Hello all,
I've got a 1991 f150 4.9l 5speed 4x4 160k that's having an intermittent bucking issue while cruising. The problem only exist while cruising down the high way between 50-60mph. I will be driving and the whole truck will just jump like its lost spark or fuel but then immediately after it will return to normal. It does this while running both fuel tanks. One weird thing it does is that when this happens if i give it more gas the bucking will stop and the truck will accelerate like normal so this leads me to believe it is not a clogged fuel filter or dirty injectors or something. Ive had the truck for about 2 years and done quite a bit of work to get it to where it is now. Just for the information new parts include.... alternator, water pump, belt, smog pump, egr, evp. egr charge pipe, thermostat, all heater and radiator hoses, battery, clutch kit, flywheel, slave, shifter kit, o2 sensor, all new vacuum lines around the motor, idle air control valve, cleaned throttle body, new sending units in both tanks, rear tank fuel pump, both gas tanks, i think thats about it. This is my first viehicle and ive tried to do everything i can to make it a reliable one. I pulled the codes last weekend due to a cel and got a 181, 327, 328, and a 334. This is what lead to the evp, egr, o2 sensor, and vacuum line replacment. This fixed the cel but the bucking still remains. Any help would be greatly appreciated thanks!

 

My guess would be TPS or MAP sensor.

 

And what spark plugs, wires, cap and rotor replacement? Never overlook the basics.

 

Sorry I forgot to mention but i did replace the distributor cap, rotor, icm, wires, and even Motorcraft plugs. Is there any way i can test the tps or map sensors? Guess i should replace both anyway just a little short on funds at the moment being a college student and all. Ive got a multi-meter so. Thanks for the replies so far!

 

Yes they can be tested, just do a search for "Ford TPS" test or something like that. You'll find millions of pages.

Bucking generally means the engine is going lean. Have you checked for vacuum leaks?

 

Map Sensor

TPS

I would not throw parts at any problem. Many aftermarket parts suffer from quality control issues. Stick with what you have, test, verify, then replace if necessary.

I agree that bucking is usually caused by a very lean condition. Eventually you may get a lean code again. If the MAP and TPS check out it's time to throw a pressure gauge on the fuel supply. It could be low fuel pressure caused by a faulty Fuel Pressure Regulator. Also keep in mind there are vacuum reservoirs that are known to rust through then vent to atmosphere.

 

I saw I had a few split hard plastic vacuum lines so I replaced all of them with back 5/32 vacuum hose. The truck idles alot better now too and even a little lower btw. I felt around the coffee can and didn't feel any rust holes but I did not test it with a vacuum gauge or anything. Should I test it if so how do i do that w/out a vacuum gauge?? Saw a youtube vid of a guy that used his old conector and a 8'' section of 4'' pvc with two end caps then used a holesaw and some epoxy with some success. I will be testing the tps and map sensors this weekend I found two very detailed articles on troubleshootmyviehicle.com and i will be reporting my findings as soon as I do. Thanks all for the help!!

 

I would put a container of fuel injector cleaner in one of the gas tanks. Just follow the directions on the bottle.

I have had a problem like that before and tried several things that didn't help. The fuel injector cleaner cleared it right up. Your results may vary.

 

Hello again, I tested the map this weekend and the middle wire grounded with the key on would only read 2.56 volts and i got the same reading with the truck running but when i disconnected it i got something totally different so im pretty sure the map is fried. The tps was really dirty so i didnt even test it i just replaced it since it was only 24 bucks. Hopefully these fix my issue i will be back tonight after I drive back to school 60 miles to tell wither or not it did in fact repair my problem. thanks to everyone that has helped me this really is a great place!!!

 

MAP output is variable frequency, not a DC voltage. Unplugging the MAP puts the computer in limp mode so it uses default values to run the engine.

 

You cannot test these MAP sensors by measuring voltage. Ford MAP sensors use a variable frequency output instead of a variable voltage output.