Temperature + Fast Idling

[smokinmyman]

My 98 pre VVti Avensis has developed a interesting habit. The idle speed varies from about 800 RPM to 1200 RPM. I have noticed that The engine speed on idle does seem to vary with the temp guage.

E.G. While driving along a straight road, no hills, easy motoring the temp guage reads about a 1\4 way up and If I dip the clutch and let the engine speed drop to idle it goes down to around 1100 1200 RPM.

If I leave car to idle at a stand still or drive up a long steep hill and the temp guage now rises to about just over half way up, and I let the engine idle it;s speed is 800 ish.

I have concluded that the coolant temp sensor's resistance is out of recognised spec and it telling my ecu that the engine is running cooler than it actually is. I think i have found it, is it the sensor that is fitted to the engine block just by where the rad top hose is connected to engine in what looks it ight be a thermostst housing?

Am i barking up the wrong tree do you think?

[Starensis]

Your sensor for the ECU & temperature gauges are seperate. The Green sensor controls your EFI ECU! If this was malfunctioning your engine light would be illuminated. Run a diagnostic just to check.

I would diagnose this as engine temperature problem, possibly thermostat.

[jamesholden]

Is it doing this when you have your A/C on as that causes the revs to fluctuate as the compressor switches on and off.

[smokinmyman]

Hi thanks for the responses so far.

When you say that the temp sensors for the ecu and gauge are seperate can you tell me where I can find them on engine? The sensor I found, if you are looking down on top of the engine is located at the back right of the block. I thought that controlled the guage, fans and sent the signal to ecu??

Just out of interest, I have been looking through other references on the net to Toyota temp sensing problems and "wandering idle". On the MR2 apparently which has a similar arrangement as far as i can make out they say that it is not uncommon for the coolant sensor's resistance value to change with age. So being of a curious nature, I did some experimenting. I have connected an old volume switch assembly sourced from an old TV (basically a variablre resistance) in parallel with the existing sensor so that the combined resistance can be adjusted by the variable resistor and yet the existing sensor which does register the temperature change accurately can stilll vary with the temp reading and it works. The fast idle drops to 800 and the auto choke is coming off.

I'll still probably buy a new sensor (£33.42 + vat from toyota garage) but it is still good to confirm the diagnosis before shelling out.

[smokinmyman]

Hi and thanks for your info "jamesholden" and "starensis".

I had discounted the thermosts in my mind as i saw it was opening after warming up but on closer inspection i think it is not closing down when the coolant temp. is dropping. Giving rise to cooler temp of coolant and therfore faster idle and drinking petrol. Thanks for advice i am getting stat today.

[smokinmyman]

Update

Fitted a new Stat which i got from Nippon Automotive (tel 01242 584967) for £12.33 inc vat and postage, made in japan and it has solved the problem. Engine temp is maintained and revs stay down. Thanks to all involved.