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Discussion Starter #1 (Edited)
Greetings All,

Fault seemed to develop overnight. Temperature gauge rises to the red after 5-10 mins of driving, STOP message appears on instrument cluster along with "Coolant Too Hot" message on fascia centre display.

Bottom radiator hose feels cold, top hose hot and under pressure. Radiator fan blows away happily as expected. Heater runs hot and turning up the interior blower reduces the temp on the gauge. I double checked for air in the system by fitting & filling a header tank on the expansion vessel and opening the heater and thermostat bleeds - both flowing well.

To me, everything points to the thermostat not opening when the block gets up to temperature. However, I believe that these engines have an electronic thermostat and not an old-school wax type.

Thermostat housing has TWO 2-pin connectors on it. Am I correct in thinking that the one on the top of the housing is the temperature probe and the one at the rear and pointing towards the bulkhead is the thermostat actuator?

Before I condemn the thermostat, is there anything else to check? Fuses? Would I see a voltage on the thermostat actuator when the coolant ECU wants to open it or does it supply a voltage to close it?

And just to be REALLY annoying, I have a receipt for work done under the previous owner for thermostat replacement only 2000 miles ago!!! :(

Advice from the gurus is much appreciated.
 

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Discussion Starter #2 (Edited)
No reply so far - might be because I'm talking so much rubbish or should be reading previous threads! :lol:

I am puzzled by the second connector on the thermostat housing - the one at the rear of the housing, pointing towards the bulkhead.

I'm thinking it's not any kind of thermostat actuator but it's a second temperature sensor (not removable) that's there so that the ECU can monitor the temperature of the water coming out of the radiator lower hose.

If this is the case then the thermostat is a typical wax thing sealed inside the housing and nothing electronic at all.

So can anyone please tell me if I'm talking more sense or less sense? - UPDATE - no, still talking rhubarb!
 

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I am sure the thermostat has some kind of electrical control, however i have code read loads of cars with electronic thermostat (or words to that effect) trouble codes and they do not overheat, I would think that the wax stat part would still operate as a fail safe but maybe wrong
 

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Discussion Starter #4 (Edited)
I am sure the thermostat has some kind of electrical control, however i have code read loads of cars with electronic thermostat (or words to that effect) trouble codes and they do not overheat, I would think that the wax stat part would still operate as a fail safe but maybe wrong
Hey Windy. I got it! I know what that second connector is on the thermostat housing. It's a HEATER!!!!

Small heating element that influences the opening characteristic of the wax thermostat. Sometimes termed a "MAP Controlled Thermostat". Explained in this fascinating video... Bloody brilliant...

Map Controlled Thermostat by MAHLE

Essentially, the ECU can cause the thermostat to open slightly more than it would do otherwise. By sensing an increase in engine load in real time the ECU can anticipate the resultant rise in coolant temperature before the wax thermostat alone would do by immediately opening the thermostat a bit wider. This regulates the temperature of the block more accurately. Very clever things our engines are:)

And, yes, the wax thermostat bit provides the failsafe as the thing will always track as well as any wax thermostat does. The additional heating element simply means the ECU can deliberately lower the temperature of the block a bit more if it wants to.
 

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Sounds like your thermostat has suffered "infant mortality". A small number of components "die young", a small number achieve remarkably long life, but most occupy the middle of the curve.

 

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Greetings All,

Fault seemed to develop overnight. Temperature gauge rises to the red after 5-10 mins of driving, STOP message appears on instrument cluster along with "Coolant Too Hot" message on fascia centre display.

Bottom radiator hose feels cold, top hose hot and under pressure. Radiator fan blows away happily as expected. Heater runs hot and turning up the interior blower reduces the temp on the gauge. I double checked for air in the system by fitting & filling a header tank on the expansion vessel and opening the heater and thermostat bleeds - both flowing well.

To me, everything points to the thermostat not opening when the block gets up to temperature. However, I believe that these engines have an electronic thermostat and not an old-school wax type.

Thermostat housing has TWO 2-pin connectors on it. Am I correct in thinking that the one on the top of the housing is the temperature probe and the one at the rear and pointing towards the bulkhead is the thermostat actuator?

Before I condemn the thermostat, is there anything else to check? Fuses? Would I see a voltage on the thermostat actuator when the coolant ECU wants to open it or does it supply a voltage to close it?

And just to be REALLY annoying, I have a receipt for work done under the previous owner for thermostat replacement only 2000 miles ago!!! :(

Advice from the gurus is much appreciated.
I know the thread Is old but I'm having same exact problem....did you ever found out what it was?
Greetings All,

Fault seemed to develop overnight. Temperature gauge rises to the red after 5-10 mins of driving, STOP message appears on instrument cluster along with "Coolant Too Hot" message on fascia centre display.

Bottom radiator hose feels cold, top hose hot and under pressure. Radiator fan blows away happily as expected. Heater runs hot and turning up the interior blower reduces the temp on the gauge. I double checked for air in the system by fitting & filling a header tank on the expansion vessel and opening the heater and thermostat bleeds - both flowing well.

To me, everything points to the thermostat not opening when the block gets up to temperature. However, I believe that these engines have an electronic thermostat and not an old-school wax type.

Thermostat housing has TWO 2-pin connectors on it. Am I correct in thinking that the one on the top of the housing is the temperature probe and the one at the rear and pointing towards the bulkhead is the thermostat actuator?

Before I condemn the thermostat, is there anything else to check? Fuses? Would I see a voltage on the thermostat actuator when the coolant ECU wants to open it or does it supply a voltage to close it?

And just to be REALLY annoying, I have a receipt for work done under the previous owner for thermostat replacement only 2000 miles ago!!! :(

Advice from the gurus is much appreciated.
I know it's an old thread but I'm having same exact problem did you ever find out what it was??
 
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