My truck been acting up a little lately. Lost bottom end power, idles funny at times. truck warming up and the idle drops and engine cuts out for a split second and then its fine. a few times, it bucked on me while accelrating. Pulled code 40 but its not in the book I have. Codes 39-42 are HEGO related. Could this be as well?

 

Im running the mustang computer...should I be referring to "CARS" in the code reader manual?!?!

 

There's not a code 40 until you get into the cylinder balance test. I would use the definitions that match the computer, so car probably would be best.

 

hmmmm.... weird...it gave me a code 40 and 91 being HEGO low signal/lean condition. Is that faulty o2 or insufficient voltage to pass a signal? I have the equus 3145 digital code reader.

When you say cylinder balance...what effects that? cuz my truck is running real ****ty now. I did the ignition tune-up and it was running awesome. it sits for a week while I'm on vacation and now it runs like crap!

 

The cylinder balance test is an optional test done at the end of the KOER test. When the last code blinks, you have to briefly go to wide open throttle to signal to the computer to do a cylinder balance test. It will then turn off one injector at a time and measure RPM drop. If there is less RPM drop on one cylinder than the rest, it will flag that cylinder. The cause could be a result of many things, ranging from a simple bad plug wire to possibly even a broken ring on the severe side of things.

I doubt that your problem is that bad. I would run the KOER test again and see what comes up. I'm guessing the "4" code is the engine code for a V8, not an actual trouble code, because there are no trouble codes that have a zero in them.

O2 sensor trouble codes in these older trucks still require some further diagnostics. It could be the sensor, or it could be another issue causing the engine to run outside the expected air/fuel ratio range. If it's been a while since the O2 sensor was replaced, it won't hurt to go ahead and replace it, but it may not be the problem.

 

Code 40 is displayed in KOER test mode only. It is not normally displayed in KOEO or CM.

Reference this from Ford Fuel Injection » Two Digit Codes 33-58

 

Running the mustang computer with factory truck y-pipe. Do I buy o2 sensor for the mustang or the truck??

 

in cylinder balance test mode - code 40 =Cylinder #4 has a problem
I don't see a code 40 under the other two digit fault code definition

 

What year mustang computer? If the mustang computer is from a model year close to the truck, the O2 sensors are probably the same but with different length pigtails. If that's the case, get one for the truck.

 

Motor is 91 HO w/stang computer. truck is a 90. I always second guess myself when buying parts that send signals because I'm not sure if I should be buying for the mustang or truck. For example...running the mustang computer with truck intake. What TPS do I buy for? if I get the truck TPS, is it compatible with mustang computer?!?!

If cylinder 4 is faulty, then im almost convinced that is my problem. I'm going to change my spark plugs from NGK to autolites or motorcraft to see if that helps. Only reason I used the ngk is because they were brand new when i did the 6litre.

Thanks guys

 

You have to force the computer into the cylinder balance test after the normal engine running test. If you didn't do that and the #4 came up first, don't worry about it. All the EEC-IV computers will flash an engine code first during KOER test, 4 flashes for 8 cylinder engines and 3 flashes for 6 cylinder engines - this is not a trouble code.

As far as sensors are concerned, Ford did pretty well at using the same part across the model lines back then. I wouldn't hesitate to use the truck O2 sensor with the mustang computer, and the truck TPS output is going to be the same 0-5V signal as the mustang was too.

If the NGK plugs are relatively new, I really doubt that they are causing any problems. I've used NGK plugs for as long as I've owned my truck and have never had any issue with them, even when severely worn.

 

Well it was acting up for a few days before I pulled codes. I got home from work and it was nice and warm. Did the KOEO first, thats when I got #91. Did KOER and got #40. do you feel that this is valid?

A lot of people say to use the autolites or motorcraft plugs with the ignition upgrade and timing bump. Another thing I'm going to look into is the plug wire. I bought universal taylor spiropro and did the lengths to my satisfaction, so hopefully its either the spark plug itself in cylinder 4 or I just didn't do a good enough crimp job.

 

Did you use a code reader, or count flashes on the check engine light (CEL)? The first four flashes in the KOER test are not a trouble code. You would have to manually enter the cylinder balance test after the standard codes are flashed - the computer does not perform this test without input from the operator, nor does it calculate cylinder balance during normal operation. This is not what is turning on the CEL.

How did you wire the O2 sensor for the mustang computer? If you didn't split the sensor signal wire to both O2 sensor inputs on the computer, an O2 sensor code would come up if one of the sensor inputs had no signal. The O2 sensor code is probably what is turning on the CEL. As soon as the computer enters closed loop for the first time, it sees the missing input and sets the code.

 

The motor was put in by the previous owner, so I'm not positive how it was wired. I picked up a new O2 sensor today anyways. I pulled the code #40 off the code reader during KOER. It did not appear during KOEO. CEL flashed the same codes as the code reader. I'm getting a little worried about cylinder 4 now....

I've never pulled these codes before when Ive run self tests. It started to act up so i plugged it in and this is what I got. I'm not really sure what i should do at this point. i will go over my ignition system tomorrow, change the O2 and check over the plugs.

Anything else guys?

 

Did the 40 come up first? If so, forget about it, it has nothing to do with cylinder 4.

The code 91 is for the left side O2 sensor, which would have to be added to the truck wiring harness to match the mustang computer pinout. The truck O2 sensor is wired to the right side O2 sensor input pin on the mustang computer. If the left side sensor is not wired, replacing the one O2 sensor won't fix anything. You will have to trace some wires to verify this is what is going on.

Here is a 1990 mustang wiring diagram: http://brembs.net/cars/maf_conversio...ang_wiring.jpg