On the 606 engine (1995 E300D) there is a thermostatically controlled device that apparently runs the diesel fuel through a small heat exchanger that is immersed in the water jacket in the cylinder head. Its purpose (I suppose) is to warm up the fuel until it reaches a certain (unknown to me) temperature. Probably a good idea if one lives in Siberia, but it never gets below freezing where I live.

It is leaking, and the replacement cost is pretty high, especially for a device that I don’t apparently have a need for, and that could in the future go bad again.

I am considering bypassing it, but I am wondering if I am doing something harmful in the long run; I am wondering if it is solely for keeping the fuel from gelling in the cold, or if the injection pump is required to have a certain viscosity fuel available. I hate to second guess the engineers at MB, but…

Any opinions? (Other than that I am a hack? )

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Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.

On my 87 SDL I needed some space for components of my vegetable oil conversion, so the fuel thermostat was sacrificed. Since I live in a relatively warm climate, and there is a coolant-filled heat exchanger right beside the diesel lines anyway, I am not too worried about gelling. I haven't had any trouble yet without it (of course I run 90% of the time on veg. oil, but the other 10% on diesel hasn't been a problem so far without the thermostat).

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Keep everything as simple as possible-but no simpler--Albert Einstein

what did you do about the fuel heat exchanger? I see that mine is leaking too (or could be the lines going into it. I'm trying to figure out how it could leak and how the fuel is routed through the exchanger... any ideas?

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Mark Herzig
1995 E300D 153K
1985 300D 142K (sold)
1979 450SL 122 miles (sold)
1992 500E 127K (sold)
1987 300SDL 132K (sold)
1986 300E 161K (sold)

The part that bolts near the motor mount with 4 hoses is the thermostat. The fuel passes up two rubber lines to the metal pipe that hangs off the head where its heated by coolant. The way to disable it would be to remove all the rubber lines to the t-stat. What's leaking? There is an O-ring on the side of the thermostat that costs <$1 to replace. The t-stat itself is $20 as I recall.

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Brian Toscano

The last message may have been slightly confusing. The thermostat is inside the housing. The housing is what has the 4 fuel lines.

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Brian Toscano

FYI – MD21722 you are talking about a different engine(s) with your 1987 cars.

Actually, on non-turbo (maybe turbo also, but not sure) 606 engines, the fuel thermostat is attached to the cylinder head towards the front, and it only has two fuel connections – So it obviously has an internal bypass once the fuel is up to temp…

Mark, I bypassed mine, no problem – but then again it never drops below 30 degrees where I live. It is more likely that the O-rings on your lines are the culprit, but if you are already in there, just buy the complete hoses instead of just the O-rings – they are not expensive, and you can have more peace of mind once they are changed out. Keep in mind that the thermostat *may* be leaking too… you’ll have to determine that once you are in there. Good luck.

__________________
Current rolling stock:
2001 E55 183,000+ Newest member of the fleet.
2002 E320 83,000 - The "cream-puff"!
1992 500E 217,000+
1995 E300D 412,000+
1998 E300D 155,000+
2001 E320 227,000+
2001 E320 Wagon, 177,000+

Prior MBZ’s:
1952 220 Cab A
1966 300SE
1971 280SE
1973 350SLC (euro)
1980 450SLC
1980 450SLC (#2)
1978 450SLC 5.0
1984 300D ~243,000 & fondly remembered
1993 500E - sorely missed.
1975 VW Scirocco w/ slightly de-tuned Super-Vee engine - Sold after 30+ years.

thank you.. That is exactly what I am doing, replacing the fuel lines also. For the record, the lines are shaped to fit so the replacement is a piece of cake (ESPECIALLY with the intake manifold removed!).

From what I recall, the main fuel line comes in (just the basic rubber hose with a clamp), gets routed through the heat exchanger, and then gets piped to the pre-filter. I don't think there is anything internal to the exchanger that could leak gas.. I did buy a replacement gasket for the exchanger but I'm thinking that would be for coolant, not gas, leaks.

__________________
Mark Herzig
1995 E300D 153K
1985 300D 142K (sold)
1979 450SL 122 miles (sold)
1992 500E 127K (sold)
1987 300SDL 132K (sold)
1986 300E 161K (sold)