W124 Temperature Woes - Running Cold

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Wishbone

New Member
Joined
Mar 4, 2012
Messages
10
Car
1990 260E
Good evening folks.

Picked up a 1990 260e last week to use as a weekend runaround and to tinker with and polish! Loving the car so far (it's a very tidy well specced 1 lady owner car with 100k and FSH) but having some trouble with the engine temp.

It never runs above 60 deg. Had a search through some posts on here and convinced myself it must be the thermostat so ordered a new one from MB and fitted it today. Ran the car for about 20 mins and it's still not going above 60 deg.

The large rubber hose from the top of the rad to the thermostat housing feels empty if I squeeze it, even after running the car for 20 mins with the expansion tank cap off so it's as if the thermostat hasn't opened to feed water into the rad.

So, do you think it's a faulty thermostat I've been given which isn't opening ? If that's the case I thought the temp gauge would quickly shoot up above 60 and that's what's confusing me.

Could it be the coolant temperature sender ? Does this have any functionality other than to feed a signal to the gauge ?

The fan runs continuously on the car - should it be doing this ? Not sure if that's normal or if this is part of the problem.

Any advice much appreciated. Looks like a great little forum here which I look forward to being part of :thumb:
 
Do you mean the engine fan or the air con fan?

The ac fan should only run all the time if the air con is switched on, or there is a fault with the temperature sender.

Make sure you have the air con off, then see if the fans are still running.
 
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The car doesn't have aircon so I'm guessing that means the temperature sender is goosed. The fan runs all the time as soon as the engine is fired up.

Is this the temperature sender on the right hand side of the engine block towards the rear, near the firewall ?
 
If you dont have aircon, then you must only have the large yellow engine driven fan. This has a viscous coupling and will turn with the engine running, when you increse the engine rev's you will hear the fan get louder too. When it gets up to a set rpm you will hear the fan cut out.

The above is normal, but with the engine off, see if you can turn the fan by hand.

The engine in your car has 2 temp sensors, the one at the very rear of the head by the bulkhead controls the engine, but the one infront of it controls the gauge.

Your fault could be the temp gauge sensor.
 
The heating inside the car seems to work fine.

If the thermostat is working I don't understand why it doesn't seem to be opening to fill the big hose going in to the top of the rad.

carat 3.6 amg - yes, it does have the large yellow fan. What will spinning this by hand tell me ? Presumably it's the temp sensor at the very back of the engine block I'll need to check, the one in front of it just feeds the gauge is that correct ?

Appreciate the feedback folks :)
 
The heating inside the car seems to work fine.

If the thermostat is working I don't understand why it doesn't seem to be opening to fill the big hose going in to the top of the rad.

carat 3.6 amg - yes, it does have the large yellow fan. What will spinning this by hand tell me ? Presumably it's the temp sensor at the very back of the engine block I'll need to check, the one in front of it just feeds the gauge is that correct ?

Appreciate the feedback folks :)

If the fan spins by hand with the engine off, it is ok. If you cant turn it by hand the viscous coupling has failed, and the fan will not cut out.

The large sensor at the back of the head (green I think) controls the engine computer, while the small sensor infront of it controls the gauge.

I suspect that the car is running at the correct temperature, but the small sensor is giving an incorrect reading to the gauge.

I have a 1990 300e which has the same engine as you (but 3.0) and this normaly only gets to 80 on the gauge, which is perfectly normal. Infact when I speed along the motorway it can sometimes drop to 70.
 
Cheers I'll check the fan tomorrow.

I still don't get why the thermostat didn't seem to be opening though....think I might whip this out and get it into a pot of boiling water just to make sure.
 
Take it for a long drive with the heater off, then see if the top hose get's hot. But it dose sound normal to me.
 
is the water level correct? might be some air in the system?? to check start the engine , both temperature dials (for inside) up to max, rev it up for a bit and check. top up if necessary.
 
Took it for a run today and the top hose is now full of coolant so the thermostat has opened.

I've removed the temperature sender and will get a replacement ordered up.
 
is the water level correct? might be some air in the system?? to check start the engine , both temperature dials (for inside) up to max, rev it up for a bit and check. top up if necessary.

Agreed. The system is low on water due to an air lock.
 
Run the engine at idle with the tank cap off and keep squeezing the bottom hose until the water in the header tank goes down then add more water as appropriate.
 
Ok a quick update.

Whipped out the coolant temperature sender and measured the resistance across it. Should be around 2.5k ohms at ambient but it was measuring around 7.5k.

Ordered a new one from Mercedes, popped it in and all is well again :)

Thanks for all the suggestions.
 

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