slight bucking at hwy speeds?

Hello all!

We recently purchased a 99 XLT 5.4L 4x4. It "only" had 151k miles on it and overall the SUV is in awesome condition. Only major issue it had was losing fuel pressure, which caused long start times. I replaced the fuel pump and the thing starts like a champ now (there was a hole in the rubber fuel line between the pump and the rest of the assembly).

One issue it is experiencing though, which is more of an annoyance, is at 45+mph, if i even SLOWLY take my foot off the gas, it feels like someone quickly pressed the brakes and the car seems to decelerate pretty quick....then if I SLOWLY press the gas again, the car bucks a little.

I can find the sweet spot too, to where if i let off and press the gas over and over (were talking moving the pedal a matter of like 2 milimeters), the car kicks back and forth... So i really dont think its anything sticking around the throttle body....Maybe TPS? Doesnt feel like a mechanical issue...more vacuum or electrical...

I've done some research, but a lot of posts end up being a known issue with that make/model of vehicle...really scratching my head on this one.

FYI
1. There is no engine light on
2. No, the engine light bulb is not burnt out
3. No stored codes

Any suggestions are appreciated! Let me know if you need more info!

I had a bucking under load at hwy speed that was a combination of two things.

A weak ignition coil, and the torque converter shuddering because of very old fluid.

I changed all the coils and flushed the trans fluid/replaced trans filter, and my 2002 with 177k on the odo drives like new. Really impressed with the difference just with these two things changed (coils and fluid/filter).

I would say its not a sensor since that SHOULD throw a code. I would check your trans fluid as suggested above, could be old or low. I doubt its spark, since you experience no problems with startup.

My first thought is to say "welcome to the E4OD transmission."

I was having the same issues you described about a month ago and ended up replacing ignition coil on cylinder 3 and 4. It never did throw a code and I had the issue for almost a month prior to switching out the COP's. I haven't had any issues since.

So i second the weak ignition coil as mentioned above.

Most likely transmission related. Shift solenoid or clutch band. You will need a repair manual for this.

Thanks all for the replies.

The transmission fluid was actually a little overfilled, but there is nothing odd with the fluid that I could see. I will look to get it changed out in the near future, as well as the filter.

The previous owner actually replaced all the COP a year or so before I bought it, but I couldn't tell you the make of the new coils...He did give me a box full of all the originals though, and said they were still good, he was trying to fix the startup issue (which like i said, was the pump). I'll look into doing the COP's soon as well.

Until I read "did give me a box full of..." I would chuckle and say that you can never trust a previous owners tales of your truck. Assume everything is original and has never been touched. at your mileage (get that, and your model, etc, into your signature file) you can assume that everything needs a "replacement" in the nearer future, so you may just want to consider listing out all the things you want to do in general. This allows you to attack a system, like ignition or fuel or transmission, vs trying to find the one bad part. It should save you time and money in the long run.

Clean your throttle? check springs and cables, its easy enough.

Ignition Coils? Would that not cause a noticeable startup problem or problems across speeds?

(not asking to be an ass, asking questions is how you learn)

I had two bad ignition coils, but replaced all of them because they were 14 years old, and were all original. One coil (#8) was flat out DOA, and made the vehicle run like total crap at all speeds. And even as bad as it was running, it took FOREVER to throw a damn code so that I could see which cylinder was acting up.

After replacing that one coil with a loaner (motorcraft) from a buddy, it would run ok at light load, but up hills and to much throttle demand in 4th (overdrive) gear and it would buck and snort in protest, but never threw a code. A coil was weak, but not dead, I assumed.

So, I replaced all the coils, and then still had a bucking issue in overdrive under heavy load, up hills mainly, that was not as bad as the ignition coil issue, but was still annoying.

So, I dropped the trans pan, replaced the filter, cleaned out the pan and magnet, then added about six quarts of fresh fluid, then disconnected the return line on the back of the transmission (4r100) on the passenger side, connected six feet of clear 3/8 hose, ran this to my bucket, and started the car. Once my drain bucket was as full as my first initial pan drain (and I could see the first sign of air bubbles), I shut the expo off, and I added another six quarts, and repeated this process for a total of THREE times until the fluid came out cherry red from my clear tube. Then got the trans warm by driving a few miles, then topped it off.

This got rid of the last bit of weird running. And made the transmission shift like new. My fluid was dark and nasty, but I could not detect how bad it was by just pulling the dipstick, and wiping that on a rag or towel. It didn't smell burnt or anything either.

I have owned my 2002 since 2004 and about 30k miles. At around 100k, I dropped the pan and replaced the filter, but only replaced the six or seven quarts that came out when I dropped the pan. So at 176k, the fluid was pretty used up.

Just a suggestion to try out... I had this happen to another vehicle. It turned out the gas tank vent was blocked and the fuel pump was being starved because of the vacuum in the tank. Loosen the fuel fill cap so it vents and see if it still does it. This will probably make the check engine light come on, but it could point you to a solution and it's easy to try.