1990 F-150, 5.0ltr, 156,000mi.
Truck runs great when dry, but when it rains, it starts to stumble and die, the "check engine" light comes on. When it drys up, it runs fine and the CE light goes off. Autozone says thier scanner won't work on it.
So, I replaced; cap, rotor, air filter, crankcase filter, plug wires, 02 sensor, and MAP sensor. I have new plugs, just need time to put them in. I used di-elect. grease on all connections. It's running great now, but Sat. night it rained and I went driving around. Happened again.
Any ideas?
Thanks all.

Kikr
take a spray bottle and spray around the coil and wires and see if you can see them breaking down,it is best to do this when it is dark easier to see what is breaking down due to moisture.good luck/my thoughts would be coil.

I beleive your 5.0l p/u has a mass airflow meter between
the intake and airbox. ck to see if you're not pulling water into
your air filter.

-tight lines
Brian

Stan, thanks for the info.
Brian, I replaced the MAP sensor that was on the passenger side firewall, is there another one that I'm missing?

Kikr Brian was talking about the mass airFlow sensor. It should be inline with your intake ducting. Since water/humidity changes the density of the air when in excessive amounts, it can throw off the reading given by the sensor, could damage a sensor, and would cause the computer to spaz.

I had the same problem on an S-10. I hd taen the fender wells off (stupid--thought I was going to dropa V8 in it). And when it rained it would stumble, die, and not start again until it dried out.

The main wiring harness was getting wet at the plug behind the fuse block. I figured this out by using a blow dryer to fx it one time. I used a glob of dielectric grease on it and it was fixed.

If I were you I would try disconecting some of your cannon plugs and greasing them with the dielectric. Being a Ford, I would do it one at a time, like do one plug, start the truck to make sure it works, and do another. Also do some research to make sure you don't disconnect something that shouldn't be. Don't know about Fords but I read a lot of manuals for GM and Delco had an audio system that would shut down if you disconnected the battery and needed codes to be re-activated, though I've never seen one myself. Not sure of Ford ever did that, but you just never know until you do some reading if there is a certain process involved with discinnecting wiring harnesses. It should be done with the key in the "off" position and the battery should probably be disconnected.

Knowing if you have fixed the problem will always be the tough part because you will not truly know if it is fixed until it rains and you can't control that. I wouldn't recommend trying to get your truck wet with a pressure washer or water hose under any circumstances.

Look at the top of the windsheild at the drivers side, then look to see where the fuse box is under the dash, if it is on the left side of the stearing wheel, you may have a leak in the wind sheild that drips water down to the fuse box.
On that era expitidion and navagitors, many had to have the windsheild replaced and or the roof repaired due to leaks.
The fuse box would also have to be replaced.
Do you have any other symptoms, like the blower motor working by its self or not working at times, radio problem simmilar to the blower motor?

No problems like that, but it makes sense. Workin' 6 days a week and no time to get into the repair yet, Thanks for the idea.