94 4.0 Misfire in the rain

[Cortcase]

Short history; I've had the truck about 6 months and have been bringing it back up to fighting shape. All new fluids, new sparkplugs, leads, coil, and MAF. When I first got the truck, it was lagging on acceleration and had no power. Zip-ties fixed a lot of the issue, but she was clearly misfiring and was basically undrivable in the wet. After changing plugs, leads, and coil there was significant improvement and changing the maf made it run beautifully.

But I still have two problems:
1. Rough, low idle. It right at or just above 500rpms and stumbles a bit. This stops as soon as you give it gas.

2. Misfire in the rain. She's much better now, but it was pretty wet on my way to work today. She lagged a bit at the first stoplight, but was running well after she warmed up. But, I have about 20min on the interstate and about 15 in, I had3 or four hard misfires, in about a quarter mile, where the truck bucked on one cylinder. I was debating whether or not to pull over(I was almost to work) and it stopped and ran smooth the rest of the way.

I've replaced a lot of parts and gotten improvement every time, but now I need some help. I have a new fuel filter to put on, which it needs, but I don't know if that will help. Any suggestions?

 

[Uncle Gump]

I would take a spray bottle with water... wait until it's dark... fire it up and spray down the secondary ignition. If you have a leaky plug wire or coil... you will hear it and see it. Just because they're new... doesn't mean they're any good.

 

[PetroleumJunkie412]

Agree 100% with Gump. You have a loss of electrical insulation in wet conditions.


Alternatively, only drive in dry weather.



I'm not a mechanic, just a nerd that ****s with obscure engines. My advice is usually half terrible.

 

[sgtsandman]

Vehicles with distributors experienced similar issues when the distributor cap was cracked. You could have something similar going on.

 

[Cortcase]

I will try that, and I certainly could have gotten a bad part, but I should have said in the first post, they're all motorcraft parts.

 

[88workcar]

Had the same issue on a 3.0, mine was the coil.

 

[Uncle Gump]

You certainly lower the chances with better parts... there is a bunch of junk out there these days. I would still start there and see where it goes.

 

[Cortcase]

Another question I have as a side bar, how do I know if my engine has a CMP or evap system? Can I tell from the vin?

 

[Dirtman]

It has an evap system. You can tell by knowing it was made after 1975...

 

[Cortcase]

Correction, EGR

 

[RonD]

1990-1994 4.0l Rangers didn't have Cam sensor or EGR unless they were first Sold in Calif

Drivers door label has other info you may want, seen here: https://www.therangerstation.com/Magazine/Summer2009/federal_sticker.shtml