Sorry to batter you guys with my problems but this is my only vehicle besides my motorcycle and its cold out. I have a 86 f150 inline six with the feedback carb(unfortunately). The truck idles like a champ and drives fine on level ground, but once i start going up hill for a bit it miss fires pretty dang bad, i have to let off the gas to get it to stop and half the time downshift and go real slow.

I just replaced spark plugs, wires, cap, rotor, EGR valve, and rebuilt the carb. I measured the resistance to the coil it has 0.78 manual says its good from 0.6 - 1.0 base timing is set at 10 degrees and when i plug the spout back in it advances past the 14 mark, im not sure what its suppose to advance to at idle, i couldnt find that. Oh and it just passed smog today, haha.

Any help will be greatly appreciated.

 

Wen you step into it, seems ok, then dies out, let off the pedal, and kind of picks up? If so, classic cat plugging symptoms.

 

Might ask the computer what it thinks might be happening.

Fuel Injection Technical Library » How To Run a Self-Test

 

penguintits,

It sounds like you have tackled many things. Might I suggest a couple more? Have you replaced the fuel filter? Check for any cracked or loose vacuum lines. See if you can rock the air filter back and forth? If so, the carburetor is likely loose and the two nuts holding it down need tightening. They really loosen on the 4.9L due to vibration and having only 2 rather than 4.

Finally, several years ago while driving my son's 1984 [very similar system to your 1986] and going up hills or trying to pass another vehicle, it would miss badly. Like yours, it had just been tuned up so I looked elsewhere. With the engine idling [make sure your e-brake is engaged] I shook the wiring harness. As I shook the factory air filter housing, the engine stumbled. There was a single small wire [14 gauge?] which ran to the feedback carburetor [I am pretty sure if my memory is correct] and it had cracked and was grounding out when moved [due to engine torque]. I wrapped the area with electrical tape and the problem instantly and permanently went away!

 

Thanks for getting back to me so quickly guys, really appreciate it. The cat sounds like a likely culprit as i just replaced the muffler because it had a 6x6" rusted hole through it, this truck has been through Alaska so some rust comes along with that.


The carburetor is on tight i torqued it down after i rebuilt it 3 or 4 days ago. I tried going through the codes but couldn't get my code reader to work on it ill take a look at that link, thanks. I will also wiggle some wires like you said.

Also I took the feedback solenoid off the carb and plugged the hole and connected it back in with the connector still plugged in drove it, not a change in difference. Shouldn't that make the truck run like crap? Because the feedback solenoid controls the fuel mixture, correct me if I'm wrong, but im thinking about replacing that as well. Thanks for the input and I'll try to look mat it later tonight.

 

Also how would you check if the cat is bad? and it should be good if i just passed california smog 2 days ago....

 

Apparently there is a device that can be inserted into the hole normally occupied by the O2 sensor, I believe it's connected to an air compressor and one measures the output at the tail pipe.

Something like that.

 

Sounds costly, ill probably just drop the exhaust and run straight headers with a test run and see if that works...unless theres a o2 behind the cat...ill have to check

 

No O2 behind the cat until OBD II I believe.

 

Your correct fellro i just looked and i can drop the whole exhaust behind the headers, looks like a tomorrow project but and easy one, except for dealing with rusty bolts...I'll keep you guys posted.

 

Well i dropped the exhaust today and it still has the same problem and its gotten worse. Im sick of it and instead of throwing more money at it ill go ahead and do a duraspark II conversion or something like that. Just passed smog so i got 2 years till I have to swap it back, worth it in my opinion. Any input on what year of trucks work as far as wiring harness and distributors?

 

If nothing above works, might also be low on spark.


Try removing all the plugs, and closing the gap, making the gap alot shorter, if performance picks up, you have low spark power.

 

Well I feel kinda stupid not checking this before, but it was the fuel filter. I kept forgetting about it every time i went to napa for parts till today. Man that woulda saved me some heart ache. Also there are 2 vacuum lines that are suppose to go to some valve with an electrical connecter, which is missing, so the lines are open. I plugged those at the same time i did the filter, but im pretty sure it was the filter i could hardly blow and air through it. Thanks for the help!