1983 300SD Turbo: Light black smoke at idle

The symptom is unburned fuel at idle. A light puff-puff of black smoke -- not heavy, but visible, at 700-750 rpm with car in gear and A/C on. Much less smoke when the tranny is in neutral, A/C off, and engine is idling at 800 rpm. The presence (or absence) of an air filter has no effect. Except for this bit of smoke, which is embarrassing at stoplights, the car runs and shifts great. 0-60 in 14.5s, and I measured 10 psi boost during hard acceleration. On long trips getting 20-22 mpg (milage verified with GPS).

What I have done: EGR valve disconnected. Valves adjusted (thanks for the nice wrenches). Injectors rebuilt in March using the Monark nozzles / pop-test kit you sold me. I timed the IP using the drip method. This checked out to be 8-10 BTDC using a pulse method (Kent Moore Tach-n-time). Recommendations on another post said timing using pulse should be a 14 BTDC -- but this does not work, so am I back down around 10 BTDC. I have pushed the IP back and forth between 6 BTDC and 10-12 BTDC and get the light smoke with either.

Cylinder compression and injector pop-test results:
#1 420 psig compression 2050 psig pop
#2 440 psig compression 2075 psig pop
#3 420 psig compression 2100 psig pop
#4 405 psig compression 2090 psig pop
#5 420 psig compression 2090 psig pop

During pop-test injectors 1, 4 and 5 showed some leaking through to the fuel return. On disassembly, there was evidence that combustion gases were getting back into the injectors. A lot more carbon deposit than I would expect for injectors installed a little over 3 months ago. All injectors were disassembled, cleaned, pop-tested and reinstalled w/ new heat insulators.

Everthing reassembled, and the car runs like a sewing machine, but I still have the visible emissions. I have not messed with the ALDA, and I can't see how it would effect fuel delivery (rich) at idle. Suggestions appreciated!

Larry

Forums

Diesel Engine Specific Aid

Chassis Model Number

W126

A little more info needed

Hi Larry, You did not mention internal engine timing. Has the timing chain been replaced and cam timing checked? How many miles on the engine? Does it smoke when you really romp on it? How much oil consumption every 3000 miles?

This info could help narrow down the problem. You should be getting 26 to 28 mpg. Poor fuel economy along with your description of excess carbon on the injector tips indicates excess fuel mixture/ poor burn cycle.

More information

Kent,

The engine has 144,000 miles and does not burn oil, just leaks a little at the oil filter housing, and turbo pipe return. No evidence of excess blow-by. The timing chain has not been replaced, and I have not checked the timing chain stretch. Yes, there is smoke when I "romp on it." I currently have the timing set at 14 BTDC using the Kent-Moore Tac-n-time, which is in the neighborhood of 25-26 BTDC using the drip method. I can tell from the noise the engine is making that this is about as far advanced as I need to go. Still getting light smoke at idle. Does the MB 617 engine use pre-combustion chambers and are they susceptible to soot accumulation that would interfere with fuel air mixing? (I have some diesel purge on order, will try that). Also, since I am inexperienced in rebuilding injectors, I have to factor this variable into the problem statement.

Thanks,
Larry

Recommendations

I would definitely recommend you check for chain stretch. This is covered in my manual on Diesel Performance Tuning and Repair. You can also use my glow plug reamer to ream out excess carbon in the pre-chamber hole. Although no guarantees I have seen this improve smoke and poor mpg situations.

Current status

Kent,

So this is what I have done since your August 4 post:
- Performed a diesel purge.
- Replaced the Monark rebuilt injectors with Bosch re manufactured injectors. They all pop-tested between 2050 and 2100 psig.
- Checked the valve clearances. No change since January. 0.004" intake, 0.012" - 0.013" exhaust
- Checked chain stretch using the Mercedes 2mm procedure. On #1 intake at 0.080" lift the flywheel shows something between 11-12 ATDC.
Have you known of EGR return gases fouling the intake manifold to the point that air flow was restricted? I can think of nothing else. Smoking is worst at idle, ~750 rpm in drive, and A/C on.