Now that the coating of black oil has been removed, I discovered a fuel line elegantly plugged with a bolt.
I have deduced from recent posts & an archive search that this is related to the fuel thermostat. This prompts a few questions:

why has this been done?

what was the factory configuration?

should I try to correct this?

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1991 300D

A lot of people are about to tell you the fuel thermostat is useless. I won't comment on the matter, but will tell you they are all on my cars and not leaking.

If you look at it, someone looks have put a rubber cap over the forward end (towards the radiator) of the fuel thermostat. That insert is plastic. Those sometimes crack and leak. You take it out by removing the circlip on it (with the thermostat off the car), on the plastic insert is a big O ring, these leak somewhat often.

Both the O-ring and the whole plastic end are available from the dealer under $20, might even be under $10, I do not have the part number in front of me. But they shouldn't be hard to find.

In effect, its bypassed because it leaked, but the leak is generally easy to fix.

I would either repair it and take it back to original, or do a better job of bypassing it. There is no need for a bunch of extra plugged lines. You can just run the feed from the tank straight to the lift pump.

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68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i

The control valve just rearward of the lift pump has a tendency to leak.

There is a rebuild kit available for the thermostat. Very easy to rebuild.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

so there should be a line to the lift pump from where the thermostat has been capped off & the line with that bolt is is the inlet, fed from the primary fuel filter?

I have a mind to fix this if it can be done for modest cost.

The way I see it those German engineers designed the system to work in a particular way for a reason.

Paul: feel free to comment (via PM if you like), I am interested.

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1991 300D

EPC or the service manual will show the routing of the lines. EPC has the part number of the thermostat rebuild kit. Not terribly expensive IIRC.

I would go ahead and change out all the rubber hoses while you are in there. Standard 5/8" fuel hose from the auto parts store will work fine here.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

found the link to 6 parts needed for rebuild & their numbers. Will consult the EPC for routing. NAPA for the hose.

Any particular savings realized through ordering through this site vs dealer 10 minutes up the road? I do like to spend a little locally, if for no other reason to flaunt my 30 y.o. $350 hoopty past the rows of shiny-new POS's they're hawking. Countermen are chill too.

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1991 300D

Parts like this are worth going to the dealer for I enjoy ordering small parts from the dealer and now get 20% discount even though I spend less than a 100 a year at the dealer.

Hi Quahog,

My thermo leaked which I think caused a drop in mpg for me and maybe letting air into the fuel sys. I am going to replace items 676,673 &670 in the photo(blow out of the 603 unit) which I could not find here on PP so went to my local dealer.

Part # are
676: 601-078-00-16
673: 011-997-39-48
670: 601-078-01-76

Total: $15. No juckyards had it locally, and the dealer wanted north of 200 for a new one IIRC. Personally, I looked into just bypassing the thermo and letting the fuel warm w/o it. Further reading revealed that the fuel actually has a cooling function on most IPs, (a CAT article about fuel coolers) which is generally why the thermo stops heating the fuel above 25C, IMO.(could be way off) Given that I plan on using the HVAC to heat this winter, I thought it better to let the system do as designed and not heat the fuel to engine coolant temp. My $.02

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5/8" fuel hose? I don't think big rig trucks that get 5 mpg use fuel hose that big.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I imagine the 5/8" was OD.

I usually try to bring a bit of the hose or the fitting with me to the counter

anyway that repair is on the list, after glowplug replacement that is this weekends fun. oh yes, good times.

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1991 300D

Fuel hose are usually spec-ed by i.d. Rigid tubing on the other hand are spec-ed by o.d. Always a good idea to bring an old hose with you so there's no mix up.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked