I need help. I've got an 89 F-150 4.9 L 4X4 4spd standard.
It cuts out between 35 and 55 mph when crusing at steady speed. When I decrease or increase the gas peddel it stops . It has no power loss. I've had the truck tuned up. Cap rotor plugs wires , took it to a dealer they powercleaned the fuel injectors and said it had carbon build up ... I don't believe it. I took it to a local truck mecanic. He didn't buy the carbon story either. He replaced. egr valve, ECU , fuel injector relay switch., throttle position sensor, throttle body gasket, Map sensor, and the cat was changed about 20k ago . truck has only 110K on it. It runs good onthe high way, above 55.. and at low speeds around town. some times the batter lite will come on when it cuts out and some time it doesnt. I could use some feed back from anyone who might have had this problem.

 

I know you just did a tune-up, but it sounds dead on a misfire problem. You could have gotten a bad set of plugs,wires, cap or, rotor, but more than likely just the wires or plugs. I would try swapping in another set of plugs and see what happens.

The battery light coming on makes me wonder a bit too. Has anyone checked out your electrical (charging) system? You may have a wire or connector that sometimes rattles loose.

I can't beleive the mechanic didn't think to check the fuel system either if he replaced all those parts. A weak fuel pump can cause a stumble problem.

It's also a chance the timing is a bit off. I really don't think it would be the timing as there are no other issues you have metioned. I can tell you the 4.9 uses a steel gear meshed with a fiber gear, no chains or belts. I have seen those fiber gears get pretty worn and the truck would still run.

 

what do you mean by cuts out? Like does it just start running poopy or just shut off?
Is your alternator running at the right charge? Have you replaced the fuel filter? Does it matter which gas tank your on?

Check your fuses...I forgot which one controls the speed cut-off but maybe it went bad...I dunno but worth a try.

 

Hey. Welcome! Some one else from this beautiful state.

Does this happen in any gear/rpm? Does it happen all the time? Has it become progressivly worse?

I really don't have a clue but I figure the more info out there, the more likely it is that some one will figure it out.

Good luck!

 

here is an update"

plugs and cap and wires had been changed twice so I think I got good ones in it. Also the fuel and air filters have been changed. what I refer to as cutting out is it has a slight bucking similar to when a tank runs out of gas but not as bad.
It only happens when I'm trying to hold a steady speed between 40 and 55 mph in 4th gear. It doesn't matter whitch tank either.
I also tried all grades of fuel and just about every brand.
Ive had this vehicle for about a year and it's been the same since I got it . no better or no worse.
Also forgot to mention the first time when they cleaned the injectors they checked fuel pressure and said it was ok a14 lbs.
the second mechanic also changed the fuel pressure regualator on the throttle body.

thanks for the imput. maybe Ill get lucky and it will be just a fuse.
still scratchin my head though.
thanks again