water separator? Help!
 

Hello all: New member here. I have an '86 SDL with 235k. I try to take the best care I can afford of "The Mighty Whitey" and have had wonderful performance from the car over the past 6 years that I've driver it daily. I've run against a little glitch, however, that I don't understand. I've had two 300Ds and two SDs, so I'm relatively familiar with these cars.

The SDL started to bog down at high speeds about a year ago. If ever I reach 80 MPH, the engine begins a "cyclical bog and lunge" and I can't get the car to go any faster without the cycle getting rougher. At normal speed I've had no trouble, so I always assumed that I was maxing out the dirty fuel filters and that I would replace them soon. I knew, however, that dirty fuel filters aren't usually forgiving at any speed, but for the longest time nothing changed. About a month ago the regular speed performance began to be a bit hesitant, no "cyclical bog and lunge", but just a little rough with no pattern. This week I had all of the fuel filters replaced (and the front end rebuilt. eek!) My mechanic took the car out and immediately felt the improved performance due to easier fuel delivery, but at 80 MPH the same symptom re-appeared. I took the car home for the weekend and while I too can tell the fuel delivery is much improved, I can feel the ghostly little variations at low speeds, less now, and expect that the 80 MPH problem will still be there (too much traffic this afternoon for me to find out for sure)

My mechanic wants to route the fuel line around a "water separator," the contents of which he said was absolutely vile when he cleaned it out today. He says he's sure this is the source, and if his experimental re-routing works he wants to route around the separator permanently. He's usually quite a purist, so I'm surprised at this suggestion and don't want to do it. In all the years I've been maintaining my diesels, I have to confess I've never even heard of a water separator. I don't know how important it may be, but don't suppose Mercedes put it in with superflous intent. Perhaps it's exclusive to the SDL, which wouldn't surprise me. I've wondered if it's something to do with the turbo charger, but my mechanic swears it's fuel delivery. Neither of us suspect the injection pump, which I had rebuilt a few years ago. O rings are new too (goodbye sulfur! I'll miss you!). There's no miss at all, and at idle the car purrs perfectly like it always has, with that lovely rattle I've come to enjoy so much.

I drive this car hard and daily and have always really enjoyed it. I want to solve this problem. Anyone recognize it? I'm stumped.

Patrick in Covington, Louisiana

algae?
 

have you run any algaecide through the car? i dont know about a water separator... i would be reluctant too. find the problem and fix it... i would say.

The car did not come with a water separator from the factory. Some previous owner must have plumbed it in. A water seperator is good to have, of course, since you may encounter crappy fuel occasionally. If you have a water separator, you have to maintain it. That means drain it occasionally and change the element on a reasonable schedule. Usually when you change the element you also change an o-ring seal. The water separator may or may not be the cause of the driveability problem, but it would be good troubleshooting to bypass it temporarily and see if the problem goes away. From the sound of it, you should know almost immediately if the problem is fixed or not.

not factory
 

if it isn't factory i would take it and throw it as far as i could. it could well be the problem.

imo it is hard to improve on mb engineering with respect to diesels. (don't start on the 603s guys)

I wouldn't necessarily throw it away, but, as I said, it needs to be maintained. The element should at least be from the current decade, and the o-ring fresh enough that it doesn't allow air to be sucked in.

Tell you what, if you're going to throw it away, just send it to me and I will put it to work.

Edit: Any chance of posting a picture of it? It might help if we could see what we're dealing with.