Diesel Tips

We plan to add to this page often; if you have a question about the operation of a car, or about an import marque in general, submit it via our contact form.  If it looks like it would be of interest to others, we’ll discuss it here. 

There are three things that a gasoline engine needs to run: fuel, spark and compression.

Diesel engines don’t fire from sparks but from compression, so it is even more important than in a gas engine. They also have to be warmed up before they fire, which is why they have “glow plugs”.

Early (pre-1980) Mercedes and VWs take about 30 seconds to warm up (don’t even try to start them until the little orange light goes off). Later cars have what they call a “fast-glow” system that takes less than 10 seconds to warm up. Bosch makes kits to convert the early cars to fast-glow. The glow plugs are not interchangeable, so always compare new ones with old ones.

Most of these cars have a little fusebox with a fusible link strip at the back of the engine compartment which is a good place to start if the glow plugs don’t glow. The early Mercedes glow plug circuit is a series circuit, like old-fashioned Christmas tree lights, so one burned-our glow plug messes up the whole circuit.

Oil changes in diesels are more important than in gas engines, as the soot and other by-products from the fuel combustion contaminate the oil fairly quickly. The 3000-mile (5000-km) interval that we recommend for gas engines is even more important in diesels. (And it’s not a bad idea to change fuel filters as well.) The API (American Petroleum Institute) rating system for motor oils has a special category for diesels. Diesel oils carry a “C” (for “compression engine”) rating.

In the past few years, oil formulations for both gas and diesel engines have been drastically modified. In older vehicles, do NOT go for the latest ratings; they do not have the same additive package and can be damaging to some engine components.

For many years we used Chevron Delo 400 15w40, but this has been reformulated and I have some information that indicates that is no longer safe for older cars. I will be doing more research on this; but, try to find oils for your older diesels with a CH or lower rating, and higher ratings for later diesels. We do carry (or can readily get) Pentosin or LiquiMoly oils from Germany, with the latest Mercedes specifications for your specific year and model.

One really bad contaminant that diesels can be subject to is sulfuric acid, which is a byproduct of the sulfur compounds in low-grade diesel fuels. We used to see cheap customers with VW Rabbit diesels go to Mexico and buy #5 diesel fuel for very cheap, but the effects on the engines were anything but cheap.

Note: Diesel fuels don’t have an “octane” rating, their equivalent is a “cetane” rating.

DIESEL SHUTOFF: One of the most aggravating things about a Mercedes diesel is a shut-off problem. Diesels don’t have spark ignition, so you don’t “turn off the ignition” like a gasoline engine.

When you turn the switch to the shut-off position, what is supposed to happen is the vacuum switch on the steering lock activates a dashpot gizmo on the injection pump that shuts down the pump. One of the real joys of the system is that the vacuum system also controls the door locks, trunk lock, and gas door, as well as the flaps in the dash for the heater/air conditioning — and a vacuum leak anywhere in the car will make all of these things malfunction.

Usually the shutoff is the first thing you notice, but you have to chase all this plumbing to find all the leaks. (There are gadgets now that inject an opaque smoke into the system so that you can see the leaks.) This means all the doors, trunk, under the floormats, under the dash, etc, etc… a real tedious job.

There is a manual shutoff under the hood, but: WARNING: PUT THE CAR IN PARK AND SET THE HANDBRAKE BEFORE YOU OPEN THE HOOD! One night when our 300D didn’t shut off, as soon as I turned it off manually it started to roll — the slight forward pressure from the engine had been enough to keep it parked, but without this, it rolled back out of the carport, across a driveway, over the corner of a planter, down the length of the side yard, and across another driveway into a field, where the brush piling up under it — and the broken piece of large pipe that it ran over — finally stopped it, with me chasing it. This car is much too heavy to stop from the outside! Fortunately, it missed the telephone pole, and trees, and block wall in the yard.

In this car, a 1975 300D, the shutoff problem was intermittent, and we couldn’t find any leaks. Checked the vacuum pump vacuum; way under spec. A new diaphragm kit ($20) and valve kit ($20) brought it back up to normal.

One possible tip: You may be able to get it to shut off if it doesn’t want to: with the car in Park (and I keep my foot on the brake!), rev the engine up for a moment and let it drop back down. It usually shuts off.

As always: Don’t stop looking when you find a probable cause for a problem. There is no rule that you can’t have more than one problem at a time, just like you can have measles and chickenpox at the same time.

Whenever someone tells me that “I’ve already done this, and this,and this, so it HAS to be this”, my answer is that there is no such thing as “has to be this”. It can always be something else. You’d think that a car is a straightforward mechanical/electrical object, but it is amazing how weird they can be. Almost as weird as people.

All information here is offered as suggestions only; the final decision as to any car repair is that of the installer and/or the owner of the vehicle. This information is the result of our own best research and experience, but we make no guarantees or warranties of any kind as to its applicability to any specific situation.