methylmarty

Hello,

I have searched the forums for an answer to this problem, but it seems there are slight differences with my issue that maybe someone knows about.

The problem is in my 99 F-150. 225k miles, stock 4.6L Triton engine, stock auto trans (I think it's an E4OD), changed rear end gears to accommodate the 40" tires. Programmer to compensate speedometer. I am not sure what and when maintenance was done on the truck being that I am the second owner and the previous owner did his own work and kept no records.

What happens is that the truck starts normally but will noticeably run rough when I first accelerate. The truck will not rev over about 2000rpm. At around 15mph the truck will bog down to about 500-700rpm before (I believe) shifting into 2nd. After it bogs at 15mph, it will rev up again, no higher than around 2000rpm. Then, as soon as the truck hits about 30mph, the RPMs will drop to about 700 and it will just kinda coast like that with the accelerator doing absolutely nothing until it falls below 30mph, at which point the engine will fire back up again and repeat the cycle at 30mph. At no point can I get it to rev higher than around 2000rpm.

This problem happened intermittently. One day, problem. Next day, no problem. And then it started lasting longer.

So my natural tendency was to replace the TPS. While I did that, I also replaced the fuel filter and sprayed the throttle body with cleaner. When I started it up, it ran great for about a week. And then, coming home from church yesterday it started doing the same thing again.

Any ideas? I was thinking fuel pump, but it doesn't make sense that it would be able to shift into 2nd but not go any higher than 30mph when it is hitting 2000rpm and at that RPM and speed should be able to shift into 3rd. Plus I don't want to drop the tank. I hate that.

Maybe TCM? Or the programmer for the larger tires? This couldn't be caused by something as simple as spark plugs or coil packs, etc, could it? I just don't want to be throwing random parts at the truck if someone could pinpoint the issue.

Oh, no codes and no service engine light at any point in time, by the way.

Thanks in advance,
- Marty

Purplehazeir

Try Seafoaming your engine. Seafoam is available at most automotive parts stores.

Pour 2/3 of the Seafoam can into your engine via the oil fill and start your truck and let it run until warm. Then pull a vacuum hose off the engine manifold and suck up the rest of the Seafoam. You will probably have to hold the throttle open because the truck will want to flood out. Once the can is empty, replace the vacuum hose and take the truck out for a hard run - several full throttles runs (be safe and courteous on the road). Come back, let it cool down and do a oil and filter change with a heavier synthetic oil weight.

Then replace the wires, plugs and coil packs. If that doesn't fix it, then there is a need to dig deeper.