engine overheats + STOP light

[jayk]

yesterday I took my car (07yr megane RS 225 PH2) for usual sunday shopping. when I was leaving the mall, the car seemed fine except air con is not cold and blowing warm wind.
I thought maybe engine is too hot so I waited. but after 10-15mins drive, the air is still warm so I click AUTO. suddenly STOP warning light went on then "engine overheating" also came up. I pull over and stop the car. I couldn't restart immediately. called the garage and said it's not worth to drive so tow it to the shop.

this morning he checks the coolant, fan, and sensor. everything seems fine. he reckon maybe the relay or resistor or cooling fan switch doesn't work properly. these parts dun cost much but i can't find the part number on web.

any one can advise me the part number pls ?

[tomalamix]

what resistor are you talking about? i think in the end you want to know the price of the radiator fan...? is that it?

cheers

[davelowe]

More detailed information is required before anyone can make an accurate guess. Did the fans spin when you engaged the A/C or when the gauge read hot?

[jayk]

the garage checked the fuse and fan. both are fine. he says i can drive but it's better to replace the resistor and relay.

i dun think both parts cost a lot but more important to know the exact part number. i gather relay has diff. size and colour.

[jayk]

the gauge read hot. he didn't mention the fan turns on or not when engage to A/C.

and the resistor refers to the cooling fan one.
thanks

[tomalamix]

cooling resistor...im sorry but m not following, im not getting what is the cooling resistor...

[AlexB]

Did it actually overheat, when the light came up? I guess you did not check. Therefore, one needs to replicate the fault by driving with the aircon. Meanwhile read the OBD codes and record the live engine parameters while driving from cold. This will help to see what's wrong. Please also review the car history including everything.

[jayk]

Hello Alex

i remember when i pull over the car and open the bonnet, it felt like oven but didn't see any steam or leaking. the engine didn't start but the dashboard seems working.

after 15-30mins, i started the engine, it's on. the cooling fan was running for couple min then stop. then i turned off the car.

on Mon, when the garage first started the car and drove for 15-20mins, it overheated as well. he checked few parts and said the cooling fan worked and no leaking. he changed the thermostat only. he drove for awhile. all seems fine but later in aftn he tried again, it happened, he reckon something to do with the cooling fan resistor or relay that didn't switch on when hot. he couldn't find the part number yet.

it's really annoying when sometimes the car work and sometimes not, cannot really tell which part fails. today i will put the car to garage and check again. let see how.

[AlexB]

As I wrote above, you can simplify everything by observing the temperature on live OBD while warming up the engine from cold. If the cold reading is different from the ambient temperature or the reading "jumps" up in no time, then you know that the sensor is at fault. It can also be any associated circuitry. Before doing anything one has to check the power and ground circuits, as usual.

These things can be done by using a cheap OBD reader, which is well under £20 from ebay. Search for ELM327. Alternatively, any garage can do it for you.

Basically, it is highly unlikely that the engine overheats at low load while the radiator is hot and fan running. Switch on the heater on full - does the temperature drop? One can actually drive at a partial load while using the heater as a radiator... If the pump has failed, then both the heater and radiator will be insufficiently hot, plus the pump will be leaking or making noise... If the engine has failed internally, then there will be evidence of burning massive amounts of energy at low load or idle, i.e. smoke/noise/revs/black oil, etc. Just look and analyse...

If your garage is grossly incompetent and you have no time on DIY, then just order to check the oil quality, the pump for noise/leaking, then bleed the cooling system, then replace the temperature sensor, then thermostat - in this order and without any diagnosis. The first three points will cost you almost nothing, the latter two are still cheaper than any lengthy investigation.

[jayk]

last week I ask the garage to look again. he checked the relay and wires then put it all back in. he drove it few times during the day. the car was fine and normal.
I really have no idea if the car is really fixed. I'm guessing the original problem maybe loose plugs/wires.

after all, only the thermostat is replaced.
french car is really unpredictable.

thx Alex for all valuable advise.