13 replies to this topic

[Minitrio]

Hi Injector Heads,

My MPI keeps overheating mainly due to fan failing to operate at the correct time. I have bought 3 Fan sensor switches in the last 12 months, but after a few run outs they fail or work intermittently (leading to partial or full over heats).

The problem is, I only notice these overheats when steam rises from under the bonnet, because the temp gauge still sits on the normal line and never moves (except just after start up).

The fan works in safe mode, but i don't want it running all the time. So i intend to fit a switch direct to the fan, so i can switch it on and off via a dash mounted switch. Obviously for this to work effectively i need to know the correct water temp.

So just need to work out if its the gauge or the sender/sensor that's faulty, but where is the sensor located?

Cheers

[KLAS]

there isn't a seperate sensor.
the brown one is the only one and it's used for all temperatur related things.

maybe your cooling system won't hold the needed pressure and start to boil

[Minitrio]

But if i run in safe mode, meaning the brown fan switch is completely disconnected, the temp needle still rises to the normal position. Therefore that means something must be sending a signal to register heat in the engine.

I'll have a look at some electrical diagrams and see what i can fathom out.

Thanks Klas

[KLAS]

the gauge is fully controlled by the ecu.
i don't know what the ecu is programmed to do with the gauge if the sensor fails, maybe just this, hold the middle and turn on the fan

[Sprocket]

The MPi ECU assumes 70c when the sensor has failed outside of either high or low limits. It turns the fan on constantly and runs the engine ever so slightly rich for safety.

Over heating can be down to several things. Do not assume that because the gauge is reading right that it actualy is.

As has been said, if the cooling system cannot hold the pressure, it will boil over. The black cap on the expantion tank is prime suspect. If the coolant is rusty colour, the cooling system is in desparate need of flushing and then filling correct concentration anti freeze.

For the price of a thermostat and a couple of gaskets, i would change the thermostat. Make sure its 88c.

Also worth looking at the condition of the radiator fins. It there is a lot of corrosion, it wont be anywhere near as efficient as it used to be.

[Minitrio]

The MPi ECU assumes 70c when the sensor has failed outside of either high or low limits. It turns the fan on constantly and runs the engine ever so slightly rich for safety.

Over heating can be down to several things. Do not assume that because the gauge is reading right that it actualy is.

As has been said, if the cooling system cannot hold the pressure, it will boil over. The black cap on the expantion tank is prime suspect. If the coolant is rusty colour, the cooling system is in desparate need of flushing and then filling correct concentration anti freeze.

For the price of a thermostat and a couple of gaskets, i would change the thermostat. Make sure its 88c.

Also worth looking at the condition of the radiator fins. It there is a lot of corrosion, it wont be anywhere near as efficient as it used to be.

The whole cooling system has been replaced, apart from the radiator and temp gauge. Pipes, expansion tank,new expansion cap, water pump, thermostat, fan switch, all gaskets (including head gasket) and fan (8 months ago, which spins the correct way). The radiator ran clear when initially flushed, the whole system was refilled with 50/50 Anti freeze mix. Squeezed all air bubbles out.

But it still overheats, because the fan doesn't kick in despite fitting the 3rd Fan switch in 12 months.

Hence i want to bypass the switch with a manual dash mounted one. Just needed to sort temp readings, but if the ECU switches to default of 70 then can i fit an independent temp sensor somewhere or fit the fan control switch that sits in the pipe and can be set independently of the ecu

Any ideas would be appreciatied

[KLAS]

even new caps can be faulty.
do you have any chance to check the real temperatur? the fan is turned on about the same temp the coolant boils without pressure

[Minitrio]

Yeah along with faulty switches! lol.

What would be the best way to check the correct temp?

[KLAS]

but the brown thing is not a switch its a NTC so it can easily be checked with a ohm-meter. the higher the temp the lower the value. and it seems that its working as the fan is turned on as soon as you remove the connector.

i use a mechanical temp gauge with a small probe i can just slip inside a hose connection like the one at the thermostat housing. anything like this will work

[Minitrio]

Ah! well I've certainly learn't something there Klas. I treated it as a switch because it switched the fan on. Glad you pointed out the error's in my thought. Unfortunately i don't have an ohm meter or even know how to use one. i now intended to obtain one and learn.

So can you explain the pressure issue and how it affects the NTC. Will be getting a new expansion cap.

Cheers

[KLAS]

without pressure the coolant boils around 100°C, with pressure it boils around 110-120°C (depending on the exact antifreeze mix and pressure).
the ecu is programed to turn on the fan at around 100°C (don't know exactly, something from 98 - 104°C).
you see that the coolant can boil (without pressure) a bit before the ecu would turn on the fan.
than you see your car overboiling but the ecu is still thinking everything is normal

[Sprocket]

without pressure the coolant boils around 100°C, with pressure it boils around 110-120°C (depending on the exact antifreeze mix and pressure).
the ecu is programed to turn on the fan at around 100°C (don't know exactly, something from 98 - 104°C).

fan turns on at 102°C

[Minitrio]

Excellent. Thanks for that guys. I now have a better understanding of the cooling system now.

I shall attempt to rectify the pressure problem, firstly by buying a new expansion tank cap

So technically the gauge will work correctly, once i've resumed normal pressure within the system?

Cheers

[Minitrio]

A bit of an update. Fitted a new expansion tank cap and sorted pressure issue, now fan kicks in with no overheating issue's. The only problem i have now is a strange one! The temp gauge goes up to the normal black line, but then goes down as it heats up, then the fan kicks in and the temp needle goes up until the fan switches off. I repeated it this several times with the same effect. What's going on and how do i rectify it?

Edited by Minitrio, 12 June 2009 - 04:14 PM.