my 91 f-250 5.0 runs rough in the rain. it skipps and shutters and just runs like crap whenever it is raining . i just did a complete tune up on it , i mean totally complete tune up. cap, rotor, wires, plugs, pcv, air filter, fuel filter, oil and filter and set timing properly @ 10 btdc. also just did new fuel tank and new sender,fdm,and float. i have been trying to get rid of a constant pinging that i have had and a lack of power in the motor ever since i got the truck and i cant seem to pin it down .


also i have a weird problem that i have never come across in my life. when im on the highway with the heater on in the vent mode and i give it gas to go into passing gear the heater switches to the defroster until it shifts back into overdrive and then it goes back to the vent. anyone ever heard of this before ?????? im thinking that whoever had the truck before me did something really really wrong under the hood and im the fool who has to go asking all these questions to figure it out.

i had a perfectly good 91 f-250 truck that i paid $250.00 for with 33.000 miles on it with a plow from a little old lady and if you look at my gallery you will see exactly what happens when you hit a flatbed tow truck when you are doing 50 mph and he is backing up in the high speed lane of the highway. its not pretty

 

In your cap and wires, try srpaying them with wd-40 at the connections and see what happens, also, check the coil very close. it may be cracked and you dont nitice it until it gets damp,

sucks about your accident, who soaked the cost of that up, you or the wrecker?? And seatbelts DEFIDENTLY save lives.

 

old wires can have cracks that et grounded out when wet, check the insulation on the wiring under the hood.

For the heater thing, it uses vacuum to hold a valve open. When you got on the gas the vacuum drops and there isn't enough vacuum left to hold the valve open. Most vehicles have a vacuum canister that stores vacuum, using a diaphram, for momentary loses of vacuum. Should be able to find on at a junk yard, doesn't even have to be from a truck or even a ford for that matter. They all work the same just look different.

 

Next time it rains or is very damp,remove under hood light bulb and watch in the dark for spark-you may need to drape a tarp over hood to seal out all outside light.I have found many this way.

 

Or you could mist it with a hose and start it, or maby even dry you may see something, worth a try.

 

i've got the same issues tonight, the damn truck just wont go, then outof the blue, as soon as the rain cuts, the truck goes.... have you figured it out ?

 

No worries
(edited to take out misdirected double post grumps )


When you set the timing did you pull out the spout plug? It's a little 1/2" grey plug near the distributor TFI ford put in to tell the computer not to adjust the timing. Basically like vac advance on on dizzies. If you don't pull it when you set the timing, you will be a couple degrees off.

It would make more of a difference in warm vs cold, and have nothing to do with rain. It would make the engine feel a little more doggy and slow to respond to throttle, even pinging at part throttle. It needs to be really close for the computer. I adjusted mine a couple times to get it set just right.

The rain could be the MAP sensor, since you have replaced everything else.

The timing will get rid of that pinging.

 

I'd change the MAP sensor as you have done everything else. You did state originally that you put on new wires, the whole shootin' match. Both you and Bucks will probably find that it's the MAP in the end.