Bucking above 40mph

[Chris P]

Lately my truck has started “bucking.” It happens after the engine is good and warm. I’ll be cruising along between 40 and 45 mph and hit a hill or rise in the road. Everything is fine if I’m lightly applying the gas pedal to maintain speed on the level, but when I press it farther to maintain speed on a hill it starts bucking. It’s like I’m getting brief intervals with no gas, cycling on and off about once a second. If I let off the gas it smooths right out. Then I can press the gas lightly and it doesn't happen again until I press it further. It makes getting uphill an adventure.

I can press the gas pedal down all I want in 2nd or 3rd gear, on hills or not, and it doesn't happen. It only seems to happen in 4th.

I have an electric fuel pump (Facet 40164) with a pressure regulator (Holley 12-804) and the old pump is bypassed. The fuel lines are routed away from the exhaust. The gas tank is nearly full and the gas cap vent is on. I’m running 87 octane ethanol free with Seafoam and lead additive. The air filter was just cleaned.

Any ideas what's going on?

Thanks

[JimC]

Timing?

[8543bob]

GOOD MORNING: I WOULD DO A COMPRESSION TEST. I HAD SIMILAR CONDITION. TOP RING ON 5 PISTIONS WAS BROKEN. DIFFERENT TRUCK NOW.
BOB

[ZGjethro]

Clogged fuel filter? Wouldn't explain high revving in second and third gears though.

[w30bob]

Hi Chris,

My Chevy Dually used to do that too. Turned out to be bad spark plug wires....they would arc to ground under load. Not saying that's the only thing....or what's happening to yours...but worth a look.

regards,
bob

[Chris P]

Thanks for the replies! My fuel filter and plug wares are new, so I'll start with the compression test and the timing. I did the timing about a year ago, but perhaps it wasn't done right.

[ZGjethro]

I am running a different 24v Facet pump and the same Holley pressure regulator. The pressure regulator is where the original fuel filter used to be, and a piece of fuel injection line runs the gas to the carb. It is never near the exhaust manifolds. I have a pressure gauge on it, and it runs at 3-4 psi and my truck is driving great. I am guessing it is a fuel delivery issue (Float?) or a governor issue. Let us know what you find

[choppa]

did ya ever find the issue?

I remeber this happening to a ww2 MB jeep I had.

If I remember correctly, it ended up being something in the distro... maybe the condensor? weird, if I remeber right, made no sense at the time to me.

[Carter]

Have you tried cleaning and gapping the points and replacing the condenser?
Years ago I had a T245 engine that would not respond to throttle and I found the points were almost closed, tuned it and no more trouble.

[Chris P]

Thanks again for all of the replies.

I have not been able to work on it yet, so the problem isn't fixed and I have yet to try any of the possible solutions. Sorry for the delay.

-Chris

[refit1701]

It could also be a sticking or worn governor.

[sturmtyger380]

I was reading something over on the G503 about a Jeep that was doing this. One suggestion was the plugs were fouled some and at the higher speed in 3rd gear the engine was sort of lugging and not all the plugs were firing.

And how many times have I cleaned the plugs on your truck.....two times? Take a look at the plugs and let us know.

Alan

[Chris P]

I finally got the compression check done and it came back with around 90 in each of the cylinders except #4, which was in the mid 60's. So I guess that explains that. Time for a rebuild.

[mix4coal]

Before jumping into a rebuild.check your valve adjustment. I pulled my head and the PO had the gasket on backwards. So before I replaced the gasket I scraped all the carbon off the pistons and from under the valves. then seated the valves with some grinding compound. ajusted the valves and now it runs like a champ.