hoytpr

Hi all, I'm reasonably good with engines and vehicles, but not a pro. My daily driver is an F150 XLT 2WD with a 351 engine that has been running well for at least a year but started "bucking" when going up hill or accelerating. Sometimes, it's a fairly hard "buck", but mostly feels like a misfire. The truck starts easy and idles smooth. It is not throwing any codes. What is the most likely cause of this (assuming the truck is fairly well maintained - which it is.) Just plugs? wires? injectors?

I know the truck has a small vacuum leak that appears to be from the vent-switch inside the cabin, but I haven't ruled out all possible vacuum hoses and connections. If I leave the switch in the "normal A/C" position, there are no signs of a vacuum leak, just when in the "Max A/C" position.

I have noticed a gasoline smell when both tanks are full. This has been present for many years, is intermittent, and I don't see any gas leaks.

Several months ago I did get a "lean" fuel bank code (sorry I don't have the exact code with me at the moment) but it only occurred after long trips at 70mph or so.

The only thing done to the truck recently was an oil change and new front shocks. I tried using higher octane fuel thinking it got some bad gasoline, and cleaned the positive battery terminal this morning because I couldn't think of what else to check, but it didn't change at all.

Any advice is greatly appreciated, I was supposed to go out of town tomorrow and now I may have to cancel.

clarkbre

It almost sounds like the motor is starved for fuel. I would guess that with the symptoms you're describing, it sounds like a clogged fuel filter...when the motor is requesting the most fuel, the pump can't deliver it. That might explain the lean code as well.

hoytpr

Hmmm. good suggestion. It could explain the fuel smell too if it's clogged so much it leaks maybe. Will change that tonight and post back any news.

Chris_1

iTrader: (1)

That's what I was thinking too. Could be low fuel pressure. Clogged filter or weak pump. If you don't remember the filter ever being changed, it's probably time. After that you will need a fuel pressure gauge to see what's actually happening there.

Crownman

Besides the above, the only other thing I might throw into the equation at this point is the pump in the tank. I've replaced quite a few old pumps that were still working, but the mesh screen had deteriorated and clogged the pump inlet, causing fuel starvation.

ericantonio

Sounds just like what is happening with mine. Same car. 95 XL 2WD. I had a broken rear tank, only front worked. Smelled fuel also. But I think there was a gab in between filler neck and hose (all fixed now, new rear tank/pump).

I just replaced vacuum hoses and at first seemed to fix the problem but not really. Still "bucks" or I call it hesitate a little bit. Just when it switches gears at like 30 and about 45mph. After replacing vacuum lines, it doesn't hesitate as long, I just give it a little more gas.

Changed fuel filter last year.

Will check plugs this week, it seemed to not do that hesitation after getting it back from the garage last year with a new Cat and plugs. Checking, after recommendations here, to check the TPS, later this week. May just change it, it's original 225K+ miles on it.

Chris_1

iTrader: (1)

Gotta tell you, I argued with myself back and forth about buying a fuel pressure gauge. Then I finally did - it was only 60.00. Cant begin to tell you how many times I've used it and how much I've saved by not changing parts that someone else thinks might be the problem.
Put a gauge on it, fuel pressure's good, move on. Fuel pressure's low - there's the problem.
We had a pump here that was 2 weeks old but only putting out 20 psi. It would run, but it wouldn't climb a hill. But it would back up a hill. That's when I bought the gauge.

hoytpr

Well the fuel filter was probably the original and definitely needed changing! It took hours to get the thing off even with the right tools. Must have had crud under the internal flared springs but they would NOT release.
Any way, after pouring out the chocolate milk that was inside the filter, I replaced it with a new one and the truck really seemed to run better. But the bucking was still there going uphill (only really happens at higher speeds but can happen at low speed when I accelerate). Then I ran the truck 200 miles and by the end of the day the bucking was reduced. I don't know if this is the computer relearning the fuel mixture, or if it's the injectors finally getting some clean gasoline. On another forum they suggested replacing the O2 sensor, which I might get to tonight.

So what is a good fuel pressure, where is the best place to hook up a gauge, and what's the gauge that you guys recommend?

Martian

Plugs... #1 cause of this problem

techrep

iTrader: (1)

Injectors may be clogged ... if you have good pressure at the rail I would remove and clean the injectors.