Just picked up a 99 f-150 with the 4.6

Owned by a older guy and was in great shape for the age

Well driving it home I noticed that once I get going it shudders when accelerating, and the faster I GO mph the worse it gets but as soon as i take my foot off the gas it goes away.

And when I floor it the thing takes off like a raped ape and doesnt do it, only when slowly accelerating

It really didnt do it on the test drive (dirt road) but on the way home I had to drive down a very ruff dirt road,

It has 205,000 miles but the engine and truck look like there is 50,000 miles

Its a ext cab 4x4 with the 4.6 automatic


Thanks

Rich

 

Rick, honestly, you bought awell used truck in terms of mileage.
It just needs TLC.
The issue sounds like ignition.
If there is no code set, you can either take a chance on replacing the coils and plug wires if equipped with that hardware or just the coils, then proceed to change spark plugs to complete a tune up.
Otherwise use a Scanner to look at live data and stored data in mode 6.
Bottom line, it's not simple either way from starting cold like you have to do after buying the truck.
Good luck.

 

Does this TSB sound like it might apply?

 

It sounds like it might be this but this builtin says from a stop and/or stopping.

My issue really doesnt happen when starting out its once I get going and then trying to speed up. I thought that it might be a u-joint since it doesn't happen upon heavy acceleration or slower speeds

I know its a used truck and has high miles but its very clean for its age

I can also test the coil packs by disconnecting one at a time correct?

 

Well no you can't test coils that way for the possible problem.
Why; the possible issue has not been explained until now.
.
When in OD and light throttle applications, the EGR system is called to open.
When that functions the air to fuel ratio goes very lean, the ignition timing goes advanced and fuel is cut back.
Under these conditions any coil that does not have full output capability can cause a misfire or stumble you will feel.
This condition will not set a misfire 'code', so you cannot tell which cylinder or coil is the cause.
Now, once the trans down shifts or you go into acceleration mode, the EGR system closes and fuel returns back richer. The faulty coil now has enough output to fire the cylinder reliably so the problem disappears until the next time those conditions are entered.
To find a possible faulty coil a Scanner must be used to look at mode 6, test 53.
At that history point you can see the misfire counts for all the cylinders.
This history location cannot set any codes.
Any cylinder that has high misfire counts out of limits is suspect.
This is why you cannot find a faulty coil just by disconnecting it.
Aren't you glad you asked?
Good luck.

 

Thank you for that indepth answer

If I dont have a scanner should I just replace all the coils as part of routine maintenance

 

You can if the expense is not an issue.
Good luck.

 

Another question is the truck had been sitting for awhile. Could it just need to be driven for awhile to get things moving again?

 

Is the gas old?
Is there water in the gas?
Is the fuel pressure in spec range of 35 psi +/-?
Are the fuel tables shifted too rich or lean?
No guessing, check these out.
Good luck.

 

Mine is shuddering too, worse under heavier loads. Seems like its the 3-OD shift. Its a 2WD and a 97 though.
I do have poor compression in 2 cyls (50 psi) and it does throw a misfire code pretty often. It also seems to have a head gasket leak, however oil's not getting water, seems to be pumping compression into the coolant for now. Is it likely a transmission problem so I can do that when I have the engine out to rebuild it (soon)