Vito 122cdi Coolant Temperature Issue

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hondaman

New Member
Joined
Dec 16, 2021
Messages
7
Location
Burton-on-Trent
Car
Mercedes Vito Dualiner 122cdi
Hi,
I'm a newbie on this forum and hopefully I'm posting this in the correct section.
I have a problem that's driving me crazy, and after many hours and much expense cannot suss out.
The vehicle details and problem are as follows:

Mercedes Vito W639 122cdi, 2015 model with 102,000 miles
OM642 3.0 V6 diesel
One previous owner with full MB service history and owned by me for the past 4 years.


A bit of a saga, this one, but please bear with me!

The coolant temperature had previously always held a steady 90C (194F), regardless of ambient air temperature, engine load or vehicle speed.

A few months ago, I noticed that whilst towing my 1.7 tonne caravan (trailer), the digital coolant temperature display rose to 100c (212F), whilst ascending a long motorway incline.
As soon as we started on the descent, the temperature dropped to the normal 90C.

Ambient air temperature was in the region of 22C (72F).

I even had the heater on full blast to see if that would help matters, but it didn’t make any difference to the coolant hitting 100C under load.
I also switched on the Air Con so that the twin electric fans kicked in, but again it made no difference.

I pulled into the next service area and checked the coolant level, viscous fan operation, top hose temperature, expansion tank etc.
I also checked that the airflow to the radiator wasn’t blocked by anything like a plastic bag or similar.
Absolutely everything was in order and nothing obviously amiss, so carried on but kept an eagle eye on the coolant temperature display.

My initial thought was that it was a sticking thermostat and that it may not be fully opening (I had that problem years ago on a Ford).

The journey was about 250 miles and each time the engine was under load it was the same story, but it never did exceed 100c which is why I decided to carry on with the journey.

When running under a light load or in traffic the temperature held a steady 90C.

Once at our holiday site and on our holiday, it continued the same pattern even when running ‘solo’ without the caravan (hitting 100C when under load).

Once back home a couple of weeks later I replaced the thermostat with a genuine MB one, and whilst at it I replaced the coolant temperature sender (genuine MB).

Took it out on my 12-mile test route and nothing had changed, still hit 100C when the engine was under load – i.e. fast motorway speeds or steep inclines.
I also plugged in my diagnostic tool and monitored the live coolant reading. This showed that when the dash gauge was showing 100, the true temperature was 97.5C.
What was slightly strange, was that when once it hit 100C on the instrument gauge, it would then come down very quickly to 90C as soon as I eased off on the throttle.

Although the viscous fan clutch seemed to be working correctly (using the folded paper test), I never the less reasoned that was the next possible cause.

Another trip down to my friendly MB dealership and the new fan is fitted (after much cursing and the making of a special holding tool!).

Didn’t hold out too much hope because the old fan clutch had a similar resistance to the new one, and the test drive proved as much – exactly the same symptoms!

By now there are only 2 more candidates for the overheating issue – namely the water pump or the radiator, (I’ve ruled out a head gasket failure or cracked block because it’s never lost any coolant and the coolant is pristine even after 100,000+ miles).

So, it’s out with the radiator and the water pump.

My hopes were that the water pump impeller blades might be corroded away or the impeller was spinning on the shaft and thus not giving sufficient coolant flow through the radiator.
Hopes dashed! The pump is pristine internally and impeller is tight on its shaft.

So, it’s got to be a blocked radiator then? (despite the pristine condition of the rest of the cooling system).
Well, the radiator is in good condition externally, nice and clean with minimal fin damage on the core.
Internally, from the little that can be seen by shining a torch into the hose connection stubs, it is pristine as per the rest of the system.
I’ve run water through it with a garden hose and it easily flows the maximum volume I can achieve with the hosepipe (not sure how this would compare with the flow rate when the engine is spinning at say 3,000 rpm).

Despite the radiator seemingly being okay, it really is the last part that it could be causing the hot running, so I’ve ordered a new one, but wouldn’t put money on it making any difference - we shall see!
If I don't replace it, I'll will never know.

I’ve been an Engineer all of my working life and worked on my own cars and motorbikes for over 40 years.

I’ve solved lots of difficult problems with cars and bikes, but this has got me beaten, new radiator not withstanding!
A mechanic friend suggested that it may be a faulty instrument display, but if that was the case, I would have expected it to read a steady high or low misreading and not to fluctuate by the 10C that it does.
Plus the fact that when I had my diagnostic tool plugged in, it mirrored the temperature fluctuations, albeit showing 2.5 degrees lower overall.

Getting into more technical realms I’ve wondered whether there’s something amiss with the ECU controlled diesel injector timing and that’s causing the hot running when under load and higher throttle openings?

Point of Note – I’ve owned 2 previous E Class Mercedes saloons fitted with the same OM642 engine unit and neither of these cars’ coolant temperature budged from 90c under any operating conditions, even when working hard towing my caravan up long motorway inclines in hot weather.
I cannot believe that my Vito, which is a commercial vehicle and capable of carrying heavier loads should be any different from those saloons.

If you’ve managed to read through to the end of this, congratulations!
So, does anyone have any ideas?
 
Hi & welcome.

A 10 degree rise under heavy load (shifting almost 4 tonnes up a long incline at motorway speeds would certainly qualify) doesn't seem excessive to me. I'm pretty sure ours goes up a bit (I only look occasionally, because on my cluster the temp. display isn't there by default), and that's only towing a 1.5 tonne caravan. It's never bothered me, and I've had the van for 14 years / 135k miles now ... why does it concern you? You would get a temperature warning light/message from the vehicle if the temp. was excessive ... the traditional gauge on my SL has a 'red' zone starting somewhere above 120C, so anything below that is going to be OK:


1639674226314.png
 
Hi & welcome.

A 10 degree rise under heavy load (shifting almost 4 tonnes up a long incline at motorway speeds would certainly qualify) doesn't seem excessive to me. I'm pretty sure ours goes up a bit (I only look occasionally, because on my cluster the temp. display isn't there by default), and that's only towing a 1.5 tonne caravan. It's never bothered me, and I've had the van for 14 years / 135k miles now ... why does it concern you? You would get a temperature warning light/message from the vehicle if the temp. was excessive ... the traditional gauge on my SL has a 'red' zone starting somewhere above 120C, so anything below that is going to be OK:


View attachment 121704
Hi, thanks for the reply.
Why does it bother me? Well quite simply because I'm concerned that it is a symptom of an underlying fault.
Note that it isn't just getting hot whilst towing, it's also behaving the same way when solo if working under load - i.e. at fast speeds and/or up inclines.
Something has definitely changed because it didn't used to do it.
I agree that it isn't getting to the danger level yet, but I want to sort it before it does so, and then leave me with a very expensive repair bill!
As I stated, I've owned 2 previous Mercs with the similar engine unit and they never budged from 90C no matter what.
Neither did an Audi 2.5 V6 diesel or a Vauxhall Omega 2.6 V6 diesel that I've owned.
In my view, if the cooling system is in good order, then unless I was towing in very, very hot conditions it shouldn't increase by 10C.
I had considered ignoring it but that isn't my nature I'm afraid! (That'll be my Nuclear submarine engineering background - there are no hard shoulders at the bottom of the ocean).
 
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Did you check your handbook? Mine (also the OM642 engine) confirms that the engine temperature can go up to 120C under normal conditions:

1639677785038.png
 
Does your vans engine have the air operated valve on it for fast warm up? Its part of the water pump on my E class. If it does they can stick partially open and this results in reduced flow. No idea if it applies in your case but you seem well capable of tracking it down if it exists!
 
Did you check your handbook? Mine (also the OM642 engine) confirms that the engine temperature can go up to 120C under normal conditions:

View attachment 121709
Yes, dug out my manual and it has the exact same page!
I must say that I'm amazed by that statement. 120c is very hot for an engine and is in warped cylinder head territory I would imagine.
Not sure at what temperature you would get a warning light on the dash, but would hope that it was well before 120C?
The thing is that mine is running hotter than it has ever been before, even without towing the caravan, and even when the ambient air temperature is down to 3 degrees as it was a couple of weeks ago.
It isn't making any sense to me at the moment.
 
Does your vans engine have the air operated valve on it for fast warm up? Its part of the water pump on my E class. If it does they can stick partially open and this results in reduced flow. No idea if it applies in your case but you seem well capable of tracking it down if it exists!
Thanks for that.
I'll have a look into it.
I've already removed the water pump on mine and there isn't any kind of valve on it, although it's possible that it may be somewhere else on my engine unit.
I have noticed that although the OM642 motor is fitted to several Mercedes models, there are differences on my vans engine such as the thermostat location and viscous fan drive take off for example.
I'm assuming it's down to packaging the unit into the various engine bays - my Vito is a nightmare for access when compared to my last W211 E class!
 
120c is very hot for an engine and is in warped cylinder head territory I would imagine.
Not sure at what temperature you would get a warning light on the dash, but would hope that it was well before 120C?

MB say in the manual that it's acceptable for the temperature to hit the top of the scale (above 120C) in high outdoor temps or mountain driving, so there's clearly no risk of damage at that point. Also that 120C can be reached driving in 'normal conditions', so that's not something you would expect a warning light for.

Is your cluster the same as mine i.e. you need to select the temp. display in order to see it? Is it possible you've just not noticed the temperature fluctuating before? As mentioned mine has done it from new (going up to about 100C under load), and the manual suggest this is quite normal.
 
MB say in the manual that it's acceptable for the temperature to hit the top of the scale (above 120C) in high outdoor temps or mountain driving, so there's clearly no risk of damage at that point. Also that 120C can be reached driving in 'normal conditions', so that's not something you would expect a warning light for.

Is your cluster the same as mine i.e. you need to select the temp. display in order to see it? Is it possible you've just not noticed the temperature fluctuating before? As mentioned mine has done it from new (going up to about 100C under load), and the manual suggest this is quite normal.
Yes, as you said the coolant temperature display isn't the default one, which I simply don't like.
Can't beat a permanent temperature display.
My wife's Seat Ibiza only has a warning light which bothers me because you have no warning of any problem until it's a bit late!
I did wonder whether I had simply not noticed this 100C business before, although I almost certain that I would have scrolled to the display when towing previously and no alarm bells rang in my head as they would have done if I'd seen 100C.
The fact that it hasn't gone over 100C is fairly reassuring, but it's the fact that my previous diesels (that all had a proper gauge), definitely didn't budge beyond 90C, and it's that that's got me going down this potential rabbit hole!
 
my Vito is a nightmare for access when compared to my last W211 E class!

I had the manifold liners delaminate on mine, dropping metal debris into the turbo. Although not that common it's a known issue with the OM642, and has affected other MB models including the ML. But with the Vito you have to take the engine out to replace the manifolds ...

That was part of a bad patch with mine, which started at 90k miles. It cost £6,500 in repairs over a 7 month period, during which it was off the road 9 times. Apart from the manifolds & turbos it also needed (all at different times):

EGR valve
Mass airflow sensor
DPF temperature sensor
Instrument cluster

Prior to that the crankshaft position sensor and DPF pressure sensor had also failed (separately) . The CPS was another known fault specific to the OM642, which was fixed as a safety recall (because it caused the engine to stop dead with no warning) on all MB models ... apart from the W639.

It also gets through glowplugs on a regular basis - the first one went at 36k miles. Access to some of those is a bit tricky in the Vito.

As mentioned mine is a 1-owner vehicle with FMBSH.
 
Can't beat a permanent temperature display.

Yes, agreed ... although oil temp is good to have as well (so you can see when the engine is warmed up, rather than just the coolant). That's almost unheard of nowadays, although the VW Sharan I had before the Vito did have an oil temp gauge.
 
I had the manifold liners delaminate on mine, dropping metal debris into the turbo. Although not that common it's a known issue with the OM642, and has affected other MB models including the ML. But with the Vito you have to take the engine out to replace the manifolds ...

That was part of a bad patch with mine, which started at 90k miles. It cost £6,500 in repairs over a 7 month period, during which it was off the road 9 times. Apart from the manifolds & turbos it also needed (all at different times):

EGR valve
Mass airflow sensor
DPF temperature sensor
Instrument cluster

Prior to that the crankshaft position sensor and DPF pressure sensor had also failed (separately) . The CPS was another known fault specific to the OM642, which was fixed as a safety recall (because it caused the engine to stop dead with no warning) on all MB models ... apart from the W639.

It also gets through glowplugs on a regular basis - the first one went at 36k miles. Access to some of those is a bit tricky in the Vito.

As mentioned mine is a 1-owner vehicle with FMBSH.
Touch wood, my Vito has been reliable and problem free apart from the dreaded rear washer and wiper motor unit.
The original lasted for about 4 years and the Euro Car Parts replacement lasted all of 18 months and needs replacing.
Will check out the price of a genuine one from my local dealer.
With the 10% discount they give me, it's probably not much dearer than the pattern one, but is more cost effective.
Can't believe what a poor design it is!
 
Touch wood, my Vito has been reliable and problem free apart from the dreaded rear washer and wiper motor unit.
The original lasted for about 4 years and the Euro Car Parts replacement lasted all of 18 months and needs replacing.
Will check out the price of a genuine one from my local dealer.
With the 10% discount they give me, it's probably not much dearer than the pattern one, but is more cost effective.
Can't believe what a poor design it is!

As mentioned I've had mine from new in 2007 - currently on 135k miles. The rear wiper motor has been replaced once (by MB), and the aircon condenser. We had problems with both front window regulators early on, but those were replaced under warranty. There was also a recall to replace the spare wheel carrier FOC. So excluding all the engine issues I mentioned before it's been OK. I do believe modern diesels are just too complicated for their own good (with the various emissions & economy restrictions they have to meet) ... fine if you're a company running them under warranty for 3 years and then moving them on, but not a great proposition for a private owner looking to keep one for years (to recoup the high purchase price). We found out early on that a good local garage with modern diagnostic equipment was unable to rectify the EGR valve issue it had - after trying for 2 weeks they gave up and I ended up driving it 20 miles down the M4 (in limp mode) to the nearest MB van dealership. Where we are now the nearest dealership (Stoke-on-Trent) is a 45 minute drive away.

The last 2 cars we've bought have both been petrol ... I wouldn't buy a diesel now by choice, but with vans you have no alternative (yet).
 
As mentioned I've had mine from new in 2007 - currently on 135k miles. The rear wiper motor has been replaced once (by MB), and the aircon condenser. We had problems with both front window regulators early on, but those were replaced under warranty. There was also a recall to replace the spare wheel carrier FOC. So excluding all the engine issues I mentioned before it's been OK. I do believe modern diesels are just too complicated for their own good (with the various emissions & economy restrictions they have to meet) ... fine if you're a company running them under warranty for 3 years and then moving them on, but not a great proposition for a private owner looking to keep one for years (to recoup the high purchase price). We found out early on that a good local garage with modern diagnostic equipment was unable to rectify the EGR valve issue it had - after trying for 2 weeks they gave up and I ended up driving it 20 miles down the M4 (in limp mode) to the nearest MB van dealership. Where we are now the nearest dealership (Stoke-on-Trent) is a 45 minute drive away.

The last 2 cars we've bought have both been petrol ... I wouldn't buy a diesel now by choice, but with vans you have no alternative (yet).
Totally agree with you re modern diesels, they are now incredibly complicated due to emission laws.
As you say, the only reason for buying a diesel in the UK nowadays is if you have to - i.e. towing or a commercial vehicle.
My wife's Ibiza is a petrol, and when we replace it, it will be another petrol car.
With the additional cost of diesel fuel, and the price differential when buying new or used, it doesn't seem worth going down the diesel route anymore unless you do very high annual mileages.
 

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