Compression-Smoke-Rough Idle
 

I have a question for you engine people out there. On my '95 S350 - 262,000 miles, the car runs rough (real rough) after starting for about a minute and produces a bluish-grey smoke out the exhaust. It just started doing this. Then it runs smoother and no smoke. I did a compression test - twice - and here are the average results of both tests.
1- 458 psi
2- 420
3- 478
4- 465
5- 448
6- 465
The book call for normal compression between 377-464 psi with a minimum of 261 psi. Maximum permissible difference between individual cylinders should not exceed 44 psi. The #2 cylinder is 55-60 psi below #3. Could this make the car run rough at idle? Could oil be seeping in #2 while the car is not running and then it has to burn it off when it is cranked? I installed new injectors when I did the compression test and can tell no difference at all - performance or smoothness. All the glow plugs are working fine and I replaced the glow plug controller. The engine has never been into at all. It uses about a quart of oil every 1500 miles. The dealer mechanic says if it was his, he would just run it. Seems like it is getting worse though.
What do y'all think?

engine sounds +feels like a "617"
 

EXCEPT FOR THE SMOKE!

If compression values are good ; is there any way that
"adjusting" the injector "pop off" P.S.I. would help ???

I am going to "dope slap" myself on this one: changing
injector configuration (even if it were only "treating the
symptoms") is just asking for more fun and games.

RINGS?
 

It's only 12:35 A.M. where are all the expert wrench turners?

O. S. I believe your mech is trying to suggest that you get all
the"Goodies" out of your old ring set that you can before you
are forced to "re-ring".

nightmare
 

Could a slightly bent rod be the cause of lower compression
on one or more cylinders?

1. try waiting 15 sec more after GP light goes out before starting. (sometimes works on my 603)
2. check for dead glow plug(s) and/or malfunction of GP relay
3. drive it with a big smile on your face :)

I tried waiting 15 and 30 seconds longer after the glow plug light went off before starting and it makes no difference. Glow plug relay is new. This is not a glow plug problem as the car shakes (but not as bad) at idle even after it warms up. Problem is internal. What I was wondering is if the lower compression on one cylinder could cause this or is it more like a bent rod. #2 is right under the crossover pipe inlet to the intake manifold which is probably the cylinder that got the most amount of egr crud before I disconnected it.
If there is a bent rod and egg shaped cylinder, can I re-build the block or is it gone? Does this engine have cylinder liners originally? I have read this from the service manual " In the event of an engine rebuild, cylinder sleeves may be subsequently installed in these engines. To do so, bore out the standard cylinder bores, install sleeves, rebore to standard size and install standard size pistons." When an engine egg shapes a cylinder hole, is it too much to bore out and re-line?
I am looking at my options here as to re-building mine or a short block.

Those compression numbers look pretty nice for a car with 265K on the clock. I doubt compression over 400psi is the source of any smoke, even if it is a little lower than the rest. I'd advise you to look elsewhere. Time to check the injectors (you already said the glow plugs are fine...)
Double check the pump timing too.
Joe

Joe, I installed new injectors when I did the compression test. Made no difference.

pop
 

have the injectors pop tested .
from what i've read even new injectors can open at the incorect PSI and if there not opening when there soposed to it can cause the problems your having

oldsouth, have you seen any of TomJ's rants about the new Bosch injectors from India? If that's what you got, the old ones might have been better than the new.

Take those injectors to a diesel indy and have them tested for opening pressure and spray pattern. I brought down 10 (from 2 motors) and all 10 were tested and 5 had tips cleaned and retested. All opened between 1950 and 2000 psi. $40. With all 6 cylinders over 400 PSI, you shouldn't be thinking too seriously about pulling that engine.

I bought them here at Fastlane. No, I have not seen any of TomJ's rants. I will do a search.
One mechanic first told me it sounded like an injector, that is why I ordered new (rebuilt) ones before I did the compression test. Then another Mercedes mechanic told me the pressure difference between the cylinders in the compression test is what was causing it. I told them when it was in the shop for an alignment to check the injection timing and that job was not done. It is hard to find a good mechanic, that is why I try to do as much repairs as possible myself. I know of one good Mercedes mechanic and I am going to try to get him to look at it. He used to work at the dealership and was their best mechanic but got fired because of drugs and now just works for cash.
I'll check around and see if the local injection place can check injectors.
Anytime anything unusual happens to the motor, I automatically think of the dreaded bent rod problem that is talked about so much. Looks like to me if there is a bent rod, the compression would not be as high as it is.

Maybe ask the guy who you think is a good Mercedes mechanic. Those compression #'s could be high IMHO because of carbon buildup on the piston tops. This can be verified by removing prechambers (which I must admit it, is probably the only way to really know) and of course the glow plugs. A good Mercedes mechanic can do this easily, check that all glow plugs are good, check for any cracks, warps or plugging of prechambers, see and maybe treat carbon buildup with something (I don't know about that except that there are products out there) All this is still way less effort and expense - most compression problems are too low and removing buildup if its there might cause the numbers to come into balance more. Good luck

oldsouth, I was just reading the EGR blocking thread and thought about you again. Didn't you disable the EGR on this car? If not, you might want to investigate a stuck open EGR valve as the cause of poor running.

Rick, EGR has been disabled for over 160,000 miles. Last time I had the intake off, it was clean as could be. ;) I fould a local Bosch dealer that said he would check the pop pressure and nozzels on the injectors for $10. Can't beat that. I am planning on removing them and getting that checked Monday. If they are fine then I can go from there.