My 88 has the 4.9 and it has been giving me a fit lately. In the mornings when it is cold it cranks and runs fine but as soon as it warms up it starts to act like it is missing. Has anyone had any problems like this and if so can you give me some insight as to which way to go. I was told it was the timing module so I replaced it, but that did not work. Need some help......

 

so you're describing the truck as "chuffing"?

have you tried replacing all the fuel filters? including the one in the tank?

lets make sure that you're getting enough fuel all the time.

next, how's the tranny fluid? new filter?

 

I have to agree. If you can't remember when the filters were replaced, you're probably overdue. Plus it's the cheapest start.

 

Thanks for the back up pat....

After the tranny fluid flushed and then the filter changed....

Change the plugs, do a dry wet compression test to make sure youre not losing to a head gasket.

 

Another consideration, at least given your descriptions of the cold versus hot running behaviors, is that you might have some significant carbon buildup on the tops of your pistons and / or within the heads. The 4.9L six has pistons that are pretty massive, and depending on how it's been run over the years, you might have the fuel detonating both before and after it gets to top-dead-center.

If you have an old-fashioned mechanic around, especially someone who used to work on tractors that were dual fuel (yes, they actually made these) where an inline engine would start on gasoline and then switch over to diesel or kerosene, they might be able to "lay hands" on this and immediately sense an issue.

In older engines, especially those run at low RPM for long periods with low load, spraying something like SeaFoam into the intake until they stalled was effective once they were at operating temperature. Upon startup, they'd clean out the neighborhood with the smoke, but that's why you also have a fine pressurized sprayer with water handy to also give the carbon a little extra help getting out by misting the intake with gentle touches to internally "steam clean" the engine from the inside to break the rest lose.

I've done this a few times, especially with older Ducati motorcycles that build carbon due to air cooling and poor fuel delivery, but use at your own risk.

Ken