Return of Code 31...

[Gary Lewis]

I don't think it is related to the driveline.  I think it is to do with the EFI, like the air/fuel ratio is going lean.  If it had a carb then I'd say your accelerator pump is faulty.  But it EFI not a carb.

Maybe the throttle position sensor is faulty of improperly adjusted.  If you ease into the throttle and don't move your foot at all after the hesitation starts does it change?  Or what if you go past the point where the hesitation starts?

[ckuske]

It is a carb though.  EEC-IV Feedback Carb system.

I have had the carb gone through and rebuilt twice now, including new accelerator pump.

I am not questioning you at all, but applying the Socratic Method here... if it was a fuel/air mixture issue, why would it only present itself under load?  I can make the same gas pedal motions in park, and it doesn't have the issue.

The truck has a new TPS also, but I haven't tried to test it for dead spots - I'm guessing that's what you think may be happening?  The TPS has a dead spot in the beginning of travel so the mixture isn't being adjusted?  

[Gary Lewis]

Oh, my bad!  

Without load the engine can easily continue to spin through a lean spot.  But under load when it bogs the load drags the engine down.

I would test the accelerator pump.  Pull the air cleaner and, with the engine off, look down the throat of the carb and open the throttle a bit.  You should see two strong streams of gas shoot into the throat.

There should be several positions on the accelerator pump lever that allow you to adjust how much stroke the accelerator pump is given when the throttle is opened.  I'd move the arm in a notch or two from where it is to give it more stroke with the same throttle opening and see if that helps.

[ckuske]

Haha, no worries!  This truck is an odd one for sure.

I'll try that again.  I've fooled with it before, and it tested out ok and giving it more stroke didn't help.  But now that I have eliminated other variables, perhaps it is worth another shot (no pun intended)

To give more throw, I should move the rod closer to the carb, or further away????

[Gary Lewis]

Yes, I'd try it again.  

The illustration below is from the factory shop manual section on the 2150, which is shown on our page at Documentation/Fuel & Air Systems/Carburetors, Chokes, & EFI/Motorcraft 2150.  On the right you can see the slots and on the left the accelerator pump.  If you move the rod to a higher #'d slot, meaning farther out on the arm, then when you open the throttle the accelerator pump arm will be forced to move farther and you'll get more fuel squirted in.

[ckuske]

Ok, thank you.  Tomorrow is another day!

[Gary Lewis]

Make sure you are getting two strong streams, and if so then adjust for more.  If not you need to find out why not.

I've seen the nozzles plugged from rust in the tank.  That's an easy fix as you can take the nozzles off and clean them out using a small piece of copper wire - copper is soft enough not to hurt the nozzles but strong enough to push junk out.

[mat in tn]

this really sounds like a carb issue. a flat spot at tip in is normally a lean adjustment and/or a vacuum leak. the squirters are not a fine-tuning component yet are a compensator. something an efi system is better at since high pressure atomization forces mechanical vaporization as opposed to a liquid. once you have covered all the bases looking for a vacuum leak it may be time to adjust the carb. using a vacuum gauge along with a timing light with a tach you should be able to tune the carb to a high vacuum reading along with the most stable rpm. at this point it will find a vacuum leak sensitivity the easiest. while testing for vacuum leaks, watch the rpm not the vacuum gauge. it will be the most sensitive.

[ckuske]

Thanks Mat.  I've been down this road before (smoke machine for vacuum, vacuum gauge to find peak vacuum at idle) but I will need to try again.

It seems my theory is debunked haha.