92 300D - leaking fuel pre heater

[kgregg]

All-
The white plastic fitting circled in the pic below is cracked and leaking diesel (or veg oil ... I have two tank Frybrid veg oil conversion system installed in my car). The car will start (on diesel) but not run for more than 20 seconds. I understand this is a fuel pre-heater. Is that correct? Will the car run using diesel that has not been though this heater? I may bypass it temporarily and see how car runs. What does running 80 degree C veg oil through this heater do to it? I sure don't need to heat the veg oil any further (it has been through 3 heat exchangers by the time it reaches this preheater). How much heat does this thing provide?

http://home.comcast.net/~kevingregg/greasecar/preheater3.jpg

Thanks,
Kevin
Baltimore MD

Originally Posted by kgregg

How much heat does this thing provide?

Up to 65*F.

[Jadavis]

Not much heat. Think of it as a small hose in hose setup. Maybe 8 inches total length. It only heats when:
1- you have the cabin heat turned on AND
2- the fuel temperature is 25 deg C or lower.

The valve starts sending fuel through the heater at 25 deg C. By 8 deg C 100% of the fuel is running through the heater.

-Jim

Originally Posted by Jadavis

1- you have the cabin heat turned on AND
2- the fuel temperature is 25 deg C or lower.

Only partially #2, not #1.

This is for the 606 but the concept is the same.
http://www.youtube.com/watch?v=Gv8_C9UYxX4

[Jadavis]

If the cabin heat is not turned on there will be no coolant flow through the heat exchanger. No hot coolant in the heat exchanger -> no heat added to the fuel.

-Jim

The coolant in the heater pipe is always hot through convection.

[Jadavis]

Ohhhh! That's why it stays nice and warm in my car when I have the heat turned off! The coolant in my heater core stays hot because of convection. I guess I really don't need a Monovalve (in my W124). The heater core will stay hot due to convection. There is no need for actual flow to come from the engine, through aux coolant pump, through the heater core, through the monovalve, and back to the engine.

Convection. Good luck with that.

If you want to heat your fuel with any meaningful result using the stock system you will need to have the cabin heat turned on.


-Jim

[sixto]

You can bypass the fuel thermostat temporarily. Internal parts are available from Phil or your MB dealer. Most common failure is the o-ring behind the plastic fitting, next is the fitting itself.

Sixto
87 300D

All useless sarcasm aside, the fuel preheater will work with the heater off because its on the inlet side, the duo valve is on the outlet side. MB wouldn't have put it that way if it didn't work.

[sixto]

Isn't it on the return side? The heating element goes into the head so no question that it's hot, but on a 124 doesn't coolant flow from the pump to the monovalve through the cabin heater core then back into the head by the oil filter?

Sixto
87 300D

Here are the diagrams.

[kgregg]

All-
I re-routed the fuel lines and bypassed the MB factory fuel pre-heater. This fixed the leak and the car started up on 4th or 5th try. I'm also now driving again on veggie oil. Life is good. Thanks for the replies. What are the downsides of not using this fuel pre-heater? Kevin

Very little. The heater is only to ensure that there is no gelling of the fuel in cold weather.

[sixto]

Originally Posted by kgregg

What are the downsides of not using this fuel pre-heater?

Same downside as ripping off matress tags.

FWIW you can buy an MB fuel thermostat to retrofit to older Diesels. I don't know the part number but sometimes you see one on eBay.

Sixto
87 300D

[easkey]

Originally Posted by sixto

You can bypass the fuel thermostat temporarily. Internal parts are available from Phil or your MB dealer. Most common failure is the o-ring behind the plastic fitting, next is the fitting itself.

Sixto
87 300D

does anyone know where i can get the complete pre heater assembly including the nipple that screws in to the block , and approx how much thanks , MK