Moisture and timing

[Ticatwo]

I posted here years ago with another nick. Still have the same truck. Lost the nick.

1986 2.9 long bed, 5sp, touch drive 4x4. Have owned it since new and it still serves me well, 279,000 miles later with only a water pump and some VC gaskets.

Recently, when it is raining, the truck has been hard or impossible to start. Sounds like it is way out of time. Starts fine when it is not raining.

Cracked dist cap? Failing TFI module? Knock sensor?
The engine compartment is dry, no leaks or wetness that I can see.

Appreciate any thoughts

 

[RonD]

Welcome back

High humidity(rain) can enable shorts or high resistance in wiring that are not shorts when humidity is low.

If you remember the high school science experiment of two electrodes in a glass of distilled water connected to a light bulb it will make more sense.
Distilled water has no minerals(salts), so electricity can not pass between the electrodes, light bulb is off.
Add some salt to the water and bingo.....light bulb lights up.

The opposite is true as well, corrosion is made up of salts, they can limit current and voltage a little when dry, may not even notice it............but add some water(humidity) and bingo, short or high resistance develops.

I would inspect the TFI wiring, maybe test it as well:
http://www.therangerstation.com/tech_library/TFI_Diagnostic.html

High humidity air also affects engine performance, reduces it, but not to the level you describe.

Not sure if humidity could affect MAP or MAF sensors that may be on the edge of failing, humid air displaces oxygen content, ask any pilot about humid air take-offs.
So it does affect the oxygen levels the MAP/MAF fuel/air mixes are based on.