Hey all, I'm having a bad surge in my truck especially under load. Its not a misfire, but more of a huge power loss. It would be like if youre driving 50 mph down the road and the engine just drops to idle, then rpms come back up. But as soon as I give it gas, the truck stumbles again . The rpms are constantly rising and falling.

Sometimes it'll run great up to 70, 75 mph, then out of no where it starts acting up. It'll surge all the way down to a dead stop then keep surging while idling. I've noticed, if I shut the truck off for about 5 min, it'll let me get up to 70 mph again but then start cutting out again. Kind of hard to drive 20 miles into town when you have to pull over 3 or 4 times.
Lately its gotten so bad that I cant even make it down the street now.

The truck sat for a few years cause it needed a fuel pump. After replacing the fuel pump, and getting some fresh gas up to the injectors, the truck ran great. But slowly over the past two weeks it's been getting worse.

The check engine light was on due to TPS sensor and IMRC's stuck open. I replaced the TPS sensor and I replaced the vacuum actuator for the IMRC solenoids. I've also replaced the plugs, wires, fuel filter, air filter, oil change, I replaced the vacuum reservoir in the passenger fender due to it being cracked. No vacuum leaks now, all hose looked in great shape.

I'm coming to an end and dont know what to do. I've never had a problem like this before. I just cant figure it out. Im hoping someone will have a fix that I might have over looked.

The truck is a 1997 f150 XL 4.2L manual transmission.

Any help would be really appreciated

 

When you replaced the fuel pump did you look in the tank for any foreign matter . There is a sock type screen on the fuel pump pick-up tube . If there is sufficient garbage in the tank it can clog the screen of that pick up and the vehicle can do just as you describe. That is one of a few things that come to mind.

 

I pulled the whole tank off, emptied it, and then cleaned it out.

 

Since we don't know at this point whether we are dealing with an ign , fuel or electrical problem, best thing we can do is to try to eliminate what is easiest to eliminate.
If you were able to obtain a fuel pressure gauge (rent) ,hook it up and watch the fuel pressure when it starts acting up. If the pressure drops, then you can confine your efforts to the fuel supply system. That can also include the electrical supply and control of the fuel pump.
If the pressure stays up and doesn't drop you might look for some type of restriction in the intake system or exhaust system.
If you can eliminate those systems , there is the electrical/ign system to go after.
From what you have experienced , would you think the problem is temperature related?

 

I'll rent a fuel pressure gauge today, and follow up on the ideas you gave me. I'm willing to take and advice and try anything I can. No I don't think it's temp related. The truck runs right under normal on the temp gauge and the fan clutch kicks in when it's suppose too.

 

Sorry for not being more specific. What I meant by temperature related was, does the problem occur when the vehicle is run only after it has had time to get thoroughly warmed up. Does it happen right after you start it or does it take a while for the problem to appear.

 

Oh sorry, I wanna say it happens after it warms up. But it doesn't seem like it has to be at normal operating temp for it to do it. Its hot as hell in South Texas so it gets to that temp fairly quickly though. I took it down the road earlier, and all seemed great, until about 2 miles later it started surging again. Turned around and and it wanted to die coming off the clutch into first. After that it stumbles about 3/4 of the way to the next shift. Every gear is like that. It's like if I hit a rev limiter at I'd say 3000 rpm. The truck doesn't come with a tach, but from listening to the motor that's what I would guess.