What Is a Fuel Thermostat In The Fuel Injection System? - PeachParts Mercedes-Benz Forum

ezrider · #1 12-26-2002, 11:38 PM

Does anyone know what the fuel thermostat is in a 350SDL? Where is it? What does it do? How can you tell if it needs replacement? Is it easy to replace?

Thank you.

mbdoc · #2 12-27-2002, 08:11 AM

IN Florida the thermostat will NEVER be needed. The design is to warm the fuel so that fuel in the tank won't freeze while driving. That happens around 15-20F unless the fuel has been winterized!

P.E.Haiges · #3 12-27-2002, 10:39 AM

The fuel is run thru the cylinder head to warm it in cold weather. The thermostat will determine if the fuel goes thru the head or directly to the injection pump depending on the temperature.

There is a similar thermostat in the oil system that blocks the oil from going thru the oil cooler when the oil is below a certain temperature.

Since I haven't had my 300SDL very long and the older 300SDs don't have this thermostat, I dont know where it is locatedor or if it hard to replace or even how to test for proper operation. You might refer to a service manual. One way to see if it is sending the fuel to the head in cold weather would be to feel the fuel line to see if it feels warm.

P E H

lrg · #4 12-27-2002, 12:39 PM

If it helps, on the OM603 in the 300Dt the thermostat is attached to the block on the left (driver's) side of the engine about half way down just behind the injection pump. It is a small cylindrical shaped metal item about 2/3 the size of one of those little containers 35mm film comes in. You can easily spot it because it has fuel lines going to/from it. When cold, it routes the fuel from the tank through metal pipes to the upper rear of the engine. It doen't look like there's much in there that could fail but it can be replaced if need be, it's pretty easy to remove. I had mine apart to replace an O ring that was leaking slightly. Hope this helps.

P.E.Haiges · #5 12-27-2002, 06:06 PM

Irg,

I'll look for the thermostat on my '87 300SDL. I actually found that the fuel goes thru the head by visually tracing the fuel lines one day. I thought, what a good idea although I never had a fuel line "freeze" in all the 30 years I have been driving MB Diesels.

It seems like it might also lower the temperature where biodiesel fuel could be used.

P E H

lrg · #6 12-27-2002, 06:27 PM

P E H,

Same here with regards fuel line freezing, never happened to me, even in upstate N.Y. While I'm sure heating the fuel and circulating some of it back to the tank isn't a bad idea, my feeling is that the heating was intended to be more of an emissions benefit. Nice as the system is, it is of zero help at startup with a stone cold engine, the most likely time you'd have fuel line freezing. Warm fuel (to a point) probably burns better so the faster it gets warm the less emissions produced. I've never examined the workings of the fuel tstat closely but my guess is it's open when the engine/fuel are both pretty cold and closes when the engine gets hot enough that the trip through the IP is enough to warm the fuel before it enters the cylinders. Either way I don't consider this a particularly critical part.

LRG

Gilly · #7 12-27-2002, 08:28 PM

We will occasionally see a car with gelled fuel here in Wisconsin. The usual scenario is that the driver filled up in a "borderline" area where the fuel was straight #2 (not blended with Kerosene or #1) such as southern Illinois, Kentucky, Ohio, or Indiana, then, due to the long range of the car, was driven to Wisconsin, and of course by then the fuel level is getting low and it gells up.

Gilly

P.E.Haiges · #8 12-30-2002, 11:57 AM

Irg,

The cold fuel at start up may not be as much of a problem as you think.

Here's what happens: The paraffin (wax) in the fuel gells into microscopic balls but they are larger than the pores in the fuel filter. They start to collect in the filter and eventually plug it. But often before the time the filter is plugged, the heat under the hood is enough to melt the paraffin in the filter and no problem is experienced.

I have an outside fuel tank for my oil burner heater. When it gets real cold, the filter will plug with a white substance that lookes like wax. All I have to do is to take the filter inside and let it melt and put it back on. For a more permanent solution, I put a light near the filter and wrapped both with a towel and plastic. When it gets down to about 5 degrees F, I turn on the light and that solves the problem. The fuel heater in the 300SDL does the same thing.

The coldest I can remember with a Diesel was one year when I left for New Orleans. It -10 degrees F and I had no trouble with the fuel system with untreated Diesel fuel.

P E H