Running Rich? Stumbling, cutting out, random idle/CEL? I have the cure for you!

[SiliconSoul]

Hey everyone. As in the title, I have had the above issues with my car for quite a while now. Everything I checked and replaced did nothing to remedy the problem. Was very random for the most part. Part throttle kept it from being noticeable for the most part, but the stumbling and high idle were really starting to bug me. I could smell fuel at times, so I knew it was sometimes running rich and hurting my gas mileage. After reading through many posts, I finally tracked down what I believe is my issue. The COOLANT TEMPERATURE SENSOR! I had no idea this component could affect the computer so much.

If you think you car has something similar, there is an easy way to test it. With the car off, disconnect your ECT. It is under the Intake resonator silencer, near the last fuel injector by the timing cover. I found it was easiest to remove the silencer to get a little more room. The hardest part about testing this is getting the damn wrap around metal clip off the ECT plug in order to disconnect it. When it is disconnected, start your car and see if there is any change. You will get a constant CEL, but the car will be drivable. If you notice a change, congrats! Replace the part and you are good to go.

Here is a good video by Ephet showing the location of the ECT: http://www.youtube.com/watch?v=rKqtcENg5R0

A little diagnosing information by FB71 : Should be about 3k ohms at 72 degF and 500-1k ohms at operating temp.

And finally a little troubleshooting advice from Kzbill for a car running rich:

1. Coolant temp sender reads the temp and tell it to richen the mixture.

2. Engine temp too low=coolant temperature sensor or wiring bad or diconnected. (temp sesor located on passenger side of intake manifold next to injector) OEM thermostat is a two stage 88C/195F.

3. A Bad O2 sensor will tell it to run in a preset factory loop which I believe is rich. (please correct if wrong) sensor wiring or connector could be bad. On mine I had a lot of corroded connectors from sitting too long.

4. There is a little Ring terminal connected to a bracket near the thermostat housing that has two black wires in it. Mine was dirty this is a ground for the engine management, dist and fuel inj. I think. clean the ground.

5. Could be a VAF but clean the terminals in the connector first. Get the haynes manual and check all the grounds in the engine compartment and clean them where they connect to the inner fenders.

Thanks to everyone for all the advice and information that let me finally get to the bottom of this problem!!!banana time

[drumnerd33]

Good write up, and yes the coolant sensor is one that frequently gets overlooked, alot of people dont realize that the computer even uses the temp sensor, some people dont even realize that they have 2 temp sensors... one for the dash gauge and the other for the computer.

[bravekozak]

Mazda uses brass terminals. They can get oxidized. If you get poor conductivity, you could use a terminal release tool to pop them out, wire wheel them with a Dremel and pop them back into the connector. One at a time.

[tac]

And a fan switch. 3 things on the engine to measure temp.

[TorqueEffect]

Yeah, when I bought my Festiva last year, it would stumble, and occasionally throw random CELs when driving along with the engine stumbling, also ran really really rich, and the gas mileage was terrible at 32MPG.


Is there a recommended brand of this sensor though? I bought the only one listed at Oreilly's, the store brand MasterPro sensor. It works perfectly, and with the new gastank I am getting 40MPG+.

[AlaskaFestivaGuy]

I take it this only applies to FI'd cars?

My pristine 88LX stumbles like you wouldn't believe and gets only 25mpg, despite replacing everything several years ago, including the car (with an overhauled one). Lacking any other ideas, I'm planning on upgrading it to FI from an existing donor.

[Movin]

It applies to feedback carb too, and boy can they go rich!!

[SiliconSoul]

Lol@AlaskaFestivaGuy... I did a double take on your signature!

You just gave me another reason to keep loving my Festival's. One day I can buy a floatplane with all the money I can save. ..

[tgentis1]

My '93 Festiva was suffering the same 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'll find out if it cured this condition next time I drive it, and will post results.

However, I have 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?

I guess I'll find out when I drive the car with the replacement sensor, because I have not performed this test on the replacement sensor.

Thanks for all the great info!! At one time I had collected 5 of these cars, now I'm down to 2 (one retired). I intend to keep driving these as long I can get 'em.

Can't beat the gas mileage.

Will post results after driving it with the replaced sensor.

Trace

[WmWatt]

Here's a link to the thread re coolant temp sender on my carburetted model ....

https://fordfestiva.com/forums/forum/maintenance/repair-help-forum/63863-coolant-temperature-sensor

I was not experiencing performance problems, just intermittent check engine light which came on more in winter and tended to go out after the engine warmed up and after the contacts were cleaned. The senders for the carburetted and fuel injected models are different sizes..

[tgentis1]

Trace

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 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.

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.

I hope this might be helpful....I'm going to go ahead a post all this as a new post in case it might help others...

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

[mexaspire]

Replaced this on my 1997 Aspire because my idle is not very smooth. Now getting P0505 code. had gotten that before and removed IAC to clean it but need to remove it again and open it up and clean it properly. Did not open before because it had some safety screws.