Truck runs poorly when weather gets warm

I have a 99 E-250 with the 4.2 six ( with 150,000 miles) and 4spd auto. When the weather is cooler (under 60 degrees) the truck has all kinds of power and mileage is around 14.5 hwy (truck does weigh 5300 lbs empty). When it warms up (over 60 degrees) the truck doesn't have any power and is a struggle to hit 65 mph. Mileage drops to around 13.2 also. It feels like I'm pulling a heavy trailer when the weather is warm. When it's cool, feels like I have the V-10 under the hood. It's a Jekyl-Hyde thing (maybe more Heckle and Jeckle?). I'm running a K&N filter but it hasn't changed how the truck runs. What obvious things should I look for? Thanks for any help!

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Sans Nom
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Ignition system. My 1990 BII with 160K on it did the same thing this fall. After replacing the spark plugs, distributor cap & rotor, and the wires it has been running fine since then. I'd say the wires are the biggest culprit, and when it is warm the extra humidity in the air allows them to crossfire. That's what make the engine buck and the lack of good spark reduces the power output.

-D

Reply to
Derrick 'dman' Hudson

No, the truck doesn't buck. Just runs poorly but smoothly. Has new wires and plugs (doesn't have a dist). It's like when it's cold, the "choke" is on giving it more fuel. When it's warmer, truck runs poorly. Seems like it's not getting any advance or something like that. Can spin the tires when it's cold and barely get across the street when it's warm. Maybe a torque converter problem? (not unlocking at low speeds?)

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Sans Nom

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