Leak in Fuel Preheater Thermostatic Valve - PeachParts Mercedes-Benz Forum

cmac2012 · #1 05-21-2017, 04:42 AM

I was going to title the thread 'what's this part - fuel leak' but I found the following in a search:

87 300SDL fuel leak, what is this?

Has some good info, his pics are good. Discovering this was actually halfway good news. I had been leaking tranny fluid a bit and I thought this was a worsening of that - it's putting out a lot, oils up a lot of the driveway. Not only will this be easier to fix, it might explain a drop in my mpg. But this won't be a walk in the park, lot of hoses attached to this.

I'm wondering if going to HPR diesel might have brought this on. Word I've heard is that if biodiesel was used previously (uncertain here) HPR may shrink fittings.

I got pics of the fuel dripping off the housing:

jay_bob · #2 05-21-2017, 08:11 AM

Very common problem on the 603.

It can be rebuilt. Undo the hoses, dismount it from the engine (it's just bolted onto the block), undo the circle clip, and pull the innards out. The O-rings get renewed and it all goes back together.

In a mild climate like N. California you probably can get by without it. At least bypass it until you get the rebuild kit, so you're not leaking everywhere.

Diseasel300 · #3 05-21-2017, 10:13 AM

Since you're in CA, just delete it. With modern diesel already being low in lubricity, heating it sure isn't going to do the IP or lift pump any good in the long run. I had the thermostatic element in mine fail in the "heat" position and refused to shut off the heating. The result being very warm fuel in the tank after some time on the highway. Not ideal.

cmac2012 · #4 05-21-2017, 10:52 AM

It's a tough one. I may move back to WA at some point. I might regret not fixing it. At any rate, the deleting looks complicated. Several hoses going in and out.

My reading tells me I might want to get the Viton variety of O-ring, I suspect using HPR brought this on as it wasn't happening when I bought it but a few weeks after using HPR the leak started.

I found the O-ring at Pelican but it doesn't specify material:

Part #: 011-997-39-48-MBZ

I submitted a question.

Diseasel300 · #5 05-21-2017, 10:55 AM

Remember that the 617's have no fuel preheater and they work fine in cold weather. Unless you're in the arctic or running WVO, I seriously doubt it would be an issue. You could always throw it in the trunk and fix it later if/when you need it.

tjts1 · #6 05-21-2017, 12:32 PM

Just bypass it. Afterglow does a lot more to help with cold start than the fuel preheated even in cold climates.

sixto · #7 05-21-2017, 02:06 PM

The hose from the small filter goes directly to the lift pump. The rest don't matter after they've leaked dry.

Sixto
83 300SE can be yours
98 E320 wagon

cmac2012 · #8 05-21-2017, 02:46 PM

Quote:

Originally Posted by sixto

The hose from the small filter goes directly to the lift pump. The rest don't matter after they've leaked dry.

Yep, I was overthinking it. I just looked at it again and of course: two of the lines are for taking the fuel to and from being heated, the third one takes it over to the lift pump, and the other is obviously from the tank/small filter.

I'll just get a new hose, a longer piece - it looks like I can just leave the thermostat thing in place. Perhaps I'll shop around for two rings of the right type and have them on hand.

jay_bob · #9 05-21-2017, 08:45 PM

Find the two hoses heading up to the back corner of the head. These are the heater loop. Ignore these 2 hoses and leave them connected.

Find the hose that is coming from the steel line from the tank to the preheater. Pinch this hose shut and remove it from the thermostat.

Find the hose that goes from the pre filter to the thermostat. Remove the end attached to the pre filter and stick this on the thermostat port you just freed up. This seals off the thermostat and preheater so that no contaminants get in there if you change your mind and fix it later (like if you move to WA).

Now take the free end of the tank line and attach this to the prefilter. The other end of the prefilter should still be going to the lift pump (the little can sticking out of the side of the injection pump at the bottom, this is your fuel supply pump).

cmac2012 · #10 05-23-2017, 04:05 AM

Sounds good. I had a bit of a struggle to twist the one hose around to plug input to the thermostat. The other hose looked like it was maybe a tad short to do the job and I was thinking new hose wouldn't hurt so I got 12 inches and cut off and inch or so to mount it.