I did a search on "hesitation" and found some good information but would like to find something more specific to my truck. I have a 1985 F150 351W and stock 4bbl carb. 101k actual miles on the vehicle. About 500 miles ago the plugs, wires, cap, and roter were replaced. About 100 miles ago the carb was rebuilt. Also replaced a blown fuse that applies to the choke heater.

After the carb rebuild and fuse replacement, the truck has started and ran great, even in 10 below zero temperatures.

But as of today, I have a hesitation problem. I can accelerate up to 25 mph or so, then it starts to lug after trying to acclerate beyond that speed. Any hard acceleration at any time will cause it to lug also.

I had this same problem before the carb was rebuilt, but only in very cold weather (below 0) and the problem would go away after the truck warmed up. Now it does not go away, but seems to get worse the more I drive.

Based upon the search I did, I narrowed the problem down to a few possible causes:
1. Bad fuel pump
2. Bad O2 sensor(i have no documentation of the sensor ever being replaced)
3. Fuel filter(s) plugged/froze up. Don't know how many filters this truck has.

I would appreicate any and all advice!!

 

If it's the original O2 Sensor, then that should definately be replaced. It is a tune up Item. This can cause your symptoms, cause the o2 semsor can foul and plug up with exaust and cause the computer to be fooled into thinking it has a cronic lean condition. Thus it richens the mixture beyond tolerence to compensate for a erronius lean condition that does not exist, causing the exact opposite to occur. When the engine is cold, the choke richens the fuel mixture even more etc... Would be the first thing I'd replace and see if it helps. It wont hurt to replace it and should be replaced every 50,000 to 80,000 miles anyway. I replace mine every four years with every other tune up I give my truck, and arrange my tune ups to coincide with the California two year smog check.

A malfunctioning EGR valve can cause a symptom like this as well under Moderate to heavy acceleration. It can cause sputtering, engine stall and severe hesitation. This should happen when the truck is warm though.

Then it could be the following parts too, if they are equipped...:

A malfunctioning ported vacuum switch.

Vacuum check valves.

Malfunctioning computer, and or sensors.

Timing out of adjustment, or lack of vacuum or electronic advance

Bad ignition module.

etc....

Then of course it could be the other things you mentioned too. Fuel filter, fuel pump etc...


Hope this gives you some ideas.

 

It sounds like a fuel filter issue to me. Get a new filter and swap it out before you do anything else. It should only be about $5 or so.

 

Thanks for the help guys. I will head to autozone this weekend for a fuel filter and an o2 sensor.

Does the o2 sensor require a special tool for its removal?

Thanks again!

 

Sounds like maybe some deicer is in order and that stuff is cheap. I had to throw in 1/2 litre before I managed to get rid of the problem which sounds very similar to yours. It would warm up outside and the problem would go away. I put 1 1/2 of those stupid little bottles in the gas tank but it wasn't enough add a few more of them.

 

Ummm, I doubt it has an O2 sensor or even an OBD computer. The OBD computer is an injector controller(among other things)but this is a carbureted engine.

The de-icer may help, these carbs are pretty good at freezing up.

It could be that there is a clogged port that didn't get completely unclogged during the rebuild also.

Maybe something here can give some ideas...
https://www.ford-trucks.com/forums/5...-question.html

 

When I was getting ready to leave for work last nite, I noticed a can of seafoam in the garage that I didn't know I had. I had some extra time so I decided to do what I read about on this site a few months ago, and sucked about 3/4 of the can thru the brake booster hose then dumped the last 1/4 in the tank.

I let it run for 10 minutes then proceeded to leave for work. Absolutely no improvement in performance. In fact, it actually seemed worse, i.e. more hesitation and even more trouble getting above 25 mph. I held the gas pedal to the floor a few times and got the speed up to 40 mph or so but still had hesitation. I was afraid I made the problem worse by using the sea foam.

Got done with work this morning and had trouble getting the truck started. Once I did it continued to run like crap with really bad hesitation. Once I got it warmed up I took it out on the hi-way and held the pedal to the floor until I hit about 65 mph and for the rest of the morning, it has been running great with no hesitation whatsoever. I tooled around town and took it back on the hi-way and it runs great.

I will continue to drive it over the weekend and if things change for the worse I will post here again.

 

Take a look at my gallery at my 1981 Ford F-150. You will find a OBD computer, called an MCU, (microcomputer control unit) with a carbureted engine, with an O2 sensor and other junk.

All feedback Carbureted engines have an O2 sensor, and OBD type computer of some kind. The most comon are the EEC-IV, TFI-IV, carbuereted engines begining in 1984.

 

Thanks 81-F-150-Explorer. I stand corrected. I haven't personally run into that on carbureted engines before 85. Maybe a lot of it is related to the California emissions.

I have read up on them a little but is the MCU system readable, just turn on a dummy light or give no feedback? Just a matter of symantics but to me an on board diagnostic (OBD) system needs to be readable in order to carry that name. Granted the early OBD's were far from standardized in the diagnostic info they gave and the way they spewed it out so anything is possible I guess.

 

A MCU is definately an OBD type computer, that uses the same code readers as the later systems. It does not store codes however in memory. They need to be read in real time.

As an example, Codes for the MCU for my truck are as follows for the ones I remember off my head...

Code 11: All systems check OK.
Code 33: KOER test operator error.
Code 41: Fuel mixture out of specs to lean
Code 42: Fuel mixture out of specs to rich
Code 44: Thermactor System malfunction
Code 45: TAB Solenoid - Or Visa-versa
Code 46: TAD Solenoid - Or visa-versa
Code 51 or 61: Temperature vacuum switch malfunction
Code 52 or 62: Closed throttle vacuum switch malfunction
Code 53 or 63: Wide open throttle vacuum switch malfunction
Code 56 or 66: Crowd #3 vacuum switch malfunction.


EEC-I and EEC-II are non OBD type computers only for the fact they are not self diagnosing. These are only found in cars though IIRC.

 

Try replacing the TFI module