Temperature gauge oddities!

[poorly_felicia]

Ok, felicity has a funky temp guage (or associated sensor)

(1998 1.3 mpi 136 engine felly)

---IDLING---
From cold: starts off at needle stop
idling: will rise to about 80 slowly, then drop to about 70 when themostat opens
Further idling: will rise to about 85/90 then stay there

---Long DRIVING---
from cold: starts off at needle stop
after 2 miles: rises to about 75/80 then drops when thermostat opens
more driving: stays around 70 constantly, with slight up/down fluctuations for the rest of the journey. Even under extreme long slow up hill runs

---TOWN Driving---
from cold: starts at needle stop
after 2 miles: rises to about 80 then drops when themostat opens
more driving all over the place. gets to about 85 in traffic jams and very quickly drops off when speed picked up. rise fall rise fall constantly.

---What she used to do---
for the first 3 years of us owning her she would start off from cold, get up to 90 and then it would stay there, rock solid under any driving conditions.

---Things i have checked---
Yes, the coolant does get HOT
Yes, the thermostat starts off shut then it does open when temp get up to about 80
No, i do not know if it does/should shut again if engine temp drops.

Recently had new coolant, but i know there is not any air locks because it circulates well without bubbles or "splurting" when the thermostat opens up. And it took exactly the amount of coolant the haynes said it would when i refilled it to get it up to max.

I know there are 2 temp sensors, one for the dashboard dial and one for the ECU, but im not sure what resistances these should be reading across their pins when cold/hot as i have no point of reference from a known working system Can someone help me out here?

Will this behaviour fail an MOT?

Is the old behaviour normal?
or is recent behaviour normal?

[anewman]

Will NOT fail an MOT, would pass if it wasn't even working at all and some cars do not have temperature gauges on the dash. But especially on these types of engines, it is best the gauge does work, as if your fan fails and you get stuck in traffic in the summer your engine will cook.

As for is it normal, after doing all the replacing the thermostat and renewing coolant my car does similar. Couldn't find anyway to stop it so have decided to live with it. I wonder if it did the same before and I didn't notice it. It doesn't appear to affect the running anyway.

I recently discovered that swapping the temp gauge itself makes it read higher, but it seems to do the same up/down thing.

AFAIK on the MPI the temp sensor is shared between dash and ECU and has more than one wire connector.

[cjb]

I reckon the thermostat is a bit sticky.

[TeflonTom]

Originally Posted by anewman

AFAIK on the MPI the temp sensor is shared between dash and ECU and has more than one wire connector.

there is only one sensor which has 4 wires

pin 1 & 3 are for the ecu
pin 2 & 4 are for the guage

pins 1 & 2 are both earthed in the loom, so the ecu and the guage both see a negative voltage (if you get my drift)

at 0 degrees you should get about 6k ohms which is approx 4 volts
at 80 degree you should be getting about 250ohms which is about 1 volt

typically anything about 4 volts or below 1 volt i would call faulty but i'm sure somebody else will come along and argue this fact

HTH

[poorly_felicia]

ok, so if the dash dial and ecu both get their temperature from the sensor above the thermostat, whats the one in the radiator for?

[anewman]

The radiator one only controls the fan.

[Mike A]

Mine is playing silly bugger right now too; I'm awaiting delivery of a thermostat from ebay to see if that'll fix it. Mine eventually rockets up to the top of the scale after about 20mins of rush hour traffic.

[anewman]

Originally Posted by Mike A

Mine is playing silly bugger right now too; I'm awaiting delivery of a thermostat from ebay to see if that'll fix it. Mine eventually rockets up to the top of the scale after about 20mins of rush hour traffic.

If it goes to the top of the temperature gauge, chances are the fan is not working. It is usually the switch, but to check, with the ignition key set to have electrics on, join the two wires. If the fan works chances are the switch is a dud.

[Mike A]

Originally Posted by anewman

If it goes to the top of the temperature gauge, chances are the fan is not working. It is usually the switch, but to check, with the ignition key set to have electrics on, join the two wires. If the fan works chances are the switch is a dud.

I really need to win one of these Ebay Felicia Haynes auctions so I can look at this stuff!

Who'd have thought there were so many people looking to buy Felly manuals?

[bobob]

Originally Posted by Mike A

Who'd have thought there were so many people looking to buy Felly manuals?

That's because they are still simple cars to work on

[anewman]

They have them in most halfords stores I have been to but I think they're £18.99 there unfortunately. Still, probably less money than an hours labour costs for many mechanics.

[richardsfavorit]

Try amazon? They are good for haynes manuals.

[poorly_felicia]

righto people, since it wont fail the MOT and she is driving like a little champ, im going to leave it well alone. cant afford any more money on her since i bought her some new shiny shoes for her birthday