Hello all. I am dealing with a reoccuring issue with my truck that has left me a little confused. I own a 1994 F-150 that has a 302 and 220,000 miles on the original drivetrain. Here is what I am dealing with and what I have replaced. I am dealing with hesitation at random moments; sometimes the MIL is on when this happens. So I hooked up a Snap-On scanner and it is telling me an O2 sensor is bad half the time and sometimes it doesn't. So I replaced it anyways and conditions were the same as the day I bought the truck, everything was great....that was in October last year when I replaced the O2 sensor and all was fine until this week. Now, I recently replaced the plugs with Autolite 25's, Excell cap and rotor, Autolite wires and cleaned and oiled the air filter. I follow the same regiment of components every time something is replaced. I even checked the fuel pressure from both tanks and it is within spec while running and KOEO. The fuel filter was replaced two years ago and It has been three years and approximately 40K miles since the previous tune up and three times a year I clean the air filter. Everything is running noticably better (except for the hesitation) and mileage has improved slightly. Now the darn MIL comes on intermittently and is acting like the O2 sensor is bad again. I have had bad luck with Bosch sensors so I purchased a Motorcraft O2 sensor and have had it last the longest. I find it hard to believe that I have had a bad luck of that many O2 sensors considdering my 1987 Cougar has both it's original sensors in it and I have no problems with it. Now, I know that I am recieving an EGR valve code since it is unhooked and in the closed position but I doubt this is causing hesitation; I have seen them stuck open and causing rough idling and poor running conditions as it did on my 1998 Escort and I have neither of these symptoms on my truck. Any thoughs as to something that is causing an O2 sensor to go bad so quickly? Any other sensor I can check in/out going voltages to help eliminate the problems? I have checked the throttle position sensor voltage(s) at idle and WOT positions and that was fine as well. I also went as far as to measure the pressure of the exhaust at the O2 sensor hole and it was about 4-1/2 to 5 psi. I considder myself to fairly competent when it comes to these Fords and I am feeling frustrated. I hope this is enough info for somebody to help me. Thanks.

 

if you unhook the EGR you have to block it off somehow. it causes problems that could be what your goin through. just make a little block plate to go between it and the intake. the plug the little green vacuum hose goin to it.

 

It's likely the O2 sensors is not bad and you're misinterpreting the code you are getting. What coce is it producing right now? If it's one that indicates " system lean O2 not switching" or something like that then it's not an O2 fault something else is causing the engine to run lean, vacuum or intake leak, air injection system leak upstream of the O2 sensor, cracked exhaust manifold, or another bad sensor.

 

Well right now I am leaning towards the MAF now after reading through some TSB's. The EGR is hooked back into the equation and nothing was changed. I checked for vacuum leaks on all the hoses as a possibilty to see if the engine was running lean as well as removing a plug wire from each cylinder at a time to see if I had a dead cylinder. Nothing. I tried a second scanner from the local mechanic's garage and there were no codes in memory even though I drove into the shop the MIL was on. But I ended up cleaning the MAF with the proper electrical cleaner and replaced the O2 sensor with a new NTK that was branded as a Motorcraft. I later drove the truck for about twenty miles with no issues of drivability but the MIL light came on periodically. What gives?

I have heard that Bosch had a bad run of O2 sensors and spark plugs so Volkswagen stopped using them. Can anyone comfirm this?

 

This sounds more like an EEC problem now, maybe source another processor to try.

 

Conanski, That is what I am leaning towards, the ECM. I really am hesitant to want to get an ECM and be stuck with it if that isn't the cause. Last week the check engine light came on while I was talking to a mechanic about another car so we connected a scanner to it and nothing showed up but was able to communicate and read out all data that could be accessed. Then the light went off. He suggested the MAF to be bad since he had mentioned seeing this on several Fords of my vintage with similar symptoms. He alsso gave me a new ignition control module just to check in case that was one of the issues that we had previously talked about. The truck runs the same, still intermittently stalling.
What about the coil pick up and armature inside the distributor? Are those prone to going bad in these? I have also heard the ignition switch in the steering column cutting out on others. I believe a lot of these theories to be plausible but I will admit, I don't know it all, I know enough to usually get me through the problem. That is why I am grateful this community exists!