On my 83 300SD, I recently started having a problem that concerns me. It has started to run rough at highway speeds especially when its warm and humid.

Starts and purrs when the engine is cold. Good power and pretty smooth running with very little grey smoke.

After it warms up and over the course of a day, it really starts to run rough. More like a flat thud miss and not a knocking or nailing. I can cruise at 60 mph but anything above that causes concern. I have pulled over and revved the engine up to rpm but can not identify anything amiss. Fuel filter is golden and full.

The fuel filters have been changed more than regularly. This is a daily driver with many stops and in Dallas-Ft. Worth traffic. Oil and air filters get changed every other month. I need to adjust the valves but am thinking that I would notice more problems when cold if they were the problem. No black/white smoke of any kind and no coolant loss.

Since this mostly occurs above 60 mph, I have had to scratch my head on this one. Am I headed in the right direction? Are there other tests?

I have read on this forum about injector heat shields. What exactly do these look like? Since I bought this car used I have found a few other mechanic "short cuts" that have caused me problems and I suspect they may be missing.

I appreciate your help. BTW this forum is the best thing to come along for DIYers.

The valves get longer as they heat up and the valve clearance becomes less. When there is no valve clearance you have a compression leak and it will cause that cylinder to run very badly, if at all.

Have you cleaned the fuel tank screen? My daughters 240D acted like this. A "missing" at highway speeds. Fuel tank screen was gummed up. the system could not supply enough fuel to the injection pump so the amount injected was short, resulting in a missing effect. Since its accureing at high speeds, I woudl suspect fuel supply problems.

300SD should not have heat shields.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

Injector heat shields could also be considered like a hard seal of sorts. They are benearth the injectors to help absorb/deflect heat from the flame away from the injectors.

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

Thanks for your replies!

Now that you mention the fact about the valves lengthening when hot, maybe I need to break free and do this adjustment even sooner. Can they be adjusted with ordinary wrenches (if I hold my mouth just right) or do I need to order those specially "bent" wrenches?

Also, I had thought about the fuel screen. In fact, I am waiting on the new o-ring to come in from Fastlane. I assumed that I might see something not quite up to snuff with the in-line filter when I revved the engine. There is a little air pocket visible but it never changes in size and no foaming appears. Is this normal?

No you don't need them but they make the job easier. You'll want two 14mm wrenches, one has to be pretty thin to work. Notice how the valve clearance is at a given temperature.

Yes air in the prefilter is normal, I removed most of it so if I ever do start sucking air in I can look at the prefilter and instantly see that it happened.

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

So far I have been unable to locate the source of the fuel problem with my 84 300SD.It seems to be getting worse. There is a loss of power and will barely get above 60 without a high speed "miss". More of a flat thud than a knock or nailing. It seems as if it is "starving" for fuel.

I have replaced the primary filter, secondary filterand the hand priming pump since it seemed to be leaking slightly. Today, I pulled and cleaned the fuel screen which showed no sign of clogging. I did notice that in order to get the tank to drain very quickly, I had to remove the filler cap. Which seemed to have a little vacuum by the way.

I did a search and some mentioned a tank vent line possibly being clogged (mud daubers?). I found the vent, for overfill, on the passenger side below the filler cap. Where is the main tank vent line?

Also, the "clear" fuel lines between the IP and the secondary filters, are very dark. Where can I get replacement tubing that is clear? This might help in my troubleshooting. I thought about the lift pump also but, in searching, have not found where anyone replaced one.

This is starting to really bug me.

The tank should be under no vacuum or pressure, I do suspect you've found your problem.

Don't just replace the tubing, sure it can be done if you have a heat gun but why not just get the new factory parts? Phil could probably hook you up with it but I doubt it is on the site itself.

Lift pumps are VERY rare to fail on these motors so I highly doubt it.

I'll go look right now to see if the vent is visible through the trunk, if it is I'll take a picture of it.

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

Here it is in a W123 trunk. I'd imagine the W126 is very similar, once you take the trunk divider out (4 bolts) you should see it right there. It is made out of steel tubing and has a rubber connector at the tank end.

Attached Thumbnails

 

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

I found the source of one of the problems. I replaced the tubing from the intake manifold to the alda this past winter. Evidently the tubing I used could not take this recent Texas heat! In the morning everything would run fine. Then as the day became warmer, the end at the intake manifold would loose its seal. Resulting in no boost enrichment. I went to a place that specializes in tubing and replaced it. Seems to be holding up to the heat.

This morning, I adjusted the valves and now she is starting to purr again! I noticed that only the intake valves needed adjusting (they were too tight). Is this normally the case?

Does anyone know the proper procedure for adjusting the alda? I can't seem to find a description of this procedure in search. I know that soon I will need to break down and purchase the service CD for this car.

I don't know if one set needs more attention than the other but they only get tighter in time so that makes sense.

The ALDA isn't really adjustable, if you pop that cap off under that there is an adjustment which you can change add or subract a little fuel.

__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k