'94 F 150 4.9 FI manual 2wd 95k miles

Hi Guys,

I've had the same problem for 5+ years, have had it in the shop 8 times, 3 times to the dealer and nothing has changed. I bought the truck new and have taken very good care of it.
When the engine is started it runs fine for 3-5 minutes and then starts to buck, jerk, hesitate for several seconds and then run okay for several seconds and then bucks, jerks, etc again for several seconds. Sometimes these intervals are longer and sometimes it runs okay for a couple minutes. On the hiway you can just feel an almost constant miss, but sometimes it's not there. The problem is much worse during the winter. If the truck is started while the engine is still warm these symtoms begin right away without the 3-5 minute warm up. It seems to be getting worse the past couple years. I get 14mpg combined city/hiway.
One mechanic said the fuel regulator could be sticking (it has 45-50psi) and to take it to the dealer and have the computer flashed. The service rep I talked to said they could change this part ? and that part ? but it would still run the same. I didn't have them work on it and don't remember the parts because it was several years ago. I recently had it back to the dealer and they change the EGR valve,evap sensor and the pvc valve and it still runs the same. About 5 years ago the temp and ox sensor were changed because of a check engine light (lean condition). The light did not go away and the next winter I think it started running rough but I'm not sure. I had it in to the dealer for the check engine light and they couldn't find anything wrong. The light eventually quit blinking, could the bulb burn out or did someone find a loose vac line and put it back on while they were working on it?
Here are the parts that have been change and didn't affect the way it runs one way or another.

Temp & Ox sensor
Fuel filter
Different brand of fuel
Accel coil, wires, dist cap & rotor, plugs
Egr valve, evap sensor, pvc valve
K&N air intake system

If you can think of anything else to check or even replace again please let me know. Thanks alot.

 

Did you try a compression test on all the cylinders? This could rule out a warping valve or head.

 

No I haven't done a compression test 'cause I thought a weak cylinder would miss all the time. Never thought about a warping head. I'll have it checked. Also, would a restricted exhaust system cause this problem? How do you determine that?

Thanks for the help.

 

Typically a restricted exhaust would cause this all the time, and there would be very little power. If you want to check it, you can pull your exhaust off from the cat back and run it like that for a while. It will be loud, but you'll know if it is the problem pretty fast.

 

It will probably be next week before I get the compression checked. Think I'll hold off on removing the exhaust sys.

Thanks

 

sounds like you COULD have a bad catilitic converter. i would do what the post above says. if thats not it, there are other things that should be checked...clean your maf sensor, make sure all your injectors are working fine, check for vaccuum leaks. good luck with it.

 

I haven't tried anything new lately. Now that two different spring breaks are out of the way I need to keep at it. But the weather has warmed up and the truck runs much better but still has noticeable hesitation, buckin' and jumpin'. When the temp is 20 deg the symtoms are really bad, but now that the temps are in the 70's it's alot better. Would the warm temps making the truck run better help narrow down what to look for. The injectors and vaccum leaks have supposedly been checked by the dealer. Looking over my receipts I realized for the past three years I had it in the shop for this problem during the month of March, after putting up with it all winter. So this is temperature related, isn't it?

Thanks

H Bill