So yesterday my 79 300td NA started dying at idle. The idle was never very smooth and it did smoke a bit before but it has increased. I was suspecting injectors and in fact the other day I stopped at NAPA to get Diesel Purge but this particular local didn't have any.

If I put the trans in N while idling the engine won't cut out. If it does cut out it restarts immediately easily. If I increase the RPMS with the throttle (and trans in N) it won't cut out. When I start from a stop the power is really low. I know there is not a ton of power to begin with but it is even less than before and I have to use the kickdown to downshift to 1st and a large black cloud will be left behind. Once I get to about 15MPH and over I don't notice anything other than a bit of smoke. Also it shakes REALLY bad at idle. Motor mounts look fine and are new within the last 20-30k miles.

It almost seems like it is not getting enough air. The PO stated there was sometimes a weird noise coming from the intake. I hadn't really noticed it but this rough idle it seemed to be there. It almost sounds like those huge drag cars that are waiting at the line (Whop....whop....whop). The filter is new and less than 1k miles on it and visually looks fine. Nothing blocking the intake.

Other things I can think of-
Valves adjusted about 1k miles ago to spec.
Oil change less than 1k miles ago with 15w40 diesel oil
PO did a compression check and one cylinder was really low but that valve was really out of spec when I did the valve adjustment
Just D2 from the pump. I had used DieselPowerService the first few fills but none the last one. I'll put some more in the new tank. Have about 4 fills since starting to drive this car.
It has been hot here lately (in the 90sF) and I drove it quite a bit yesterday and the problem started occurring about half way through the day.


Thoughts on what to try-
Diesel purge
Is there any thing that could impede air from flowing in other than clogged/blocked intake?
Clogged exhaust

Here is a video showing what is happening. The smoke isn't terrible until the throttle is depressed for hard acceleration.
http://www.youtube.com/watch?v=h1tDMmNsDIo

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

The first thing to try before doing anything else is eliminate air in fuel. It sounds like fuel stavation, not air starvation. The easiest way to see if you have air in fuel is to put a short section of clear fuel line between the cigar hose and the fuel filter. You should have a solid stream of fuel there. If you see foamy fuel or large pockets of air bubbles, find and fix the leaks.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I found some diesel purge at another NAPA, do the purge and change the filters. I will stop and get it later today but the more I think about it the more it seems like a clogged exhaust. Thoughts? I guess I could take the exhaust off and see how it performs.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

Check to make sure you're not dumping crankcase oil into the intake via a holed diaphragm in the vacuum pump. The vent line from the pump to the air cleaner should not have oil in it.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Your video is private and unviewable

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I'll put a section of clear fuel line on the return. But black smoke is unburnt fuel so it seems like I'm getting plenty of fuel, it just is not combusting.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

Should be fixed. I just watched it and the shaking is much worse in person. The whole car shakes along with the engine and that is not seen in the video since there is no point of reference.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

I can see no oil in the vent line. The clear inline filter is however dark so oil may be finding its way in to the fuel or I just have a dirty filter. I will find out tonight after the diesel purge and new filters.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

What's underneath that foam around the intake hose to the air cleaner. Is that a clear passage?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Nothing. I think two POs ago put it on there to dampen the sound that I am describing, at least that is what the last PO thought. Yes it is a clear passage. The foam is directly on the black plastic tube.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

The more I think about it the more it can't be air in the lines. When it kills, it starts immediately with no delay. It is slow and sluggish but does not seem like an air leak.

I think the issue is oil mixing in the fuel. After driving it today the problem is better but still there. Yesterday when the problem arose it was near a n empty tank and probably diltuted by oil. Then I filled last night and now it is not as diluted with oil. Plus the clear filter is dark indicating either dirty fuel or oil in the fuel.

Where does oil enter the fuel?

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

In the IP. You can pull the return line and feed it into a clear bottle and see what color it is. There was a thread a few years ago about this topic--engine oil contaminating fuel. My vague memory is that it was something associated with the lift pump.

Have you tried running it with the air filter removed?

I tried to find the thread but I don't think the search was going back far enough since it was only giving hits since 2010 and I think the thread was before that. One thing you could do is get a clear gallon container, fill it with diesel and run the input and return lines into it and see if it turns black.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

That engine has a breathing problem. It's either a bent, tight, or leaking intake valve, or possibly a flat lobe on an exhaust valve, causing the cylinder to exhaust into the intake. A cylinder that is not getting enough air will also give you your smoke.

In that thread the Little and only O-ring that is in the Lift Pump was no good and it was sucking Oil from the inside of the Fuel Injection Pump into the Fuel.

But, if you have read the other threads putting Lube Oil in you Fuel dose not make the Engine run worse. Although it could cause smoking.

__________________
84 300D, 82 Volvo 244Gl Diesel

I did put some fuel into a clear container. I took a picture. It looks a bit yellower than the greenish hue diesel fuel here but it really wasn't dark. Not quite sure this is my issue. The clear filter is really dark so next I'm doing a diesel purge and will change the filters.

Tonight on my last drive of the night it almost seemed as if there was no issue.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD