OM651 - Coolant Temperature Sensor Problems

[Dickie Fix]

Has anyone any experience with coolant temperature sensors please. How do they usually fail: open circuit or short circuit, or do they become inaccurate as they warm up and would a resistance of 2KOhm be reasonable at room temperature? It's a (A 099 905 37 00).

Just trying to figure out why my temp gauge has started rising much slower than usual, indicating that the engine is taking a longer time to warm up, even in the high temperatures we've been having recently. Don't think it's the thermostat as the radiator to thermostat hose stays cold until the gauge reads over 80 degrees.

 

[toolman1954]

Diesels should heat up quite fast, by their very nature they are achieving high cylinder head temperatures to detonate the fuel within minutes of starting.
If your engine is taking longer than normal to heat up, and therefore the thermostat is opening later, then I would think that the thermostat needs changing.
How long after starting the car are you putting your hand onto the top hose? You need to get the engine up to temperature as swiftly as possible to achieve optimum fuel economy. You do not want it staying cold for too long.

Steve

 

[clk320x]

Diesels should heat up quite fast, by their very nature they are achieving high cylinder head temperatures to detonate the fuel within minutes of starting.
If your engine is taking longer than normal to heat up, and therefore the thermostat is opening later, then I would think that the thermostat needs changing.
How long after starting the car are you putting your hand onto the top hose? You need to get the engine up to temperature as swiftly as possible to achieve optimum fuel economy. You do not want it staying cold for too long.

Steve

Steve, my e320cdi takes longer to heat up than my CLK320 (petrol) (But still not longer than 3-4 miles)

Is it normal for petrol to be quicker to reach operating temp? or is it worth doing the stat on the e?

 

[toolman1954]

Abs,
Don't want to hijack this thread, I will message you.

Steve.

 

[Dickie Fix]

Diesels should heat up quite fast, by their very nature they are achieving high cylinder head temperatures to detonate the fuel within minutes of starting.
If your engine is taking longer than normal to heat up, and therefore the thermostat is opening later, then I would think that the thermostat needs changing.
How long after starting the car are you putting your hand onto the top hose? You need to get the engine up to temperature as swiftly as possible to achieve optimum fuel economy. You do not want it staying cold for too long.

Steve

If I start the engine from cold and let it idle, it takes 45 minutes to get up to 60 degrees and the bottom hose (RH in the attached diagram) from the radiator to the thermostat is still stone cold. The hose from the coolant reservoir is hot as is the return hose from the heater matrix. That the hot hoses are not as hot as usual makes me think that the gauge is correct when it says 60 degrees and the cold bottom hose indicates that the thermostat is still shut. So what could be causing the slow warm up?
The diagram is a bit strange as it shows the temp sensor (B11/4) to be between the thermostat and the circulation pump at the bottom front of the engine when the physical location of the temp sensor is on the cylinder head as usual.

 

[toolman1954]

Hi,
in my opinion, your thermostat needs changing, your engine is taking far too long to warm up. Get it changed and review your temperatures then. I have no idea how easy it is to change a thermostat on your engine or even whether it is a DIY job. If nothing else you should see a marked improvement in your fuel economy with a new thermostat fitted.

Steve.

 

[Dickie Fix]

Yes, all the symptoms indicate classic thermostat failure, but on all my previous cars it's been obvious when feeling how hot the radiator hoses are while the engine's warming up that the thermostat is not doing its job. On this one, however, the the hose to the thermostat is stone cold, so it's stopping the hot water from getting to the radiator. I've ordered a new MB thermostat and it should be arriving today. It's not a difficult job to change, but I have this aversion to "rectification by substitution".

I first noticed the problem last weekend when towing our caravan. The engine did not fully warm up until about 15 miles into the journey, even though it was tugging a 1250Kg box behind it. It hung around 70-80 degrees for about 10 minutes or so then suddenly jumped up to its usual 90. Our (indicated) fuel consumption for the 80 mile journey was in the mid 30's, which is what we usually get. Then on Monday we did a 20 mile round trip solo and it never got up to beyond 80, but fuel consumption was the normal high 50's. We're off out again today, so I'll see what happens.

 

[pembssurf]

Has anyone any experience with coolant temperature sensors please. How do they usually fail: open circuit or short circuit, or do they become inaccurate as they warm up and would a resistance of 2KOhm be reasonable at room temperature? It's a (A 099 905 37 00).

Plus another for thermostat failure extremely common on these engines, had to replace mine at 30k miles this year the n same engine. Exact same symptoms.

Just trying to figure out why my temp gauge has started rising much slower than usual, indicating that the engine is taking a longer time to warm up, even in the high temperatures we've been having recently. Don't think it's the thermostat as the radiator to thermostat hose stays cold until the gauge reads over 80 degrees.

 

[steveq]

I have noticed similar symptoms to you -- after about 5 - 8 miles the temp gauge slowly gets up 70 - 80 deg and after 10 - 12 miles it then suddenly jumps to around 90 where it stays.

I had a thermostat issue on my previous W211 E220 CDI with the OM646 engine but the symptoms were more normal. The water temp never got beyond about 60 degrees unless I was sitting in heavy traffic. Replaced the thermostat and the temp went to 90 deg quickly and stayed there.

That schematic is good -- it is a complex system though. Having a 'Coolant Pump' and 'Coolant Circulation Pump' is a new one to me.

 

[Dickie Fix]

A quick update. Did a 16 mile round trip this morning, mostly A roads @ 50-55MPH. The temp gauge moved off the stop after about 5 mins/2 miles, reached 75 degrees after about 12 minutes/8 miles and crept up to 90 after about 20 minutes/12 miles. Checked the thermostat to radiator hose when I got home and it was barely warm but the heater matrix return hose at the top of the engine was hot, so thermostat seems to be functioning correctly. Indicated fuel consumption was 63MPG, so nothing wrong there.

I'm not convinced yet that it is the thermostat. Unlike the simple thermostats of old, you can't test it by hanging it in a saucepan and watching for it to open! So I'll just keep an eye on things to see if and how they change over the next few weeks. Thanks for your help Steve. I'll add to this thread if I do decide to change the stat and share my lessons learned.

@steveq, I think the coolant circulation pump is an electrically driven pump to provide continuous heat in the cabin, useful when the stop/start system has stopped the engine.

 

[Mr Filipov]

@Dickie Fix the symptoms you describe certainly spell stat to me, but your call at the end of the day.

I had very similar behaviour on the E350 I had and it keps getting progressively worse till the point it didnt even want to reach 70 indicated. Thermostats are more common failure on Mercedes than you can imagine.

 

[paul73mt]

I have only just got mine and sometimes it takes 16 miles to get up to temp (90 degrees) then other days it seems to get to temp within 4/5 miles so not sure if it right or wrong

 

[paul73mt]

sorry to bump up a tread again but on talking about the temperature gauge normal temp I decided to plug in my code reader and monitor the actual temp compared to my temp gauge. So started from cold live data reading 25degrees C started and slowly went up @60 on the gauge mine real temp was only saying 51 now this slowing increased and the gauge was around 8/10 ish degrees out from the actual temp on the live data. Now I have been saying that when my temp gauge goes just over the 80 mark the thermostat opens as the temp will then drop to around 70ish (this all by looking on the gauge ) goes backup to just over the 80 mark then drops again and so on and so on until it reaches the norm of approx. 90 degrees c.(this could be between 10 and 20 miles ) Now looking on the live data it seems the thermostat goes up to 72 degrees then thermostat opens temp then drops to as low as 60 then up again then does the same thing again. So with this in mind I'm thinking the thermostat is faulty ??? It has been into MB but they say nothing wrong with it the tech even come out looked at my video of it and said that's normal. The part is only £55 but the next point / question is how easy are they to change ??

Thanks

 

[Dickie Fix]

@paul73mt : I did the same as you with my code reader and have had similar results, however, it doesn't happen all the time. Over the last few weeks the temperature gauge and code reader have shown a normal warm-up to operating temperature (an indicated 90 degrees) in about 3 - 4 miles. Then yesterday it went up to 60 as normal, then hung about a bit before going up to 80 after about 10 miles before continuing up to the usual 90 after another couple of miles. Unfortunately, it was one of those times when I didn't have the code reader plugged in, so I don't know what its readings were. I've got a new MB thermostat sitting beside me and I've almost decided to change it at the weekend just to see if it makes a difference. Doesn't seem like a big job, but on our B Class it's at the bottom front, so it's a ramp job. Your E may have it on the top of the engine which should make things easier.

While looking into the problem I came across this Youtube video which explains how these ecu-controlled thermostats work:

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Every day's a school day!

 

[paul73mt]

@paul73mt : I did the same as you with my code reader and have had similar results, however, it doesn't happen all the time. Over the last few weeks the temperature gauge and code reader have shown a normal warm-up to operating temperature (an indicated 90 degrees) in about 3 - 4 miles. Then yesterday it went up to 60 as normal, then hung about a bit before going up to 80 after about 10 miles before continuing up to the usual 90 after another couple of miles. Unfortunately, it was one of those times when I didn't have the code reader plugged in, so I don't know what its readings were. I've got a new MB thermostat sitting beside me and I've almost decided to change it at the weekend just to see if it makes a difference. Doesn't seem like a big job, but on our B Class it's at the bottom front, so it's a ramp job. Your E may have it on the top of the engine which should make things easier.

While looking into the problem I came across this Youtube video which explains how these ecu-controlled thermostats work:

Every day's a school day!

That's interesting (good find)!!! but i can see that mine shouldn't get to normal temp within say 5 or so miles i know its not working hard as i do approx 3miles till im on the motorway for about 30 ish miles then around 3 miles on a/b roads. I have seen one video on youtube but it shows the oil filter out also (but not sure if there doing a service ?? ). Mines does it prob 90% of the time, there was a time i got it to go straight up to normal operating temperature by turning off the ECO., But this was just wishful thinking as now does it when its off or on. Today coming in to work it took 18 miles to get to normal temp but im only going along the m25 at 55/60mph that why i got rid of the C350 as just didn't use the power. I have heat inside the cabin when i turn the heat up. So i have heat within the system.Just not sure weather to buy the thermostat or just wait till its buggered. When we had the really hot weather it done the same where you would of thought it would heat up quicker ? Or perhaps the ecu is at fault ????

 

[Dickie Fix]

This Youtube vid may give us a clue as to what could be happening:

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[Justintyme]

That's interesting (good find)!!! but i can see that mine shouldn't get to normal temp within say 5 or so miles i know its not working hard as i do approx 3miles till im on the motorway for about 30 ish miles then around 3 miles on a/b roads. I have seen one video on youtube but it shows the oil filter out also (but not sure if there doing a service ?? ). Mines does it prob 90% of the time, there was a time i got it to go straight up to normal operating temperature by turning off the ECO., But this was just wishful thinking as now does it when its off or on. Today coming in to work it took 18 miles to get to normal temp but im only going along the m25 at 55/60mph that why i got rid of the C350 as just didn't use the power. I have heat inside the cabin when i turn the heat up. So i have heat within the system.Just not sure weather to buy the thermostat or just wait till its buggered. When we had the really hot weather it done the same where you would of thought it would heat up quicker ? Or perhaps the ecu is at fault ????

Since last Monday morning, my Merc is displaying the exact same symptoms as described above... from a cold start, external temp@ 12c, staying at low 60's for 10-12 miles before climbing up to the 90 mark, but staying at that temp from that point onwards ( unless stopped for awhile and allowed to cool down ) Same thing happened last night, stayed at the 60 mark, despite climbing a serious hill, before hitting the 90's. So Thermostat I guess. Does anyone have the correct part Nr for the 2010 E Class W212 2.1 diesel, with OM651 engine? Watched a video on You Tube last night, showed a Thermostat being replaced on a OM651 Engine...does not look that complicated, but would be nice to talk to some one here on the forum who has actually done a replacement. There is always something you will meet when doing the job yourself, that was NOT shown on You Tube .. LOL As another possibility, could be faulty sensor?

 

[HITECH]

In my own experience I find that the first thing people do when the car takes longer than usual to get up to temp is blame the thermostat. So they change it and hey fault fixed. But what they actually done was replace the thermostat THEN put in fresh clean new coolant fluid to the exact mixture that it is supposed to be. So majority of time it isnt the actual thermostat. People tend to top up with coolant and keep doing it. The mixture ends up that lean that its almost 100% coolant. The chances of that getting up to any kinda of temp close to your 80/90 are quite slim. What I do when I get a car in with similar complaint is flush the coolant system a few times and simply mix the proper coolant concentration according to the car and the problem is solved 99% of the time. The ones I have had to change thermostats on are ones that are stuck closed and cause more serious problems. I suggest flushing system and replacing ur coolant 1st properly before moving onto the thermostat.

 

[Dickie Fix]

Problems persisted today with it taking 9 miles to get up to 90, so decided to change the thermostat this afternoon. Printed out the instructions from the WIS and got the car up on the ramp. Then I hit problem number one. Taking the front underpan off so that the radiator can be drained requires the bumper to be removed. It must need a special spell because it did not want to be removed, so removed the rear underpan instead (easy) and found where the thermostat was. Problem number two, the three standard torx-headed bolts (according to the EPC) holding the thermostat on are actually torx machine screws with a security pin in the centre. Problem number three, there's very little room around the thermostat to undo the heater plug or the hose connections or even get to the 3 screws. So, after spending the whole afternoon on this and getting nowhere, I put everything back and called it a day. I'll now happily pay for someone to change it for me.

After 50 years of maintaining my own cars, this is the first time any car has beaten me. So if anyone can recommend a good indi near Peterborough, I'd be grateful.

 

[Justintyme]

In my own experience I find that the first thing people do when the car takes longer than usual to get up to temp is blame the thermostat. So they change it and hey fault fixed. But what they actually done was replace the thermostat THEN put in fresh clean new coolant fluid to the exact mixture that it is supposed to be. So majority of time it isnt the actual thermostat. People tend to top up with coolant and keep doing it. The mixture ends up that lean that its almost 100% coolant. The chances of that getting up to any kinda of temp close to your 80/90 are quite slim. What I do when I get a car in with similar complaint is flush the coolant system a few times and simply mix the proper coolant concentration according to the car and the problem is solved 99% of the time. The ones I have had to change thermostats on are ones that are stuck closed and cause more serious problems. I suggest flushing system and replacing ur coolant 1st properly before moving onto the thermostat.

Good Post, Hitech. Because the fluid is going to be replaced anyway, do the change and flush first, drive and see what are the results. In my case though, since I got it last January, twice the low level warning came on, and each time I topped it up with 500 ml water, so I presume that the mix has been thinning rather than becoming more antifreeze pure. I had already bought 5 ltrs of Comma Xtream 48 concentrated, as a coolant change was planned, ever before the temp started to act up. So now I will drain, flush and refill. On the U tube video I watched, the drain is shown as being on the bottom of the radiator, (Red Cap) with outlet beside it to take a plastic pipe. Will ALL the water drain out via this pipe? Or is there other outlets as well? Far as I know, the total capacity is 10 ltrs, so if I measure what drains out, and it comes to 10 ltrs, it will be a complete drain?