Intermittent stumble in my '94 F-150

1994 F-150. 167K mi. 4.9L I-6 engine.

The problem: for a while now, the truck has been occasionally stumbling (misfiring), in an isolated fashion–once, then not again for several seconds or minutes. The problem seems to be worse on the highway, generally only happens once the truck has been running a few minutes, and is most likely when goosing the throttle from a cruise setting–like when adding throttle to maintain speed while climbing a hill. (WOT, however, is not any more likely to have this problem, and probably less likely.) If pressed, I’d say the problem is somewhat less severe on the front tank vs. rear–happens on both tanks, though, and fuel level seems to have no impact.

Upon initial troubleshooting, I observed distributor rotor and cap to be worn and carbon-ed up…so I replaced them, as well as the wires, to no effect. (Plugs have 12K mi on them and look good.) Next, I replaced the coil, which also had no positive or negative effect.

Now, I’m thinking it might be a fuel delivery issue…either the fuel pressure regulator or maybe two roughly equally-tired fuel pumps. How should I approach this? I can buy a fuel pressure gauge to connect…but I’d really like “real-time” data vs. hood-up-and-idling. Do they make something like this?

How would y’all approach this? I’m inclined to break down and buy a half-dozen copper-cores, on the premise of “plugs have no shelf life,” and keep them in storage if no effect. Beyond that, though, I’d actually pay a financial penalty for continuing the “shotgun” approach.

The only ‘real-time’ data on the fuel pressure is to hook up a gauge and tape it to the windshield so you can see it as you drive. Most '96 and newer OBD-II system don’t monitor the fuel pressure. But, have you considered the fuel injectors? Dirty injectors can cause a stumble like you describe.

You can monitor fuel pressure while running, but I suspect that of the fuel pressure was not being regulated properly you’d see it when revving the engine while parked, and it the pump(s) is(are) dying, you’d feel a definite loss of power.

I’d see if you can rent an 'Event recorder" from a parts place. That’ll allow you to monitor the operation of the ignition system while the truck is running. I’d also strongly suggest that you get someone else to drive while you do this. An accident would not fix anything.

My gut is suggesting a failing crank sensor. Don’t change it based on my gut, however, just keep it in mind while you’re troubleshooting.

Yeah, I was kinda hoping not to have to fiddle with the injectors. It would make sense, though…did a head gasket 3,000 miles ago and obviously had to disturb the injectors to do that…who’s to say they didn’t pick some schmutz up?

I just REALLY hope it isn’t an ECM on the way out!

This doesn’t have that “failing ECM feel to it”. To me it has more of an “erratic spark under load” feel. I thought of the cranks sensor because it’s one of the few ignition system components (term used loosely) the operation of which might be subject to engine (crank) loading, like when you accelerate.

AN UPDATE: Problem was happening around once per 5 miles or so. Drove a 1500-mi round trip recently; in addition to an isolated, intermittent miss, truck twice misfired for 15-30 seconds at a time. Testing showed switching tanks during misfire had no positive or negative effect, and truck could maintain speed on a slight grade, suggesting (at least to me) only one injector affected.

After the second time, problem resolved itself and has not returned at all in last 1200 miles.

So…would it make sense that an injector could do the equivalent of “passing a kidney stone”? 'Cause that’s how I’m going to score this one, unless somebody thinks I shouldn’t: some largish foreign object was interfering with the opening/closing of the injector, and was ultimately passed into the cylinder and out the exhaust, resolving the problem.


Does your fuel rail even have a test port?

Yeah, but I can get the $50 back for the gauge if I don’t use it. That’s why I’m asking–no symptoms remain, would you just call it a day?

Not a mechanic but I have had the same problem with my 91 F-150 300 straight 6. Went through the same repairs. Replaced plugs, wires, distributor cap and rotor to no effect. Recently have started adding a fuel injector cleaner to the gas and there seems to have been some improvement. I have used the more expensive cleaner not a cheap version.