I have a leaking fuel thermostat on an 87 300d OM603.
Instead of fixing it, I have temporarily bypassed the thermostat. Runs fine, no issues.
So I had an idea to reroute the fuel lines so that the fuel is always running through the heat exchange circuit. Full time winter and summer.
My thinking is that warmer diesel fuel will be more efficient with more power year round. (theoretically)
Or is there a sweet spot for fuel temperature?
Am I thinking wrong? What say ye?

Bypass the fuel thermostat (go straight from the tank to the lift pump) and be done with it. Unless you live in Siberia, it isn't really doing much anyway. Plumbing the fuel through the heat exchanger full time is detrimental to your injection pump. Not only does hot fuel have less power density (it expands), but hot fuel also has less lubricity so you're tearing up your lift and injection pumps.

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Not true.

Effects of High Temperature and Pressure on Fuel Lubricated Wear

On my 87 I bypassed the thermostat, ran it through the heat exchanger full time and no heat exchanger full time, couldn't tell any difference. Since I'm in a hot climate it's all bypassed now.

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I agree with the general sentiment of just bypassing this device, and run the fuel hose directly to the prefilter --> lift pump. I have the exact car you do, and I've bypassed this on my car, and on two previous 190D's without issue. I'm also tempted to bypass the redesigned version on the om606 engine. Really not needed if you live in location with a more temperate climate. Also, this gives you an opportunity to really clean up and reroute the fuel and coolant hosing in that area to make it look much more streamlined.

"So I had an idea to reroute the fuel lines so that the fuel is always running through the heat exchange circuit. Full time winter and summer"

But that's how it came stock perhaps you are refering to removing the fuel thermostat which regulates the fuel temperature?

I ran a 617 for 22 years with outdoor temps into the -40's without a fuel heater or additives. The challenge isn't keeping it running but starting it can be. My current 602 has the fuel heater I rebuilt it 8 years ago just because but it likely would operate the same without the heater even in extreme cold.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

I say spend the $10 on the parts to fix the thermostat and put everything back to original spec. I did that job on my '87 wagon with dealer parts about 11 years ago, and it is still running just fine.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Anybody who has ever poured diesel fuel in freezing temperatures will agree that it gets noticeably thicker. Maybe in San Diego it doesn't matter, but I think the fuel preheater might be useful elsewhere.

What cracks me up about the fuel preheater is that it seems that the time you need warm fuel the most in a cold climate is during starting. But, the engine is still cold then so the preheater doesn't start working until after the engine has warmed up, but by then I imagine the fuel has already been heated to some extent by the radiant heat from the engine compartment. And then the rest of the year, while the engine is running, the fuel is getting too hot.

I suppose that the preheater could be useful if a block heater is utilized, and if it in fact heats up the coolant that is in the heat exchanger so the fuel will be warm before starting.

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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles

It looks like there are no performance gains in the summer with heated fuel.
And there may or may not be an advantage to having heated fuel in the winter months in areas that actually have a winter.
It will occasionally get down to 5-10 degrees F in my area.

So I will spend the $10 on parts to make it not leak.
I guess I can put the thermostat in the freezer and see if it works as it should. Don't really want to spend $35 on that part.

Thanks for all the input.

The fuel heater thermostat is included in the $10 parts kit.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Please post a link if you can. I only saw separate parts.

Way too long ago to recall any links.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Here's a link for the parts but you may have this already

OM601, 602. 603 Diesel fuel thermostat REBUILD parts list

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Yes, that's the list I saw. Not available as a kit. O-ring and cap are inexpensive. Thermostat not so much.

Thanks