with ony one bad cylinder, and it doesn't misfire after warmed up, you could probably drive it as is. If it gets to a point where it misfires when warm, then it needs repairing.

P E H