Im having this problem in my 98 5.9, I recently put a 3rd of sea foam into the tank with higher octane gas. Then this hesitation started. Its almost exactly the way you said your truck was acting...

 

I have the issue. ended up being a clogged cat. so i'm replacing the cat and the o2 sensors.

 

I am curous if you found a solution to the timing problem as I have had a similar situation. I replaced the intake gasket, cap, rotor wires and plugs and the crank shaft sensor; the timing still goes out once and a while and causes a backfire (at which time it then seems to reset itself). It seems the ECM is correcting for something that it should't be. Next on the list to change this week is the IAC valve.

Next I guess would be the distributer in its entirety.

If you solved the problem please post it and let us know. Cheers!

 

Did you replace at least the pre cat o2 sensor and check or have the cat checked?

 

All O2 sensors checked out okay (cold and at temp). I have not checked the cat.

 

Well, check the cat, and what do you consider "All O2 sensors checked out okay" ?

 

Okay - reset the PCM and the truck is running fine. It appears that the intake manifold gasket was the problem.

 

Which intake gasket, upper or lower(plenum) ?

 

Lower plenum was changed - internal vacuum leak. The problem is back and slightly different now. I think there was a couple of different problems. I am going to check the TPS and have the O2 sensors rechecked. The problem seems more prevelant when the engine is cold. When it warms up it seems to run better then before. Also the gas mileage is up a little.

When previously checked the voltage levels from each upstream and downstream O2 sensors were check at start-up and as they heated up. Both sensors were within their original voltage specifications. The sensors were also switching correctly. An interesting note though is that there is no left bank O2 sensor in the truck (nor was there a tap to connect one) even though the computer shows two (upstream). The local mechanic is also looking into the ECM to see if there is a firmware update for it.

Anyone want to buy a truck with 95,000 miles on it . . . . because that might happen if i cannot get this figured out.

 

Only California models will have 2 upstream o2's. Unscrew the precat o2 and see how it runs(leave the plug connected). And I can't tell you how many times I have seen o2's check out ok with a scanner, only to replace them and find out they were bad. I'd order a couple NTK's from rock auto, and go from there.