4.0 won't start when it gets humid

[ak47smurf]

Howdy members. I live on the Texas Gulf Coast (Hurricane Ike visited us), and am trying to chase a problem with my son's '97 Ranger 4.0l. It intermittantly just refuses to start, and the problem appears to be when the humidity goes up. At first I thought it was just a fluke with the fuel pump relay and replaced it, but the problem manifested itself again this afternoon. In reading other threads, I'm wondering if, because the problem occurrs randomly and seems to be tied to higher air humidity, the MAF sensor could be the culprit? Any feedback or clues would be greatly appreciated so I can resolve the problem, get him his Ranger back and regain possession of my F-150. Thanks in advance..."ak47"

 

[racinranger]

take off your coil, flip it over and look for a crack in the plastic filler. I've changed about 30 coils while working for the dealership, all from crack insulation underneith steming from a bolt hole. just flip it and check takes 5min

 

[rkyroadrcr]

I know this may sound crazy. I few years ago my B4000 ran fine and it started raining on my way home. The truck started running like crap. I didn't think it was going to make it home. Replaced the plugs and it was fine again.

 

[MAKG]

On older vehicles, one blames a damp dist cap. But you don't have one.

The coil pack doesn't have bolts anywhere near a place that can crack, unlike the remote-mounted coils on 2.9Ls.

What I'd suggest is leaving it sit all day, and then once it's dark, have a buddy try to crank it (or better yet, try this before sunrise). I suspect you'll see some arcs from the spark plug wires or coil pack, due to surface tracking on the dew.

There is also some possibility that one or more relays are sticking in the humid weather.

One might think that MAFs respond to humidity due to the vastly different heat capacity of humid air. But it turns out it affects the cool wire as well as the hot wire, in the same way, and the effect is removed by keeping them 100 deg apart. I once tried an experiment with an EEC-V Explorer, a missing air filter housing, and a spray bottle. I thought I could make it stumble. I couldn't. No discernable change in idle quality or speed, or anything else.

 

[Ford97Ranger]

I've seen this a lot before, most commonly plug wires. Also relay sticking too like MAKG stated

 

[Big E]

I have had many problems with this same problem..... I eventually had to move my relays inside the cab.... there are three relays over near the power distribution block.... check to make sure that none of the contacts are corroded....
Caused me much grief

 

[ak47smurf]

OK folks....finally got the doggone thing to do it's "I'm not starting thing" last night when the wind shifted to blowing in off of the Gulf. Ran the coil across the multimeter and all was good, visual yeilded no cracks or other indication that might be trouble. Hit O'Reilly's this morning for a set of Motorcraft OEM plugs & "waars" (as it's pronounced down here in Baja Oklahoma). It cranked right up and really hauled the mail when I took it out to the freeway for a test drive. I'm tenatively declaring "Victory" on the gremlin. My sincere thanks to all who provided input and a very merry Christmas to all.
"da Smurf".