E250 temperature gauge issue?

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DrNick

MB Enthusiast
Joined
Nov 25, 2003
Messages
1,320
Location
UK
Car
SL500
So driving along the motorway yesterday, I look down to see that the temperature gauge is off the top of the scale. Very alarming, but everything seems to be working fine.
Start thinking about what to do, surely there should have been some other kind of warning, when a big red message appears saying too hot, stop immediately, or some such.
This message then disappears and I carry on looking at the gauge, now thinking that there is a real problem, when before my very eyes, the gauge drops very quickly back to normal.
Car behaves normally after this, and has done today as well.

In the previous week, one two occasions I have noticed the cooling fans running after I've stopped the car, but the gauge has been normal on those occasions.

Car is an E250 2014 petrol with 25k miles and service history.
Anybody experienced this before? Its going for a service in a week (if it makes it that far). What would be the culprits to advise they check. Thermostat? Sensor? Water pump?

Nick
 
I would have them check all the cooling system, could be a number of things from low coolant due to a leak, a faulty temp sensor, faulty Thermostat or some other damaged component.
Makes sense to have it all checked out. Could even be an air lock in the system.
 
Happened again at the weekend on a 30 mile motorway journey. Stopped to check things out. No evidence of overheating, steam, fully smells etc. Left to cool and checked the water level. Its normal.
Drove back 30 miles. No issues at all. Looking to me like a wiring or a sensor problem. I'll ask them to check it out but I can imagine it will come back as no fault found.
Interestingly, although it went right into the red, I didn't get the the big warning to stop immediately. I do think it went into limp mode though as it was reluctant to rev. As i say though, on the way back, no problems at all - completely normal
 
As said could be several things, thermostat, water pump or temperature sensor. Sounds like something mechanical is jamming and then under extreme temperature is freeing itself. difficult to check because its intermittant.
 
So its been in for its service and came back nice and clean.
However, as expected, no fault found for the cooling system. No codes stored, no evidence of overheating, coolant all fine, couldn't replicate the fault. Just have to see if it gets better or worse.

Does anybody know where the coolant sensors are located on the 2.0 petrol engine (W212 E250) so I can at least plug and unplug it (or them) to clear any dirty connector worries?
 
Can't help with sensor location, but a similar thing happened to my 2015 E200. Ended up being a faulty water pump.

D
 
Can't help with sensor location, but a similar thing happened to my 2015 E200. Ended up being a faulty water pump.

D
When you say similar, how similar? Did it go right up into the red and then back down again, but with long periods of normal operation? Or was it doing something different?
 
Thanks for this. I think this might be a different fault as mine has no trouble warming up to normal. Its only after its been normal for a while that it misbehaves, and then only occasionally. If its the thermostat, the implication is that it is operating normally, is suddenly closing, and then after a few minutes, opening again. Doesn't sound likely, but I'm sure stranger things have happened!
More likely to be a water pump. I can imagine that the impeller may slip occasionally, but I would think this would fail hard very quickly.
My money is still on a duff sensor/connector issue, or possibly the dashboard doing weird things (which sounds equally dubious and horribly expensive!)
 
When you say similar, how similar? Did it go right up into the red and then back down again, but with long periods of normal operation? Or was it doing something different?

It went up to the red from cold. Then came back to normal temp after a few minutes. Then started to creep up again. Basically a 'yo-yo' effect. I didn't run the car for any extended period of time as a result, so couldn't comment on your long periods of notmal operation. Car was recovered to MB as I didn't want to cause any long term damage.

D
 
An update: Its doing it more and more. A quick investigation has revealed that the top hose is cold, the radiator is cold but the bottom hose is hot. All hoses easy to squish. No obvious hot spots on the engine except where you'd expect. Internal heater works ok so this is looking less like a water pump and more like a thermostat problem to me.
So its going in hopefully on Thursday for a thermostat change. Looks like MB is the only source of thermostats for this engine at present.
I'm beginning to suspect that the gauge is very non-linear. Once it moves above the 90 degree point, it moves incredibly quickly to 120 (exactly 120!) and then is equally quick to come back down. I've seen it go above 120 once but it waited at exactly 120 for a while before it went there. Still no other indication from the dashboard that there is a problem except once when the big red stop-overheating warning came on and then went away.
 
An update: Its doing it more and more. A quick investigation has revealed that the top hose is cold, the radiator is cold but the bottom hose is hot. All hoses easy to squish. No obvious hot spots on the engine except where you'd expect. Internal heater works ok so this is looking less like a water pump and more like a thermostat problem to me.
So its going in hopefully on Thursday for a thermostat change. Looks like MB is the only source of thermostats for this engine at present.
I'm beginning to suspect that the gauge is very non-linear. Once it moves above the 90 degree point, it moves incredibly quickly to 120 (exactly 120!) and then is equally quick to come back down. I've seen it go above 120 once but it waited at exactly 120 for a while before it went there. Still no other indication from the dashboard that there is a problem except once when the big red stop-overheating warning came on and then went away.

If the top hose is cold and engine is obviously not overheating, I will be changing the temp sensor as well, which is not too far off the stat, or might even be a part of it.
 
A bit more digging reveals that the cooling system is quite complicated, with the thermostat being electrically actuated (thus adding another failure mode), and the pump also having a vacuum operated valve in it.
The reason for doing more digging is that the thermostat is buried under the inlet manifold, and its a 4hr+ job to swap it. Thus a bit more investigation is in order before pushing that particular button.

The pumps appear to have known issues in terms of the actuation arm failing (metal replacements available from China), but also the valve itself can stick.
The video link below shows exactly the temperature gauge symptoms I'm seeing, and is linked to the pump valve not working. If the pump valve then unsticks itself, then everything will behave normally. My guess is that its sticking more and more.

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Still being investigated on Thursday, but due to the prices involved, we're being more detailed about it. Pumps are not cheap either
 
Hopefully the solution.
Water pump changed. The valve in the water pump was seized in the closed position, which explains everything. New water pump just over £100 from MB. Don't have the final bill yet, but I think a few hours of labour went into it.

Now it behaves itself, warms up a bit slower and stops at 90 degrees (although I need to go up the motorway to be sure its fixed)
 
All good now. M274 engine water pump has a vacuum operated valve that starts off closed to aid rapid warm up, and this opens under ECU command. If this doesn't open, then no water can circulate. An obvious test for this is to feel the top hose. If cold, then no water is circulating. In a normal engine, this is strongly indicative of a thermostat failure. However, in this engine, it could also be the water pump valve. The issue is compounded by the over complex nature of the thermostat, as it also has electrical operation with an embedded heater, which makes it expensive. Its location also means that the labour to swap the thermostat is also an issue. So just swapping the thermostat to see if it makes a difference is not the trivial cheap exercise you might think, so its best to check everything (including the water pump) before embarking on a "cure".
The water pump is also not a simple job either, due to its location, and isn't the cheapest item in the world of water pumps either!
The whole cooling system is a complex nightmare, and is so for no good reason I can see, other than some minor gains in speed of warm-up which is probably only significant in really cold countries.
It also means that there are potentially some other weird failure modes. For example, if the vacuum system develops a leak, the symptoms may be an overheating engine!

I'm also convinced that the temperature gauge is very nonlinear and very sensitive above normal.

Hopefully this will help some people to sort out similar issues with this engine.
 

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