I have a 94 4WD with the 4.0 engine. When driving for 1 to 2 hours straight, the check engine light will come on. It will disappear if I coast or stop and turn off the engine, but will usually return after a short period of time. I am wondering if the cat. converter could be causing this, as I can sometimes smell it.

Thanks...

 

Probably not. If you can feel exhaust at your tail pipe your cat is probably ok. It takes alot to stop them up and I don't think your truck is old enough. I would suspect your O2 sensor. Has your gas milage suffered any?

 

Gas mileage has been about the same. I just brought the truck back from Germany where I would drive at speeds up to 90 mph. If it is the O2 sensor(s), is this a do-it-yourself fix?

 

It could be the O2 sensor but it could be many different things. I would suggest getting a scanner, which is only $35 or so, and pulling the codes and running the pinpoint tests. Doing this might save you some $$$ since it will tell the exact problem and avoid replacing good sensors.

 

I had/have this exact same problem.

Around town/short trips no problems. Long trips of 3-4 hours or more, interstate speeds...problems. Also, just turning it off or slowing down doesn't turn it off except for temporarily. If I shut it down for an hour and it cools completely down, then I can do another 3-4 hours before it comes on again.

Took it to the local Ford Dealership and was told by the service man that it is a malfunctioning sensor. I don't know shi# about all this new-fangled computer/sensor stuff, so here is what I can best remember him saying before my eyes glazed over:

There are two sensors (don't remember if he named them). It has something to do with driving for an extended amount of time at a set speed (without much speed fluctation). The two sensors work in harmony, one doing it's job for a while, then transfering the job over to the other sensor and vice versa, all without you knowing

Evidently, one of the sensors is a little slow in taking up it's duties, so it is registered that it is malfunctioning. The delay is supposedly only microseconds, to small for anyone/anything but the computer to notice. Computer then causes the light to come on.

He further stated it cost about $240 to replace the sensor (I think, it was a couple months ago), and he couldn't promise that the 'good' one wouldn't need the exact same thing done 2 months later (for the same fee).

He then said that since there wasn't really anything wrong with the sensor (it hadn't 'stopped' working, it just would occasionally react a little slow under the conditions listed above), then he wouldn't spend the money to replace it unless it started causing problems.

Problem for me is, when I'm on a long 3-4 hour trip, that is the last time that I want an idiot light coming on. Even worse is that you have to 'wait and see' if it's the sensor acting up, or if its some other problem that's going to seize up my engine and leave me stranded on the side of the interstate 4 hours from home.

At the time, I opted for the cheap route and am just ignoring it. I rarely travel far, so it hasn't been to much of a problem. If I do, 4 hours is usually a good time to stop to eat and take a break while the engine cools.

I have not noticed a change in mileage, but on one occasion, on a long trip (8hr), the light would come on, then go off, about every 15 minutes. Then it came on and stayed on about 30 minutes, when it went out, the truck began running poorly and I began to wonder if it was going to leave me stranded. It was like it wasn't getting enough fuel and began running rough and with no power. Could barely maintain my speed. I managed to get to a place to stop (where there was civilization) and did the old 'let it rest an hour' and then I was able to drive the last 3 hours without problem.

Bottom line............I don't know, could just be FOMOCO service dept mumbo jumbo, maybe one day I'll save up enough money to have it fixed.

Bama40