That is a picture of the thermostatic valve, it has no connection to any coolant circiut, only fuel circuits.

When operating above a certain temp the fuel flows into one fitting of the valve and right out of a second other fitting, below a certain temp the fuel flows into that same fitting iand the temp sensative wax pellet acts on the valve elements causing fuel to be directed out a third fitting, fuel flows up to the preheater/exchanger and is warmed, then back down into a forth fitting and ia directed out the original output second fitting, heated and on to the lift pump and the secondary filter.

The primary function of the fuel preheater is not so much to raise fuel temps for combustion reasons (although there may be some benefit) the fuel temp is raised to prevent fuel clouding or the precipitation of waxes within the diesel fuel that would clog the filtration media of the secondary low micron filter.

The actual fuel preheater/exchanger which is connected to the coolant flow from the cylinder head is pictured here in this thread in post #11

http://www.peachparts.com//shopforum/showthread.php?t=288198

How many o rings in the fuel thermostat?
Is there any effect from running the car with this bypassed?

In Moderate Sub Tropical Climes it will not matter.

(All bets are of if WVO/VO are used)

__________________
'84 300SD sold
124.128

The fuel thermostat can be replaced completely for a whopping $159.00 in the aftermarket. It may be a lot more from MB. The part number is 6010700126.

Or, you can rebuild the thermostat by replacing the o-ring: $2.85 from MB. Part number 0119973948

Roy from PeachParts suggested that I should replace the plastic cover because they tend to crack. Mine is still good, but I got a replacement anyway. It was $4.95 from MB. The part number is 6010780016

You will need some snap-ring pliers to get the cover off.

I run B100 in the summer. I may switch to Viton o-rings if the one from MB fails soon. I measured the o-ring from MB and it is 19.5mm I.D. and 3.0mm C.S. You can order these from oringswest_dot_com using the metric size M3X19.5.

Be sure to clean off your engine mount if this happens to you. I think it could dissolve.

__________________
1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

There is only one o-ring. See parts list below.

__________________
1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

So it is easy to assume that if it is disconnected it probably just had a bad oring and someone didnt want to replace it so they opted to disconnect the unit.

If you look online for this part it is $159 to replace the whole thermostat. You really have to dig to find the replacement cap and o-ring. It took me 30 minutes on the phone with the MB parts counter for them to believe that they even have a part for the o-ring and cap.

Someone should sell rebuild kits as a standard part online, like in PeachParts.

__________________
1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

There are two kits.

Diesel fuel thermostat REBUILD kit, huge application list
OM601, 602. 603 Diesel fuel thermostat REBUILD parts list

Diesel fuel thermostat repair kit, huge application list
http://www.peachparts.com/shopforum/showthread.php?t=289787






Have a great day.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

The fuel tank usually a small amount pressure in it. If you open the filler cap the pressure is released and the amount of fuel that leaks during the job is minimized.

If you can start with an almost empty fuel tank there is less head on the fuel. This will allow you to more easily raise the fuel lines above the level in the tank to prevent the fuel from spilling out while you change / rebuild the thermostat.

The fuel lines leading from thermostat to the heater were original and very brittle. Bending one of them resulted in a crack in the line. Be prepared to replace any original lines with the appropriate _diesel_ or biodiesel (viton) fuel lines.

Be prepared to cap / plug the fuel inlet to avoid emptying the tank on the garage floor while you are rebuilding your thermostat.

I cleaned up my thermostat in carburetor cleaner before putting it back together. I did not put the valve with the expandable wax or the plastic cap in. You should only use carburetor cleaner on metal parts.

__________________
1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

I just by-passed it altogether. I'll deal with it later if the cold impacts performance.

__________________
http://i284.photobucket.com/albums/l...aman/Fleet.jpg

Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic

Been trying to track down my fuel leak issue on my '91 350SDL (no bent rods yet - but only 115K miles on her). This is the culprit - calling the local dealership tomorrow to see if they have the parts I need, in stock - my guess is that they won't. Wondering if NAPA would have viton o-rings in this size. But sure as I don't order the o-ring AND the cover, the cover will crumble when I go to pull it out. So may as well just replace both and be done with it. Was going to order them out of NY (regular o-ring, not viton) but $13 for shipping was a bit steep for $7 worth of parts. SMH. Would rather have viton in the event that biodiesel becomes more readily available around here - the Q-T near me runs a biodiesel blend - which is probably what is causing the current leak. But I'll get it buttoned up. Nice to see what that part looks like disassembled.

Yep I found the culprit on my 87 wagon this evening. I replaced all the rubber fuel lines and thought I had it but noooo. As always thanks for the pics, instructions and the parts number information now I can try to fix it this weekend.
I bet this is where I get a slight air leak miss from when idling that has had me stumped. I'll find out soon enough.

__________________
If it ain't broke take it apart and find out why.

1983 300SD, 4 speed
1994 C280
1987 300TD wagon
1996 HD Road King

Ride in Peace Eric Peterson, Harley of Macon

I realize it's 11 years later, but I am dealing with these issues on my '92 300D and this is all very helpful. Thanks to all.

Same here in Australia. UVO fuel so want this to work in winter
Cheers