QuestionI have a 95' mustang GT with 150k miles on it! Sometime this past December of '04, my mustang started idling and riding rough.....intermittently. I've noticed that it will occur bewtween 5 to 7 days, then go away and it idles perfect and runs perfect. When it idles bad(and I mean if you want a back massage,sit in my mustang while it idles, especially if a little gas moves it to 1k rpm.), the car seems to have power and no loss! I've replaced the fuel pump, regualtor,pcv valve, spark plugs and wires, MAF sensor, o^2 sensors and EGR valve. I've had the computer checked and it seems fine. I've even had a diagnostic check for codes and no codes are being pulled up and shown, no engine light when it runs bad! A friend (who is a mechanic) has a computer and tested the firing of each wire to plug, no problems! I've changed the distributor cap, rotor, coil pack....just about every damn thing I can do! But...I have not tested the injectors! If I have a bad injector, will it throw a check engine light? If I have a bad cat. converter, should it throw the check engine light? Many of times, I have put gas treatment and fuel injector clean to check for bad gas. As you can see, this is troubling me for I'm tired of this intermittent rough idle and drive. Oh yeah, it still shakes bad when driving, but it is dampened by the higher rpms, but you can still feel it. Please help before I completley have tear down the engine to look for this problem and fix it!
Sincerely-
A mustang enthusiast and no money havin' college student!
AnswerWell, the only way to accurately diagnose the problem is to perform tests while the engine is acting-up. I could only offer information and maybe a few guesses. The check engine light comes on when an emissions sensor goes out of its normal operating range. A sensor can fail, but still be within its normal operating range, and not trigger the check engine light. A bad fuel injector could cause an intermittent problem; I have seen injectors intermittently stick open or closed. The EEC on this vehicle is not as sensitive to engine mis-fires as on newer vehicles (newer EEC's will trigger a check engine light after 3 mis-fires). I doubt that your converters are bad; a bad converter would usually cause a constant, not intermittent problem. It sound like you have replaced nearly everything on this engine that could cause a rough-running concern. I would guess that a fuel injector is bad. You would need to isolate which cylinder is mis-firing, and test processors injector output first. The injector driver in the processor could be going bad, so replacing the injector(s) would be another waste of money. Problems like these are sometimes hard for professional mechanics to diagnosis. It looks like you have already spent several hundred dollars on parts. I would highly recommend taking the vehicle to a Ford Dealer for diagnosis before spending any more money or time on this problem. They can usually diagnosis a problem like this for about $80, and you could make the repairs yourself.
I hope this helps,
Dave
