Symptoms of a failed CHT Sensor?

[WydRyd]

What's the typical symptoms of a failed Cylinder Head Temp Sensor?

Black smoke? Hard to start, too rich idle & stalling?

Is there any way to bypass this state to put the DME into a limp mode so the car is at least drivable?

I think I read somewhere that a paper-clip can be put into the DME side of the socket/harness to bypass this state?

Does the CHT sensor typically go into an open circuit state when it fails completely?

[ChrisBennet]

See my post in this thread for faking out the CHT sensor.
engine cuts out.
-Chris

[WydRyd]

Thanks Chris. I have a spare NEW two wire sensor at home, but it's going to be a PITA to do this swap in my car park at work

[PatrickB]

When my CHT was failing, My engine would cut out abruptly for about a half second when under full throttle at high RPM's... This equated to a violent thud when the engine suddenly started again. Not to good for the drivetrain....

[Svaha]

I had been experiencing a variety of minor running problems, though once warm the car ran strong. During cold start-up my car fired up nicely, but within a few miles it usually stalled or the idle would surge, or when accelerating hard once in a while it would hesitate some. Once warm, my car was very hard to restart. I'm talking 7-8 seconds of cranking before it would fire up.

In my effort to exorcise these gremlins I had:

1. Replaced the fuel filter
2. Cleaned Idle Stabilizer Valve
3. Replaced both pre-cat oxygen sensors

None of these DIY's had been the solution to my problem, so I had chalked them all up to routine maintenance that probably really needed to be done anyway (My car had 80K when I bought it and had never had a new fuel filter or O2 sensors)

But today, lo and behold

4. Changing the Engine Head Temperature Sensor has exorcised all of my car's gremlins in one fell swoop. No more stalling, surging, hesitating or hard starts. Also, my cold start idle is above 1000 for the first time, it had always been the same as warm idle, around 800. And I sense a marked performance increase (above and beyond that which I normally experience when I've gone to the trouble to work on my car myself).
The sensor was bad, I tested it with a multimeter before taking it out and it read 1 regardless of engine temperature. The new one registers correct ohms based on engine temperature.

So just putting this out there as one more 993 data point, these sensors can fail and they initiate an array of glitches in the proper operation of our cars.

Happy Driving,
Mark

[WydRyd]

Thanks Mark for the detailed write-up. Sounds like yours failed at the other end of the scale (i.e: when it warmed up). Mine seems to have failed completely and won't run properly on cold start

I tested my CHT a few months ago and it measured OK at the time.

I just recently (over last couple days) had my AFM replaced because my original one was screwed (it was causing an ultra-lean condition) causing the engine to stall and surge violently at steady throttle. Once the AFM was replaced, that problem went away.

Now however, another "gremlin" as popped up and I strongly suspect the CHT has completely died cause the DME is dumping WAY too much fuel in causing black smoke, poor idle and stalling. I won't drive it in this state from fear of the excess fuel washing down the cylinder walls and causing even more troubles

I'm going to test the CHT resistance shortly to verify whether it is in fact faulty. If not, then I guess the replacement AFM I've just installed has malfunctioned in some obscure way

[PatrickB]

Merv~

As this part (CHT) fails on an intermittant basis, testing the resistance might be a wated effort... Unless you get lucky, and it happens to fail when you test it... If the CHT sensor is original, you should change it anyway.

With my Carrera, the engine would completely cut out for maybe a half a second around 6k RPM. Definitely not a surge though, as the engine would actually cut out... After replacing the CHT, the problem never happened again.

Several years late, I started having rough running, and surging problems though... I replaced the flywheel sensors again, along with many other parts to no avail... The last step was to replace the injectors... It might be something for you to consider...

[WydRyd]

Just went outside and made the car idle high for a few minutes to warm up and then I shut it off. Tried to start it up again and it would not fire up. Just keeps cranking. Definately seems like it's temp related. I'm waiting for an Ohmeter to arrive in 1/2hr so I can test the CHT's resistance to ground. I'm almost certain it's my CHT. I had been having cold start issues (too rich) for a while now, so maybe the CHT has finally given up and died completely

P.S: I have an old style one wire CHT sensor. Most probably the original sensor from factory. ~17 years old now, so perhaps time to upgrade it anyway

[WydRyd]

Just as a test, can I just plug a new CHT sensor into the harness connection point (i.e: without actually screwing the CHT into the engine) to see if the car starts and idles correctly?

[ChrisBennet]

Originally posted by WydRyd
Just as a test, can I just plug a new CHT sensor into the harness connection point (i.e: without actually screwing the CHT into the engine) to see if the car starts and idles correctly?

Sort of.
If the motor is cold then it will work.
However, once the motor warms up the sensor will still be indicating "cold" and it will run too rich - but might get you home.

The CHT sensor has a resistance of around 3-3.6K ohms "cold". (Say 3600 ohms in Canada and 3000 ohms in southern California )

When the motor warms up, the CHT resistance drops to around 270-300 ohms.

Testing the resistance of a suspect sensor is pretty useless IMO. The problem with the old single wire sensors is that they ground through the engine and when that ground goes bad the resistance goes to infinity. Instead of recognizing that sensor is giving bad readings the DME (apparently) orders up a mixture to match what the CHT is indicating for a temperature.
I.e. a mixture appropriate for zero degrees Kelvin.

For a couple of bucks you can pick up a pack of resistors, read the color code on the back of the package and be in business. I'd carry 3 resistors in my tools kit with the leads prebent and ready to go. That way you'll be ready for any conditions.
-Chris

[WydRyd]

Thanks Chris.

I measure the Ohms of the old sensor and it's 2.8k, cold. My new one shows ~2.3k ohms, cold.

I connected the new sensor to the harness. Car is cold as it's been sitting overnight. Just kept cranking. No start whatsoever I can smell fuel real bad, so it's not firing the plugs properly.

I pulled out all the spark plugs and they are extremely fouled & wet. Doesn't look like spark is getting to them properly. I bridged a screw driver to a plug lead and I can see spark coming out to the body, so definately have a good coil and distributor/rotor.

I was using NGK BP8ES plugs and they are totally fouled. I'm going to pick up some W3DPO's now and will replace them all and see if that fires the engine up. Bizarre!

Might even pick up a DME relay and replace that also whilst I'm at it

[WydRyd]

I think I may have found the culprit!

See this thread!!!