1987 300TD - What is this part and where does it go????? - PeachParts Mercedes-Benz Forum

Chewable · #1 07-24-2013, 11:33 PM

I have this piece that is floating around in my engine bay and is obviously missing 2 vacuum lines.

Where is it supposed to bolt up to and what lines are missing?

compu_85 · #2 07-24-2013, 11:36 PM

Hmm. Is the fuel pre-heater installed properly? The lines on that look too large for factory sized vacuum lines.

-J

Chewable · #3 07-24-2013, 11:38 PM

I think you are right and they do look too large for vacuum lines. I regretted typing that as soon as I posted it.

Where do I look for the fuel pre-heater?

This is my first diesel so I am figuring this out as I go along.

ROLLGUY · #4 07-24-2013, 11:44 PM

That is the fuel thermostat bypass valve. It is hooked up to the fuel heater loop, and bypasses the fuel when it gets up to temp (not sure what temp). It is useful in cold climates if working properly, but I usually abandon it and the fuel heater loop and run the hose directly from the hard line to the lift pump.

Chewable · #5 07-24-2013, 11:48 PM

My guess is that the last owner bypassed it.

I know that they thought it had a fuel problem and were playing around with the fuel system. I think that they replaced the lift pump. I will tie it out of the way for now and try and track it down better this weekend.

Thanks for the information.

ah-kay · #6 07-24-2013, 11:57 PM

It is the fuel pre-heater. It is mounted on top of the engine mount with a bracket. If your car runs then it is by-passed already. The 2 hoses are connected to the coolant circuit. It is routed behind the oil filter. You cannot remove them unless you loop the coolant round or block it at the cylinder head.

The other 2 ports are the fuel inlet and outlet. Primary ( small elbow ) filter -> pre-heater-> lift pump. I think the primary filter is now connected directly to the lift pump. I by-passed it on one of my car because it is leaking. It does not do much.

Chewable · #7 07-25-2013, 12:01 AM

The car runs fine so I am sure it is bypassed.

I will just tie it out of the way for now.

ROLLGUY · #8 07-25-2013, 12:20 AM

Quote:

Originally Posted by ah-kay

It is the fuel pre-heater. It is mounted on top of the engine mount with a bracket. If your car runs then it is by-passed already. The 2 hoses are connected to the coolant circuit. It is routed behind the oil filter. You cannot remove them unless you loop the coolant round or block it at the cylinder head.

Sorry Bob, but I don't believe that is entirely correct. The heater loop is just a metal tube welded to the pipe that comes right off the rear of the head. The fuel and coolant do not mix, nor does any coolant hoses need to be bypassed or plugged to delete the fuel portion of the hoses. Like I said, only one fuel line (supply) needs to be connected from the hard line to the lift pump. The return remains as it is. All the other supply hoses and fuel thermostat valve can be removed. The fuel heater loop stays, but the two rubber hoses that are clamped to it can be removed. It all makes for a much simpler pluming job.

The pipe is shown on the left of this photo:

ah-kay · #9 07-25-2013, 12:27 AM

Hi Rich,

The fuel pre-heater is like a heat exchanger and the coolant and fuel never mix. What I am saying is that the fuel hose in the picture is by-passed already. If the OP wants to remove the fuel pre-heater assembly and cleans everything up then he can loop the 2 hoses in the picture, or plug them, or plug the hole on the cylinder head.

ROLLGUY · #10 07-25-2013, 12:53 AM

Quote:

Originally Posted by ah-kay

Hi Rich,

If the OP wants to remove the fuel pre-heater assembly and cleans everything up then he can loop the 2 hoses in the picture, or plug them, or plug the hole on the cylinder head.

I am saying there is no need to loop or plug the two FUEL hoses connected to the fuel thermostat valve in the photo. The coolant portion of the heat exchanger (fuel heater loop) is not affected by the removal of the fuel hoses. The coolant portion remains hooked up and not disturbed in any way, but the fuel hoses can be removed (along with the abandoned valve they are connected to). After another look, I can see that the supply hose is directly hooked up to the lift pump.

jay_bob · #11 07-25-2013, 08:09 AM

My preheater valve is bypassed, but I live in SC so it's not really needed here even in the winter.