OM642 thermostat replacement

[traser]

Hi. I think I have a thermostat failure in a W221 S-class, which I believe is the OM642 engine: 2011 350 Bluetec.

The symptoms are that the engine doesn't appear to get up to normal temps, especially worse on very cold days, the temp gauge hovers around 40c but occasionally will drop to zero (an engine restart resets that), and the heating doesn't warm the cabin even on max blower and temps.

From googling, the thermostat could be the culprit and it's nicely located at the front of the engine. However, Mercedes thought it would be funny to use a longer bolt at the back, which is blocked from removal by the fuel rails and other parts of the engine.

Firstly, do those symptoms sound like a thermostat fault, and secondly, are there any guides for replacing the part on that engine without having to take lots of the engine apart first?

I have had a search here and it seems like people have done it, but I haven't seen anything like a step-by-step as to what needs to be removed to make way for the new part.

 

[TeddyRuxpin]

You need to remove the intercooler pipe (the big silver thing coming out of the turbo) to do the thermostat on a later OM642

Make sure to get new green and black o-rings for each end of the pipe when refitting

The thermostat is expensive, unfortunately

 

[AndrewOl]

As above with loosening the fuel pipe clamps there is just enough clearance to get the long bolt out. Just check you don’t have a temperature sensor fault though as the dropping to zero on the gauge doesn’t sound right.

 

[MrGreedy]

I echo @AndrewOl in thinking.

Usually if it were just thermostat, you would notice a small shift in achieving and adhering to the required 88-90°C steady state, and usually getting worse and worse over days, weeks or months. It's not the normal failure mode to see rapid temperature fluctuations during driving if the thermostat isn't working correctly; more like just unable to reach the correct steady state temperature.

 

[OneForTheRoad]

Get an OBD2 scanner and find out what real time temperature is first .

 

[Smiley]

I agree with the others, I am not convinced that the thermostat is definitely the problem - I have had an e class where the thermostat was definitely the problem - engine very slow to warm up and only getting to about 40 degrees - but even at that temperature there was plenty of hot air in the cabin. The fault codes need to be read to avoid replacing unnecessary parts. Lack of hot air in the cabin is usually the duo valve but that would not explain the low engine temperature reading.

 

[suty455]

Sounds like the stat to me, mine did exactly the same thing on my E350 would never get warm always hovered around halfway and if sat in traffic it would actually cool down with the AC on coasting also causes it to cool, pain to do but not massivly, replace the seals on the air feed to turbo(orange), PCV (orange) and the intercooler pipe(green) at the same time and really anytime they are disturbed really only a one break serviceable item

 

[traser]

Thanks all. It sounds like to avoid taking large amounts of the engine apart it's just loosening the fuel pipe clamps then?

I've taken it around to a couple of garages, one who said it's the thermostat but doesn't know how long it would take to replace it, and a specialist who is intrigued by the gauge activity, but they are a good distance away. Mercedes will replace it for between £400-£700, which is frustrating for a part that looks so easily accessible and that 'only' costs £100 for non-OEM...

 

[E55BOF]

Feel the radiator after a run. If it's cool, there's a thermostat problem. Also, check the top hose; if you can compress it easily with your hand, the cooling system isn't up to temperature.

 

[traser]

It's taken a while but I ordered a replacement OBDII scanner. It reports the coolant temp is 77 degrees after a normal run out.

The thermostat is very warm to the touch, but not hot enough that I can't hold onto it, and the hose is easy to compress.

Any thoughts as to whether it is the thermostat? Another symptom is that when the engine temp needle drops to zero the engine fan can be heard going 100%. Again, turning the engine off/on restarts the gauge and the fan back to normal.

Besides these kinds of problems, I love the car!

 

[OneForTheRoad]

With the water at 77 what does the dash tell you it is?

 

[E55BOF]

Replace the 'stat. The coolant temp should be 90 degrees or so; you wouldn't be able to hold on to the housing, or the hose, if it was up to temperature.

 

[Gnasher141]

Intake air temperature 174 degrees C ????

 

[traser]

After another run today, the gauge was pretty accurate and stayed around 40 degrees before dropping to zero. OBD reader confirmed the temp matched the gauge.

Then, a restart and further drive and the coolant gets to 69 degrees, and the dash gauge reports almost 80 degrees, so some way off there.

If it consistently had a fault I'd take it to Mercedes but I know when I turn up there they won't see or find a fault. There are no error codes to go on either.

 

[traser]

Intake air temperature 174 degrees C ????

Let's ignore that lol

 

[OneForTheRoad]

Let's ignore that lol

Your meant to reach 90 before the start opens up. Yours is knackered. The wind rushing over the radiator at 70mph will soon cool a system that's opening at say 40 degrees .

 

[E55BOF]

1. Replace the thermostat. You will never sort it until you do that.

2. See how it is after that; if there's still a (second) problem, you can set about fixing that then.

 

[traser]

Thank you. That was the conclusion I had come to as well but it never hurts to have other opinions.

Next step though - I'm comfortable replacing the thermostat, but not sure how and what to remove to get that long bolt out. Does anyone know of a workshop manual or a step-by-step for this? I've searched Youtube but have not found anything relevant to this engine.

 

[suty455]

Dont take this the wrong way but what is the point of asking the same questions over and then not taking the advice? Remove the intercooler pipes and fuel filter pipes and all is open before you do it buy the turbo inlet and pcv valve seals to replace them also buy yourself the green intercooler seals and replace those, thats it, its an easy job just make sure you buy the correct stat some have a preheater which is a black plug on top some dont

 

[E55BOF]

If there's no electrical connection to the top of the 'stat, you do not have the heater.