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OM603 Fuel Thermostat

5K views 1 reply 2 participants last post by  BenzHacker 
#1 ·
I need a technical explanation. I just had my fuel thermostat fail. I had a large fuel leak at the plastic fuel inlet. I have a new part on order and will have no problem installing the new one. My question is, what is the fuel thermostat for???
 
#2 · (Edited)
This is an old thread, but since many of the diesel questions go unanswered and we ALWAYS direct new people to use the search function, this question needs answering.

These units don't really fail - they leak. Fixing the leak requires replacing the o-rings (MB part number 0119973948) which happens to be a nitrile/buna-n elastomer seal.

The purpose of a fuel thermostat is to pre-heat the fuel. Unlike gasoline, at very low temperatures diesel fuel (particularly #2 ultra-low sulphur diesel, thicker than #1 diesel fuel, if you can still find it) can gel up and become waxy, leading to hard starts and rough running for several minutes until the engine is well warmed. The thermostat works like the one in your cooling system - recirculating the fuel until a preset temperature before opening to improve the fuel flow characteristics and thus easier to inject.

Usually they leak from a hard/cracked hose, replace the hose and refit the hose clamp. If they leak from the body, it's one of two o rings. In such cases, remove the fuel thermostat from the car (unbolt it, remove hoses) and remove the snapring/circlip at one end. Withdraw the actual thermostatic valve from the body with a good tug. Match the size of the o-rings to replace them.

While nitrile/buna-n seals will work, flourocarbon (Viton) is the preferred choice as it is compatible with all liquid fuels you could run in a diesel (all petro diesel fuels, biodiesel and WVO/SVO) and can operate at higher temperatures for long periods without degradation. Re-insert the valve into the body. You may find that the fresh o-rings will make going in more difficult than withdrawal, lubricate with a little diesel fuel or ATF, though make sure your hands and the oil is extremely clean to avoid contamination. Refit the snap ring/circlip and bolt back up, fit the hoses and hose clamps.

-bh
 
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