hondaman
New Member
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?
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?