OT; 89 F150 won't start when humid

MartyinVa

Member

Told my son I'd ask the brain trust here what it could be.
89 F150 5.0 On real humid or days like yesterday when its been raining for over 24hrs the truck will just not fire.
Motor turns over great (new battery)
you can hear fuel pump run for a couple sec. when you turn the key(on both tanks)
Truck just ran good Sat. when the weather was perfect.

Told him I'd ask, he is trying to get ready for hunting season so thanks for any help you can give.
 

Humidity is affecting the distributor or ignition. Many times I pointed a hair drier to the distributor for a few minutes and got engines to start up.

Ground a spark plug. Do you get any spark?
 
Tune up rotor cap or coil wire are white an
And crusty if so clean and try and tune up later. I had a 1996 would do the same thing . And it would flood with fuel I would,d half to pull fuel relay turn it over to clean fuel out. Same thing if battery in not cranking fast enough. Usually coil wire at coil emery cloth
 
Years ago I had a 1971 GMC pick up that would do that. My dad took it to many shops and no one could figure it out. Then one day I was with him at a shop and the guy was trying to start it and I garbed the distributor cap and got the heck shocked out of me. That is when I told my dad he had a bad distributor cap. The place wanted to charge my dad for figuring out what was wrong and he said heck no my son did you work I'll pay for the cap and that is it and that is what happened.
Almost bet it has a bad distributor cap and you can test it the way I did to be sure LOL
 
(quoted from post at 17:40:29 09/10/14)
Told my son I'd ask the brain trust here what it could be.
89 F150 5.0 On real humid or days like yesterday when its been raining for over 24hrs the truck will just not fire.
Motor turns over great (new battery)
you can hear fuel pump run for a couple sec. when you turn the key(on both tanks)
Truck just ran good Sat. when the weather was perfect.

Told him I'd ask, he is trying to get ready for hunting season so thanks for any help you can give.

Change the ignition module on the distributor.
 
Ditto on the cap. I had a few vehicles over the years that used Ford type caps and it seems in humid conditions condensation forms inside and causes them to short out temporarily. I just wiped the inside of the cap and it fired up. The drier trick works good also. MJM
 
When it's running use a soapy water squirt bottle to mist the wires and work your way to the cap. Wires and/or cap would be my best guess.
 
Thanks everybody.
Changed ing module last night and that did not help.
It dried up enough last night(low humidity) that it should have started.
We pulled a plug wire and did not have spark also pulled coil wire and laid it next to post and didn"t see it jump fire either.
Tried to crank it after dark and didn"t see anything jumping under the hood.
We also don"t smell gas when cranking over.
Would the brain box allow the engine to turn over and not give it fuel or spark?
 
Will be checking that when he gets home, run out of time last night.
The coil sits on the top of manifold but does not look like our round can coils on our N's.
square frame with round windings in the middle, 2 wire plug going in bottom with coil wire to dist coming out the top.
Guess if I pull the 2 wire plug and turn the key on I should have 12volts there . Right?
 

If the module on the outside wasn't the problem then it is probably the pickup inside the distributor. I've seen one or the other go bad and sometimes if the pickup inside goes bad it can cause the one on the outside to go bad. The are recommended to be changed together although most people dont. The EEC IV system is pretty bullet proof except for those two modules.
 

Have you checked the wire coming off of the solenoid? There should be a wire coming off for the start position of the switch.
 
Well we did some troubleshooting on the coil.
12v coming to coil
while cranking over we did not get a signal from the pip (position sensor)
Went back in the house to do some more reading.
Matt went back out and laid on ground just looking and reached up and just wiggled the ground wire from battery. This is a new wire, it grounds to the frame about half way down then over to engine.
He heard something buzz or click. He went to check to see if he left the key on/he did but he tried to start it and it fired right up.
The only thing we did was tighten the clamp around the wire where it grounds to frame.(it would slide in clamp)
Not totally convinced that was the problem.
Thanks to all who have offered help.
 
(quoted from post at 08:51:25 09/12/14) Well we did some troubleshooting on the coil.
12v coming to coil
while cranking over we did not get a signal from the pip (position sensor)
Went back in the house to do some more reading.
Matt went back out and laid on ground just looking and reached up and just wiggled the ground wire from battery. This is a new wire, it grounds to the frame about half way down then over to engine.
He heard something buzz or click. He went to check to see if he left the key on/he did but he tried to start it and it fired right up.
The only thing we did was tighten the clamp around the wire where it grounds to frame.(it would slide in clamp)
Not totally convinced that was the problem.
Thanks to all who have offered help.

Being an old techie, the first question you always ask" Is it plugged in?" meaning start at the very basics.
Since you originally posted that sometimes it worked and sometimes it didnt, I was not convinced that any electrical part went south. Humidity and water wreak havoc on any connection, expecially a low voltage DC connection.

I would remove the clamp and clean it up as well as where it mounts to the frame.

"The only thing we did was tighten the clamp around the wire where it grounds to frame.(it would slide in clamp)"

Was the same guage wire used in the replacement?
That should be an absolute clean and tight connection.
 
Thanks GW224,
This was an aftermarket cable so I can't be sure it was the same gauge and I had to strip the cable and use the same clamp.

Still don't understand why it grounds in 2 different places. Actually it is 4 places as it has 2 little pigtail wires coming of the battery post clamp. MY little pea brain does not understand if the ground was bad how the starter could spin the engine over like gang busters. I'm trying to remember if we had this problem before the cable change.

I guess only time will tell if that really is/was the problem.
 

Dont know how the frame of the F150 is constructed. Could possibly be that not all points make a metal to metal contact with the engine so that all electrical components can be subject to improper grounding. Remember for the sake of a comfortable ride rubber mounts and rubber grommets are used just about everywhere. Those little electrons don' flow through rubber.

So to eliminate any issues multiple ground points are used.
Just as a note most if not all engines have a ground strap from engine to firewall and possibly to frame.

Dont know where exactly those other grd wires go from the battery, but my assumption is to a fuse box or an electrical component that needs to have as much ground noise eliminated as possible....Just my guess.
 

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