Hi All

I am stumped in trying to figure this one out. Its my brothers 91 124 with 102 Euro engine. Recently the car is running high temps when in traffic. Conditions include:
1) High cooling system pressure, hoses feel rock hard. System also has residual pressure when engine is cold in the mornings. Is this an indication of a head gasket leak or crack? Water is very clean with no oil.
2) Electromagnetic fan turns on, high speed electric fan turns on. There is a low speed but that comes on with the A/C. Low speed does not come on without A/C. Is there a low speed fan that is suppose to kick in from an engine temps sensor?
3) Overheat problem is unpredictable - one happens once in a while.

Any advice is greatly appreciated. Thank you.

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1992 124 E200 (Euro)

have you checked the theromostat you could also do a pressure test on the system to see if it is pressurising. also have the radiator checked to see if it blocked

cooling system pressure is controlled by the rad cap .. check that ... eratic overheating could possibly be a sticking thermostat.... If the coolant is old , you might want to change that too. If you had oil in the coolant and/or oil seeping out of the back of the head/block that would be a head gasket.

Stefan.

Hi

Thanks for the replies...A bit of history
1) All 3 sensors on the engine are new.
2) Thermostat, expansion tank and pressure cap are new
3) Tested everything for the fan circuit - relay, resistor, fuse

I read on the archives that if you have residual pressure in the coolant, it may mean that compression is somehow leaking into the cooling system. I'm thinking that it only happens at extended idle conditions so it may be in the high speed fan circuit. Will try another fan switch to see what happens.

Anymore comments? Anyone?

Thanks

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1992 124 E200 (Euro)

My memory isn't what it used to be but I seem to remember a diagnostic procedure where a liquid was added to the coolant. If compression (exhaust) gasses were present in the coolant it could be detected. I don't know if the coolant changed color or if there was another result. Does anyone remember this procedure?

I've got a tool that draws vapor from the cooling system and passes it through a blue liquid. If the liquid turns yellow, it indicates the presence of combustion gasses.

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1989 300 SEL that mostly works, but needs TLC

Yes - that's what I need. Something to test for CO in the coolant tank! Thanks

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1992 124 E200 (Euro)

Posting an update to the intermittent overheat problem...

Basically after thinking about it for a while, decided that it had to be electrical. The only weak link seems to be the high speed fan relay. Did a swap with a known good one and the car immediately started to run cooler. Frankly, I'm not sure how the high temp aux circuit works. The coolant sensor is a NCT type and the black relay is a 40A. BTW The low speed silver relay is a 30A.

On opening it up, the 40A relay has a resistor built in for the solenoid action. I resoldered all joints and sanded the 2 (?) contact points. The contacts are also a puzzle, 2 contacts, one larger than the other. The smaller one contacts earlier then the bigger one. I studied the wiring diagram but can't make out if the signal to trigger the relay is a on/off type or by a variable voltage (from the NCT sensor) . If it is the latter, then the relay coming on and off times can vary according to the above mentioned resistor. Also I tested the relays with a transformer and the 40A seems to come on at a lower voltage.

Just thinking out loud if anyone want to add on....

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1992 124 E200 (Euro)

I'm not too familiar with the euro cars but the aux fan pre-resistor should be checked. With age the resistive element breaks down leading to an open circuit to the aux fan.

Thanks duxthe1 - I have checked it and it was OK. The low speed runs with the A/C.

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1992 124 E200 (Euro)