OM603 CHASING DOWN THE STARTUP SMOKE

[AUDIBLE]

OM603 CHASING DOWN THE STARTUP SMOKE

'87 300SDL OM603

Near Los Angeles, temps always between 55 and 85


The mighty 300SDL had been smoking and clattering on startup until warm. I replaced the glow plugs and reamed out the threads for carbon. Also removed & cleaned injectors (which weren't very dirty, I think they have less than 50K on them), but didn't pop test and rebalance. Was quite careful to not mix parts. Car ran without much smoke after this for a few thousand miles, even on kick-down acceleration. but still clattered on startup and somewhat on idle, and still has one cylinder nailing when cold.

Because of an oil leak, I just replaced the front crank seal and the vac. pump gasket. Oil leak is gone, but now the blue-white smoke on startup is back. It smokes very badly, enough to attract attention and complaints, gradually subsides until the engine is fully hot. The smoke smells only of diesel and seems to be unburned fuel. Smoke on kick-down acceleration is now pretty bad again.

The glow plugs all test at about 1-2 ohms at the glow plug relay. The relay tests good for all six glow plug circuits(delivers 12v to all).

Compression COLD front to back is 350, 380. 360, 320, 350, 350. Compression HOT front to back is 390, 410, and then my piece of junk Harbor Freight diesel compression tester crapped out on its EIGHTH cylinder test. But you get the idea.

Haven't been able to establish if there's excess oil use yet.

The Crankcase ventilator tube from the valve cover to the air intake broke from age when I pulled it off. Does anyone know what effect that hose being missing can have?

With the above glow plug information, can I get away with not taking the damn things out and trying them on the bench? It seems that I need to check the IP timing and pop test, rebalance and/or replace the injector nozzles next. Any suggestions of things to look at before I do those things?

PS, this is far and away the best repair blog/group that I've seen for Mercedes.

thanks everyone.
Bruce S

[biopete]

Originally Posted by AUDIBLE

OM603 CHASING DOWN THE STARTUP SMOKE

PS, this is far and away the best repair blog/group that I've seen for Mercedes.

thanks everyone.
Bruce S

You can say that again ! Does your relay afterglow? Put in an afterglow relay if you don't have one already. You could also make a cheap afterglow setup to test with. That should help. The only thing i can think is 1-2 ohms is high resistance on plugs. Mine usually read .3 to .6 ohms i think. But could be your meter. What does it read when zeroed out? Also, what does a new plug read with your meter?

[ah-kay]

I would suspect it is the injector over dispensing as glow plug will not affect smoke when tranny kicks down during acceleration. You can try this as a quick test for After glow.

1) Remove 'small' connector inside the GP relay.
2) Open up the cover. It is quite easy as it is clip on.
3) Lift the purple wire, make sure you do not mess up the other wires.
4) Replace cover and plug the connector back in, i.e. all wires connected except the purple one.

There is now After-Glow for about 45s. This will help to have complete combustion in cold start, at least for the first 45s anyway. If it does not help then put everything back and nothing loss, nothing gain. At least you tried.

Good luck.

[piccolovic]

Don't forget the possibility of bad fuel. Read the warning to all diesel owners on the sticky threads.

The problem with not having the recirc tube is there will be a mess of oil on top of the engine and hood.

[AUDIBLE]

I reset my meter and found more like .5 to .7 ohms resistance on each glow plug, which should be OK, so I've heard. The plugs stay on about 25-30 seconds, and I usually start them over right away for a total of near a minute of pre-start glow. I agree that rigging it to glow after start might help, but it begs the problem of fixing what's really causing it, so I think I need to keep looking.

thanks
Bruce

[supercub]

Mine was smoking and new injectors and timing helped considerably.

[1977busman]

Im having the exact issue you have minus alot of smoke on heavy acceleration. My maintenance records from PO show that the glowpolugs and injectors were replaced and rebuilt within the last 12 months. However, I have noticed a few items so far that he had paper work for, but were definetely never done to the car.
Oddly the car starts better at 20degrees than 40. It always fires instantly, but smokes alot and shakes and studders. Im going to try the purple wire afterglow trick tommorow, and will be follwing this thread closely.

[AUDIBLE]

I tried disconnecting the purple wire at the GP relay. It didn't make a big difference, but it seems no harm leaving it that way. The only thing that helps the smoking from a cold start is after a long GP warmup, to start at immediate high revs (start with accel. pedal pushed way down.) Keeping the revs above 16-1800 for a half a minute or so results in somewhat less smoke. The smoke on startup WHEN HOT, after a minute or two rest, without depressing the fuel pedal at all, has gotten much worse. Smoke is still bad on kick-down. I don't see much else to try besides re-balancing the injectors, possibly with new needles. Comments always welcome, though.

thanks
Bruce

[bustedbenz]

I've never seen a diesel that didn't smoke a little bit at startup. I wouldn't let it bother me, personally. Everything you've described just sounds like normal diesel startup to me for everything except the absolute newest stuff. Combustion is poorest when the engine is cold, and the first few revolutions on the starter pump fuel into the cylinders which is then not burned until a few seconds later when the engine actually starts, at which time it is running "rich" to get rid of that extra fuel, so there's a little smoke as it burns away. No harm done. Eventually as the engine gets hot enough to completely burn 100% of the mixture (if that ever actually happens, anyway) the smoke fades out as the unburned fuel starts becoming burned fuel.

No engine is 100% efficient. Every "factory" attempt at reducing smoke in future versions of the engine all centered around reducing the available fuel to the engine -- in other words, removing horsepower just to make the smoke go away. Hardly a beneficial trade.

[sixto]

While it's clattering, loosen the injector lines one at a time and see if you can isolate a culprit. By chance, is there an oil leak by the #1 exhaust runner?

Sixto
87 300D