I got my 4.9 running after putting in a new engine, I set the initial timing with the computer disconnected. When it hits 10 degrees with my timing light, the timing will fluctuate from 10-15 degrees and won't really rest on 10. When under light load the truck will hesitate and runs in a bit of a limp mode.

 

Do you mean with the spout plug disconnected? I doubt the motor would run at all if the Computer was disconnected.

 

yeah that's what I meant

 

Was this problem present before the engine was replaced?
Is the ignition module and distributor new or used?
Is the replacement engine new, used or rebuilt?
What about the plugs, wires, and ignition coil, are they new or old?

 

Engine is a rebuilt one, distributor is the one that originally ran in the truck and it worked fine until the swap, I replaced the cap, rotor, spark plugs, and wires with the swap. Engine starts right up and idles really smooth

 

What year is this truck?
Check fuel pressure.

 

Do 300's have a timing chain that could be old?

 

Sounds like either an issue with the distributor not being set correctly or even as far as how the gear is pinned on the shaft because it could be set 1/2 tooth out or it could be an issue with the distributor bearing even the installed height of the distributor could be too high.

I know you said the engine was new...Is the distributor new too and is it a Ford OEM distributor or one from the local AP store because theres a huge difference in the qualities in both.

Bought another truck dirt cheap yesterday. Tore into it today and found out the brand new reman distributor they put in had a shutter wheel that was way out of whack,,,It caused every cylinder to have different timing and fire differently causing the TFI module to go on the fritz making the computer do alot more corrections and calculations than it can handle...

It had bent vanes also the vane widths and the gaps in-between the vanes were all different and non compliant to Fords requirements. Spent 1.5 hours blueprinting the shutter wheel and also put a new Halls Effect pickup and TFI module and its running killer now.

I hope you get your problem sorted out...

The last thing that enters my mind as a possibility is the rebuilder put the old thrust plate back in between the engine block and the cam gear that has too much wear allowing the cam to walk.





Good luck

 

Gears, the big one usually has plastic coated teeth, that disintegrate.