Hello, I need some advice. I am the original owner of a 89 mustang gt 5.0 5 speed, 90k miles. The car is complete stock, no mods. The car has never been in the shop and the few problems I have had with it I fix it myself. The car was running great until I parked it for a couple of days and when I crank it up a couple of days later it is running very rich and back firing. I figure it was time for a tune up and I replace spark plug wires, spark plugs, and rotor. This made no change to performance. So I broke out Chilton’s service manual and started trouble shooting. I check the MAF, MAP, and the TPS sensors and they check good. I did not have a way to check fuel pressure so I change out the fuel pressure regulator. No change. I replace the fuel pump and the fuel filter and gas tank about 2 years ago. So I ran the codes with a analog meter and receive both O2 sensor faults. I did not receive any other codes. I was suspicious that both O2 sensors failing at the same time. So I pull the computer out so I could measure the voltage coming out of the o2 sensors that feed the ECU and measure 0 volts on each output. I measure the impedance for the ground at the ECU feeding the O2 sensors and it measure very low which is good. So I try to run engine run test but it aborted and the ECU came back with a code 12 which is unable to test upper limit.
If the O2 sensors output is 0 volts that should make the ECU run the engine richer? I look for vacuum leaks and I could not find any. I did not feel like changing the O2 sensors, so I brought the car to a local shop. Well they have had the car for a week and I am starting to have some doubts about them. They first told me the MAF was bad. I told them I check that and I thought it was good. But I could I have made a mistake. So I told them go ahead and replace it. They told me today they put the new MAF in and it did not correct the problem. They said they are going to install the original back in. They were honest about that. Now they think it is the ECU is bad. They said they notice it was laying out on the floor. I told them the reason for it being out was that I was making the O2 measurements and I told them the problem with running rich was before I took the ECU out and left it laying out of the side panel of passenger side. They told me they are having trouble finding a replacement ECU. He said they even tried the ford dealer and they did not have any. Do you think the ECU is bad when it outputs codes and run engine tests? What items on my stock car should I watch carefully for showing up missing are being replaced with lower grade parts? Should I try a different shop? Can someone give me some suggestions for sources for a ECU for my car. Thank you, Tunanut

 

You should be able to get a mustang computer on Ebay for pretty cheap.

did they change the o2 sensors?

I would suggest checking the Engine Coolant Temperature Sensor and Air charge Temperature Sensor you can check the voltage coming into the ECU to see if it's in the ballpark.

 

I looked on Ebay, but I was nervous about buying a used one not knowing if something is wrong with it. When I drop off my car at the shop, I told them I thought it was the O2 sensors and shop lead said the computer shuts down the O2 sensors and that is why they are reading 0 volts coming out of them. I have not read any thing that confirms this theory. I know there is heater elements on some o2's but I would not think that would be shut off via ECU. I am thinking about calling the shop today and asking him to change the o2 sensors out any way, since they have never been changed out before. When I crank the car in cold or swarm condition, it does not raise the rpm at idle for the swarm up period because I think the ECU is running in open loop mode. When in this mode I would not think it will care what the engine coolant temp is. Am I wrong in thinking this? Thanks for any suggestions, Tunanut

 

Take car from shop now. Should be key on 12v to warm the sensor. Ground wire. O2 creates small voltage for computer to read and adjust fuel trim. Computer does not send reference voltage to sensor. Getting old but that is how I remember it. Remember what? Oh-huh 02

 

If the O2 sensors are bad, that could easily lead to a too rich problem. The computer falls back to a 'fail-safe' program so to speak, when a sensor reads bad. After 21 years, you could easily have two bad sensors. And, that is what the computer is telling you is bad. Napa has a Bosch O2 sensor for your car for $45 each. I would suggest you replace what the computer is telling you is bad.

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