General Temp light on, but no signs of overheating

[EPa]

Hi everyone,

3 days ago, for the 1st time in a year that I own the car, the temp light came on. I have searched the forum and believe have read all the revelant threads, still I would like your opinion.

The facts:

1. car is a '99 Seicento Sporting SPI engine

2. the temp light comes on after about 10 minutes of starting a cold engine. Does that every time (from a cold engine)
3. if I turn off the engine while the light has come up and then back on, the light comes on immediately, so its behaviour is consistent
4. if I wait for the engine to cool down before I turn it on again, then no light, so still a consistent behaviour

5. no coolant leaks
6. no oil in the coolant, at least from a quick look and feel after opening the cap of the coolant tank and checking (dipped my finger in, cold engine of course!). Didn't do a chemical analysis though

7. the radiator fan is working fine as before: after a while it comes on, blows for 20-30 seconds, then off, then on again after 30-60 seconds, and the cycle continues

8. both tubes (there are 2 right?) coming in and out of the cooland tank get hot while the engine is running
9. the tube connected to the coolant temperature sensor at the right of the engine below the coils is also becoming hot while the engine is running
10. the tube above the radiator also gets hot (this is one of the tubes going to the coolant tank already mentioned at 8)
11. the tube below the radiator also gets hot

12. haven't seen gasses or smoke coming out of the engine (so far ), coolant tank or otherwise. Have driven the car for about 20 minutes with the temp light on before reaching my destination.

So,

what can it be?

5 and 6 rule out leak (and therefore system pressure I hope), coolant tank cap and head gasket problems.

7 suggests fan and fan temperature switch is ok

8-11 suggest (I hope) the water pump and radiator are fine

12 suggests that the engine is not actually overheating

Am I right about all this? If yes, then the only thing left is the temperature sensor that sends the signal for the dashboard light (thats behind the engine block right?). But then again its behaviour is consistent as 2-4 suggest.

If I have gotten it right, there are 3 components related to coolant temperature: one that swithes the fan on and off, one that sends a signal to the dashboard light, and one to the right of the engine block under the coils. That last one is enabling cooland circulation by opening or closing, right? If that's the case then because the coolant tubes get hot everywhere that would suggest it is not the problem, right?

What do you think?

 

[rallycinq]

Bleed the system, juts to make sure, and/or refill with fresh coolant if you don't know the age of the current coolant.

It could be as simple as a faulty switch, but do check first.

Cheers

PD

 

[arc]

i think its a fault sensor. it is simply switching too early, suggesting overheating when the car isn't.

this sensor is on the back of the cylinder head, behind number 1 cylinder, just down from the oil cap. they only cost a few quid from FIAT - it's worth replacing that imo.

There are 3 electrical sensors on the coolant system in total. The one on head for the gauge, there is one on the inlet manifold (right hand side when view from the front of the car, blue sensor) for the ECU and one on the radiator for the fan.

Below the coils is the thermstat, that opens to let water to the rad once it has warmed up. The fact the raditor fan kicks in and out as it's meant to suggests that the thermostat is fine.

 

[EPa]

Boy are you guys fast! That is the quickest replies I have ever gotten!

Yes, I'll do the bleeding first, as I 'm not even sure the coolant system has *ever*(!) been bled.

And then yes, that's exactly what I was thinking about the sensor, that it behaves as if it has gotten more sensitive, as if it fires up at a lower temperature, but I have never heard of such a sensor malfunction. Maybe it touches something very hot when it shouldn't?

Thanks for the answers so far!

 

[rallycinq]

Coolant does 'go off' after a while. Its well worth draining the system, flushing it through and refilling with fresh coolant.

Cheers

PD

 

[EPa]

'go off' how do you mean? Coolant is only water and anti-freezer right? Being in the middle of the summer one would think even just water would do the job. So how could water 'go off' ? Do mean calcium deposits maybe?

 

[rallycinq]

'go off' how do you mean? Coolant is only water and anti-freezer right? Being in the middle of the summer one would think even just water would do the job. So how could water 'go off' ? Do mean calcium deposits maybe?

Ah, the alcohol in the water evaporates leaving just water. It also picks up the corrosion from inside the block, hence rusty water. Water alone does not inhibit corrosion which in the alloy head can lead to furry arteries.

The muck from the engine can not get through the small passages in the rad leading to a silted up rad.

On so many levels---- Get it changed!

Cheers

PD

 

[EPa]

Get it changed!

That's for sure!

 

[Ralf S.]

The hot tubes don't conclusively let the water pump off the hook. It will still be circulating coolant, though not necessarily at a sufficient volume for it to be cooled.

The fan doesn't seem to stay on long enough.. I'm speculating that the fan gives the radiator a boost, in that rather than circulating the water quickly enough, the car is relying on "super cooling" the coolant that is being (too slowly) circulated through the rad' instead.


Ralf S.

 

[EPa]

It's the sensor that's faulty. The dashboard light now lights as soon as I turn the ignition key, even after the car was parked all night. It is always on, so there goes the 'consistent behaviour'.

 

[fingers99]

Don't rule out a simple wiring fault. Should be easy enough to test the sensor in a pan of water bought to the boil.

 

[EPa]

Thanks for the tip. I tried moving the sensor and its cables, disconnecting and connecting it, but didn't think of that. Will try it before I get a new one.