I have a 99 F250 (4.6L) that runs rough during wet conditions. It runs perfectly fine when dry, but even just road spray will cause it to misfire so badly it barely runs.

It's been to the dealer multiple times they say it only needs tuned up and can find nothing else wrong It's been tuned up twice. Immediately after the tuneup it's marginally better: road spray's no longer enough but rain or wet snow and it's back to running rough.

Does anyone have any idea what may be causing this?

Also, the rear end just started getting a vibration in it on take off. Are there any known problems for this? It doesn't have enough miles that it should be wearing out.

Thanks.

 

Hmmm 49 veiws with no suggestions. Only thing I could think of is bring it to a different dealer. Maybe bring it to a good mechanic if you dont have a warranty left. Good luck.

 

Is it a 4.6 litre (as stated)? Or is it a 5.8 V8? I didn't think you could buy a '99 F250 without a 5. 8 at a minimum?

It sounds to me like you may either have a contaminated wiring harness, or a bad ground, or both which are aggravated by wet conditions. Next time when it does it, open the hood and see if you notice any electrical "arcing" in the engine compartment. Look for sparks, or unusual sounds. The motor must be running to do this, so watch any loose clothing around the belts/moving parts.

Look also for any frayed electrical lead wires to your spark plugs.
Good luck

 

the only engines in a 1999 f-250 are 5.4 v8, 6.8 v10, and 7.3 v8 powerstroke.

 

Is it a '99 F-250 Light Duty 4.6L was probably available for them since I think they were basically just a HD F-150

 

His truck may not be the SD, it may the the 7700lb GVWR version, I believe it was available with the 4.6L V8.

As to your problem, I have no idea. It's obviously wiring, take a big swig of patience and start at the beginning, eliminate everything till you find your problem.

A dealer/mechanic will do it, but for the same reasons you don't want to do it, it usually gets really expensive.

Good luck,

Waxy

 

I had to ask but it is a 4.6L. It may be a light duty. That probably gives away I don't know a lot about mechanical things and I usually rely on the mechanics (I think I need a new one). It's never too late to learn. So I really do appreciate your patience.

I thought it may be electrical. I have a starting point now and it sounds like I have my work cut out for me though. Do you have any suggestions where's the best place to start with the wiring? Could there be any other electrical components in a place where moisture could but shouldn't be able to get to them?

 

Go to the self service car wash.

With your truck running try to "re-create road spray" to see if you can replicate the problem. (It's what I do with motorcycles)

If nothing else, you may be able to figure out which side of the truck to start looking for wiring issues.

Make sure you check your main grounds.

 

Next time you have problem wait until night and raise hood and take a look. I know you then can see any short that may be sparking. I have seen engines with bad wiring look like Christmas tree lighting at night.

 

Hey, no problem, that's why we're all here, none of us knows everything, and we're here to help, and by the way, welcome aboard.

Does your truck look like a 2002 F150, or a 2002 F350? My guess is it looks like the F150, in that case it is the 7700 GVWR version.

Monsta's idea is really good, he's a bright one that guy.

I hate electrical problems, they are almost impossible to diagnose, even with the truck right in front of you, let alone over the net. Your problem could be as simple as a bad ground or a wire, somewhere in the 200ft of wire on your truck, that rubbed through and is causing a short when wet. The problem is it may take hours to track down that wire.

Do like Monsta said, and concentrate on the coil packs on each cylinder. Lightly spray the truck till the problem starts, one area at a time.

If you can at least localize the problem that's a big step.

Let us know what you find out.

Waxy

P.S. If it's -40C where you live like it is here, you may want to hold off on that trip to the car wash.

 

On the same lines as da "monsta" use a spray bottle filled with water. Spray one foot sections of harness at a time. Also do it at night. Any little arc or spark can be easily seen. Used this method many times, works like a charm.



On edit, Well if I had read the entire post before replying, I would have seen the suggestion to do it at night, Sorry folks !

Mark

 

I've had 4.6L engines misfire because of spark plug wiring. I know, it's been tuned up. I had a 4.6 come in and it missed. It was damp and rainy out. Looked in the engine bay, everything seemed fine. Brand new aftermarket wires. Talked to the Ford techs down the street. Advised to change to Ford wires. Said they've had that same problem with aftermarket wires. Changed wires and miss was eliminated. Just something else to check. Ford Coil pack ignition systems are very touchy with plug wires. Same as the GM DIS systems.

Just some info.

Jimmie
2003 F250SD SB SC XLT 4X4