Sudden bursts of Acceleration

[Vaughn MacKenzie]

Starting last Saturday my engine will suddenly start accelerating more rapidly than where I have my foot on the pedal. I'll be accelerating gently and then it'll start rushing ahead with a surge of power. It usually happens soon after starting out in the morning and during light to moderate acceleration.

Is something up with the injection pump? Luckily when I let off the pedal it stops. Never seen anything like this before, although GM diesels are still a learning process for me.

Vaughn

[DmaxMaverick]

You could be getting air into the system when cold. The air will skew the timing due to buffering under pressure. Did you change a filter or line recently? Old/tired injectors can leak air back into the injector lines with the engine off. Usually, the lower the fuel level in the tank, the worse it is. It is more noticed at this time of year, just coming out of Summer. The cooling off can cause the fuel in the tank to contract rapidly, creating vacuum in the system. Try loosening the fuel cap after shut down. If the problem goes away, you may need to replace the cap (not venting, or the wrong one).

[G. Gearloose]

When my truck would get those symtoms, fresh filters would cure it for another year. It would always surge stronger than it could ever run at WOT, too bad it was out of control.

[ 11-02-2005, 04:05 AM: Message edited by: G. Gearloose ]

[Subzilla]

If it's within the first 5 minutes or so, then it's the HPCA kicking in, otherwise known as the "housing pressure cold advance". This is an injection pump feature used to help in cold starting and will give those very symptoms of surging. Mine feels like someone is squirting nitrous into the system. Once the engine warms up, the sensor kicks the HPCA off. It's also tied with the high idle solenoid. If it occurs longer than 5 minutes, then check out one of the above suggestions. Looking at your signature, you may already know all of this! Good luck.

[Dezman125]

Hi
My truck had similer symptoms,when i was driving i would get surges,at times you could hear a differtent rattle from the engine.Turns out my HPCA sensor in the head was bad and would kick in the advance at times even when the engine was warm.I would disconect it after it warmed up and the different sounds and surges were gone when driving.

[5.7L oldsdiesel]

I had that kind of problem with my 5.7L diesel 2 yrs ago.It would accelerate on its own pretty much.Turned out that the fuel filter was almost plugged,which was just mentioned above in a previous post.That fixed my problem.

[Vaughn MacKenzie]

Thanks for the info guys on the advance timing device. I can feel the exact moment that kicks off, the rattle gets a lot quieter. When it surges the pitch of the rattle doesn't change.

I am also wondering if there's more blowby gasses lately feeding into it and causing this. I'll check and see how much vapor is coming out the line.

Vaughn