Coolant temperature sensors - failure/degradation modes?

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Hi,
Question for the experienced people here - as engine temperature sensors age do they get a little doddery or do they generally fail fairly obviously (very high or very low resistance?)?

Trying to understand a cold start problem here. Maybe the sensor is gone a bit weird in a subtle way (tbh the values over OBD2 look fine).

OR... if an engine was a little worn out would it need a little bit more autochoke than it would have needed first day. IE the sensor is fine, and the ECU is giving what it thinks is the right enrichment for the cold engine but it just isn't enough. What would needing more choke than usual be a symptom of?


Yes, it's a vague and meandering "question". Let's call it a thread where people can relate their tales of strange coolant temperature sensor / fuelling happenings.

Waterson

If you hook up a scantool to your car from cold and watch the coolant temp sensor output as the car reaches operating temperature, (say 90 degrees) you should see a fairly steady proportionality between temperature and voltage output. Best viewed on a graphing scanner. In my experience they can fail both gradually and abruptly.

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Appreciate your input as always Waterson. Youve replied to a few of my threads over the years.

I'll have a bash! There's a first time for everything. Cts is in a b@lls of a spot but I'm tapped in since last week. Tried adding some resistance in parallel and it made things way worse. I'll log things in standard spec before I do any more.

Was happy enough to tell the car it was warmer than it was.... fooling it that it is colder worries me when the fans are ecu controlled. These things can be worked around I guess but when you're at the nasty hacks stage of just trying something and it's your first time at this craic on car electronics and it's not your car... there's always a worry :-)

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OK, early this morning, OBDII reported coolant temp as 3degC. Seeing as the frost had just cleared off the cars that seems about right.
Made very quick progress from 3degC to 50 or 60 degC.
Noticed a change in engine sound around the 15-20 mark.
Clacking evap solenoid running intermittently from 60degC on.
Somewhere around 90ish the stat opened enough to reduce temp for a while and generally slow temp increase.
Fan on at 105.


No problem with idle as long as car is left run without accelerator input.
Idle stays high and gradually winds itself down to 800 as engine warms up.

BUT...
If you try to drive it before it's showing any movement on the temp gauge, it feels like it runs on a completely different map, it snaps out of the "gradual reduction of idle rpm" and seems to go straight for "yeah I'm all warmed up idle" - so it ends up being very very rough for the first few junctions you stop at. I'm guessing it tries to go for 800rpm but runs so rough it ends up dropping to about 500 and very almost stalling completely.

Is it usually the coolant temp sensor or the intake air temp sensor dominating at that stage?

Irish Steve

The coolant temperature sensor is most likely to be the main control, as the intake air temperature may well stay well low if ambient air is cold, there's very few modern vehicles that have any sort of divert on the air flow to take air from around the manifold, as used to be the case on some older vehicles.

You mentioned auto choke, so I'm presuming petrol engine, so this comment won't apply to your specific case, but what I'd be more concerned about is that a bad temperature sensor on some of the later diesel burners that have DPF systems on them could mean that it may not go into the necessary regeneration cycles as the engine temperature is not being correctly reported, which could result in a badly blocked DPF, or worse.

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Irish Steve wrote: »

The coolant temperature sensor is most likely to be the main control, as the intake air temperature may well stay well low if ambient air is cold, there's very few modern vehicles that have any sort of divert on the air flow to take air from around the manifold, as used to be the case on some older vehicles.

You mentioned auto choke, so I'm presuming petrol engine, so this comment won't apply to your specific case, but what I'd be more concerned about is that a bad temperature sensor on some of the later diesel burners that have DPF systems on them could mean that it may not go into the necessary regeneration cycles as the engine temperature is not being correctly reported, which could result in a badly blocked DPF, or worse.

Yep petrol. Plastic manifold so it doesn't even have any coolant line running through it as some older cars had.

Might try a resistor in the IAT line. Way happier doing that than the CTS. TMAP sensor is in the freezer now..

Seeing about 2.2k ohms at indoor room temperature.

Waterson

May I ask what car this is? The problem as I understand it is a hesiation under load in the warm up phase?

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Curiouser and curiouser.

I just don't know what you'd have to do with this car to prompt an EML light.
Pull MAF (TMAP actually) - runs like crap, no EML light.

Pull Camshaft position sensor.... no EML light.
But...
The car reverts to a best guess of where the cam is based on the crank signal, and goes to semi sequential injection

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car is much better when cold. No more dipping down to 400-500rpm when stopping at junctions. Tried this 4 times with 8 or 9 hours between them to reflect the real world situation where the problem has happening - seems to pretty much cure it. Car is "off" alright when warm or at higher revs. Replugging the sensor fixes that.

Contact cleaner into plug and socket, a little dielectric grease - the car seems fine now even without doing the above! Lets see how long this lasts!

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Nah, problem is back again once I stop trying to fool the ECU.

At the back of this there was always the sneaking suspicion that a series of foul ups by one specialist after another had led to some wear and tear due to marginal oil pressure... is it even worth my while getting a new sensor (€80?) and fitting it (awkward, engine mount needs to be removed)?