Stalls after succesful start

So you were just right.

I kept the engine alive for a while in my garage until coolant warmed enough and then went for a test drive, where the car behaved normally.
Time to order a new sensor.

Thanks so much everybody

Pedro

Well done, Pedro; that’s excellent news.   Thanks for taking the trouble to report back.   I, for one, was very keen to know how your test was going.

if you have a thermometer and a digital voltmeter to measure resistance, and you are interested in looking at why your car behaved as it did, you could connect 2 wires to the new sensor and then boil some water and drop it in and then measure the resistance as it cools down, say, every 10 degrees.   Then swap over the new sensor for the old one and do the same with the old sensor, or, even quicker, put the old one in the freezer for 10 minutes.   My guess is that you will see the resistance of the old sensor fixed at a constant value, one that  corresponds to very hot on the new one.

Thanks again for the feedback.

Thanks so much again, Martin.

I´ll do that when I get the new sensor (although it will take some time to do it)



 

One quick comment on this. Testing a new item against an original can be misleading for two reasons. Firstly your original will be an Elmwood device & there is a possibility that the replacement may also be made by Elmwood. If that is the case & you see a difference then you have a result. However it could be a pattern part & although the perimeters should be the same they may be a country mile away. You may ask me how I know this but the language of my reply may not be to your taste.
The second reason is an earth return problem. If there is corrosion on the earth, a high resistance will affect the effective value of the device, as far as the ECU is concerned.
An example to illustrate my comments; these are arbitrary values.
The ECU might be anticipating a cold value of the sensor at 10k & a hot value of 1k for optimal performance. If there is a poor, high resistance earth of 10k, then the ECU will be seeing a cold value of 20k which may not be much of a problem but when hot it will be seeing 11k which the ECU interprets as cold & adjusts the mixture accordingly.
As I have said these are arbitrary values to illustrate a point.
With electronics taking over decision making on cars, the test voltage may not be 12v but a lower value & the current which flows through the circuitry is also low. If you consider ohms law, a dirty contact can adversely alter the voltage/current. & throw the entire system haywire.
Please don’t think that I’m trying to diminish your thoughts in anyway, I only wish there had been such “mini lessons” as these, when I was building my knowledge base.
M

Thanks, yes, you’re dead right about the effects of corrosion on circuit resistance (and the weird symptoms that can result); having once lived near the sea, I recall only too well how corrosion can affect circuits as every electrical connection soon turned green.  

I stress I know nothing about you lovely cars, but if your MG cars are like my ugly Rover 100 (MEMS 1.6 version), the coolant sensor would have a negative temperature coefficient (NTC), so when cold it has a high resistance (eg 6k), but at 100C the resistance is only around 100 Ohms, and because Pedro’s ECU thinks his engine is hot even when it’s stone cold, his circuit would be indicating a low resistance.   But I stress I know nothing about your cars or whether or not the coolant sensor has an NTC characteristic.

Going back to your observations about high resistance reminded me of how the sea-air corrosion created a high resistance in the high-current starter circuit.    The braided earth bonding cable between the block and the body glowed and fizzled when the key was turned to the start position but nothing else happened.   The corrosion caused a high resistance, so the cable behaved like an electric heater, before crumbling into brown copper oxide powder.   But that was a very small price to pay for the privilege of living by the Moray Firth.

Thermistor temperature coefficient, positive & negative, are on year two syllabus.
Those who have covered year one should read Hughes Electrical Technology, until the rest of year one catch up.
M