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My '93 Festiva was suffering these symptoms:
occasional CEL, stumbling, running rich, high idle etc. Every time I had these symptoms, which became more and more frequent, I would check for codes and get the #9 (ECT sensor) code every time. I changed this sensor (coolant temp sensor mounted in the intake manifold) with a good one from a car that never had these problems (thankfully I have two, and I was switching from one car to the other).
I had connected the old (alleged faulty) sensor to an ohmmeter and placed it in an electric teapot of water for testing. My ohmeter showed room temp resistance at around 1.3 ohms, and at boiling it drops to a minimum resistance of .285 ohms.
Without specs from a manual, I wonder if these resistance measurements are within ranges specified for a normal good sensor?
Replacing the sensor fixed it. I am so Delighted!! I really love these cars, and now this one purrs like a happy kitten.
I live in Alaska and took a nice pleasant test drive (about 80 miles) to see if I had cured it.
I also adjusted the throttle position "Switch" (not a sensor or potentiometer) so that the contacts closed at closed throttle. Easy to do if you remove the cover off the switch before adjusting it. It's only purpose in life is to close two contacts at closed throttle, and close two more contacts (with a common contact in the middle) at wide open throttle. Incredibly primitive, but effective. Now when I release my foot off the gas at speed, the fuel is properly shut off to the injectors for better deceleration.
FYI, The Vane Airflow Sensor (VAF) does the work of telling the computer how much demand you are placing on the engine, not the Throttle Position Sensor, (TPS) which on most cars is a potentiomer not a switch.
Some added information for those that are interested, I measured the resistance of the replaced sensor at the engine connector (above the thermostat housing) immediately after shutting off the engine after my lengthy test drive.
The sensor resistance at operating temperature (hot) was .358 ohms, which was a bit higher than the one I measured at boiling water temperature (.285 ohms).
This information isn't much use because I don't know the temperature of the coolant in the engine at the time of measurement, but I DO know that the water was boiling when I measured the one I took out of the car and put it in the teapot to measure it.
Also, for what it's worth, the highest measurement I could obtain from that sensor I removed was approx 1.8 ohms when I touched the ice in my freezer with it for a few seconds while measuring the resistance.
I did this just to get an idea of the maximum range of resistance that this sensor was capable of across the range of temperatures I could expose it to.
IMPORTANT:!
I was never able to get the sensor I removed to show any kind of anomalous behavior while testing it out of the vehicle.
However, changing it cured the driveability condition that was driving me nuts.
So, if you are going to test a sensor's resistance, do it while it is mounted in the vehicle at the engine harness connector (you have to unplug it and measure between the black wire and the red/blue wire). I even went as far as to measure the resistance at the Engine Computer (Engine Control Module, ECM) from red/blue wire to black wire.
Doing this did NOT help me to diagnose the problem though, it just gave me an indication of the sensors resistance at a given time or temperature.
I'm sure it's probably possible to monitor the behavior of the sensor while the vehicle is being driven, and I believe that is the only way to actually catch it and see it failing in real time. I didn't try to rig the digital multimeter probe wires such that I could see the voltage or resistance at the sensor while driving, but I did consider it.
I just wanted to fix the damn thing, and I had nearly gotten stuck in traffic a time or two when the vehicle started acting up, so I just replaced the sensor.
Good Luck all, I will continue to keep checking this forum.
I spend many years as a driveability technician, but I can always learn more!!
Trace
occasional CEL, stumbling, running rich, high idle etc. Every time I had these symptoms, which became more and more frequent, I would check for codes and get the #9 (ECT sensor) code every time. I changed this sensor (coolant temp sensor mounted in the intake manifold) with a good one from a car that never had these problems (thankfully I have two, and I was switching from one car to the other).
I had connected the old (alleged faulty) sensor to an ohmmeter and placed it in an electric teapot of water for testing. My ohmeter showed room temp resistance at around 1.3 ohms, and at boiling it drops to a minimum resistance of .285 ohms.
Without specs from a manual, I wonder if these resistance measurements are within ranges specified for a normal good sensor?
Replacing the sensor fixed it. I am so Delighted!! I really love these cars, and now this one purrs like a happy kitten.
I live in Alaska and took a nice pleasant test drive (about 80 miles) to see if I had cured it.
I also adjusted the throttle position "Switch" (not a sensor or potentiometer) so that the contacts closed at closed throttle. Easy to do if you remove the cover off the switch before adjusting it. It's only purpose in life is to close two contacts at closed throttle, and close two more contacts (with a common contact in the middle) at wide open throttle. Incredibly primitive, but effective. Now when I release my foot off the gas at speed, the fuel is properly shut off to the injectors for better deceleration.
FYI, The Vane Airflow Sensor (VAF) does the work of telling the computer how much demand you are placing on the engine, not the Throttle Position Sensor, (TPS) which on most cars is a potentiomer not a switch.
Some added information for those that are interested, I measured the resistance of the replaced sensor at the engine connector (above the thermostat housing) immediately after shutting off the engine after my lengthy test drive.
The sensor resistance at operating temperature (hot) was .358 ohms, which was a bit higher than the one I measured at boiling water temperature (.285 ohms).
This information isn't much use because I don't know the temperature of the coolant in the engine at the time of measurement, but I DO know that the water was boiling when I measured the one I took out of the car and put it in the teapot to measure it.
Also, for what it's worth, the highest measurement I could obtain from that sensor I removed was approx 1.8 ohms when I touched the ice in my freezer with it for a few seconds while measuring the resistance.
I did this just to get an idea of the maximum range of resistance that this sensor was capable of across the range of temperatures I could expose it to.
IMPORTANT:!
I was never able to get the sensor I removed to show any kind of anomalous behavior while testing it out of the vehicle.
However, changing it cured the driveability condition that was driving me nuts.
So, if you are going to test a sensor's resistance, do it while it is mounted in the vehicle at the engine harness connector (you have to unplug it and measure between the black wire and the red/blue wire). I even went as far as to measure the resistance at the Engine Computer (Engine Control Module, ECM) from red/blue wire to black wire.
Doing this did NOT help me to diagnose the problem though, it just gave me an indication of the sensors resistance at a given time or temperature.
I'm sure it's probably possible to monitor the behavior of the sensor while the vehicle is being driven, and I believe that is the only way to actually catch it and see it failing in real time. I didn't try to rig the digital multimeter probe wires such that I could see the voltage or resistance at the sensor while driving, but I did consider it.
I just wanted to fix the damn thing, and I had nearly gotten stuck in traffic a time or two when the vehicle started acting up, so I just replaced the sensor.
Good Luck all, I will continue to keep checking this forum.
I spend many years as a driveability technician, but I can always learn more!!
Trace