Leaking Fuel Thermostat 300SDL - PeachParts Mercedes-Benz Forum

Togorashi · #1 10-05-2008, 03:25 PM

I've got a leaking fuel thermostat on my 86 300SDL that started a couple days ago. Seems like something that always has to be replaced at some point.

I ordered a new fuel thermostat (the $20 plastic piece). It should be here in a few days. Any way to temporarily stop the leak? Glob it up with silicone caulk? Plumbers putty? Right now I've just got an oil-change pan under to catch the drip.

sixto · #2 10-05-2008, 04:29 PM

Did you get a new o-ring as well? Usually it's just a bad o-ring but sometimes the plastic fitting cracks.

In the meantime you can bypass the fuel thermostat. Connect the line from the small filter directly to the lift pump.

Sixto
87 300D

Togorashi · #3 10-06-2008, 01:46 AM

No, I didn't buy an O-ring. I assumed that it would come with one. It probably doesn't though. Crap. I'll just go down to Carquest and have them get me something that will work. An O ring is an O ring right? As long as it's the right size?

Really, you can bypass the thermostat?! So what does the thermostat actually do? How does bypassing it affect functionality?

a2t · #4 10-06-2008, 10:14 PM

Ive been thinking about removing it all together.

Mine broke (OM603) and it was $125 part. Dont know how you got it for $20!

Anyone know if its needed ?

sixto · #5 10-07-2008, 12:07 AM

I don't know the setpoints of the thermostat so I don't know under what conditions it uses coolant to heat fuel. US spec 617s do fine without a thermostat though the sense is 603s perform better in cold weather or maybe can perform in colder weather than 617s. I don't know how much that has to do with the fuel thermostat or how necessary that is in Atlanta.

MB sells individual thermostat components and it's easy to rebuild. Most of the time it just needs an o-ring. If it can be repaired for a couple of dollars and a half hour's work, why not?

Sixto
87 300D

bob_98sr5 · #6 10-07-2008, 12:50 AM

hey guys,

i did this several months ago. sixto's right: it was the o-ring. i tried an viton o-ring from a kit i bought from harbor freight but it was too thick.

see my thread here: W124 87 300D Fuel Lines: have I identified them all?

good luck if you have big hands!

Togorashi · #7 10-07-2008, 02:39 AM

Thanks for all the info. I think I'll just bypass it for now. It seems like many people are running without them. If it did create problems, any speculation as to what might pop up?

redassag00 · #8 10-07-2008, 02:45 PM

I just bypassed mine until the parts came in to rebuild it. Happened on my 87 300D when my dad was driving it, 3 times until I figured out it was the fuel thermostat.

Togorashi · #9 10-07-2008, 02:59 PM

I just bypassed mine. Seems to be working fine. Parts should be here later this week, but I'm not even sure I want to mess with it.

a2t · #10 10-07-2008, 10:20 PM

Let me know how it goes bypassing it. Its a whole mess of tubing that I'd love to ditch. Wish I knew it was a rotable component rather than a throw away. Have to go look for the old 1, prob just a few dollar o-ring...doh!

Togorashi · #11 10-07-2008, 11:22 PM

Well, I did a bunch of drastic stuff tonight. I bypassed the fuel thermostat and I completely removed the ALDA. I went for a long test drive and it's running REALLY well. We'll see what the long term performance is. I'm concerned about how high the revs are when I'm cruising at highway speeds, but I post about that in another thread.