My 91 F150 has developed a hesitation and chugging at slow accelleration. It idles fine and runs fine at strong accelleration. I never get a check engine light. And since the weather has turned colder here it seems to run better before the engine warms up. I have recently replaced plugs, wires, rotor, cap, and fuel filter. Does this sound like the EGR valve? Thanks, Steve

 

I have the same problem. The engine used to hesitate when the wheather gets colder, but this winter it is abnormal. It really HESITATES at low speed range. High speed is without problem. I have serviced everything in the emission control system, ignition system and of course tune ups. Next thing I has to investigate is the fuel system. Does anyone know where to start without burning hard to find money ?
Fuel pressure regulator? Injection nossles? Fuel pump?

>My 91 F150 has developed a hesitation
>and chugging at slow accelleration.
>It idles fine and runs fine
>at strong accelleration. I never
>get a check engine light.
>And since the weather has turned
>colder here it seems to run
>better before the engine warms up.
> I have recently replaced plugs,
>wires, rotor, cap, and fuel filter.
> Does this sound like the
>EGR valve? Thanks, Steve

 

For both of you,scrub the throttle bores & plates clean with a tooth brush & solvent,even if they look clean. The brush does the trick for the edges of the plates. Hold the throttle open and get busy.Also clean the air bypass valve and check for continuity at the bypass solenoid.Someone here has found a dead one on a truck that otherwise ran decent at high speeds but was sluggish at low.
Phillip

 

I have now cleaned the throttle body. There was a lot of buildups but now it is shiny clean. There was also a lot of buildups in the beginning of upper intake manifold ( appr 5 mm, 1/5 inch !).
However, the action is done without any noticeable effect, except for the blinking check engine light.
The engine still runs very bad at low range. The idle speed is ok, the top speed is ok, but in low range with load the engine does hardly not run at all.
As earlier told, the problem occurs when the weather is cold, i.e. 0 degr Celsius and below (freezing point and below).

 

Now it's time to check MAP sensor operation as well as Coolant Temp Sensor operation.
Phillip

 

How do I check the map sensor? thx PG

 

PG !

I hate to tell you, but last night I proceed with the detection. I was wondering if I had a "black" sparkplug. And see - number 7 was in the right colour, but a crack in the insulation. I possibly cracked it when I put it in last time - the first tryout to cause the bad thing.
With a new plug the engine works well !
Conclusion - scrubbing the throttle bore solved my problem. The engine works better the ever !

Merry Christmas !

 

how did you clean the throttle? what solvent did you use? did you remove it from the engine? please explain

 

Dear John !

In my case, it was impossible to clean the throttle when it was mounted. It was a lot of buildoups.
I simply removed it and cleaned it. I tried a couple of different solvents but none was doing the job. Maybe the best one was wat we called here in sweden, 5-56. It is a pressured can with normal penetrating oil, the one you are using when removing rusty bolts. However, the best result was to scrub it with at tothbrush. I won´t tell you, but i used a really sharp knife also. Take care you don´t scrath the bore to much.

Before cleaning, I also removed the throttleplates. They are fixed with two screws, and can only be reinstalled in one way.

The knife was usefull even to cut out the buildups in the upper intake manifold inlets. I was careful, and so should you, that nothing falled down in the manifold.

The action really improved the engine, and today I have been out in the mud in a small local trial competition. No hesitations, and no stalls - maybe therefore I won the race !!!

 

thanks for the info & advice! have a great new years holiday!!