1999 Silverado bucks and jumps on decelleration

[Curth]

I have a 1999 Silverado. It has 112,000 on it. Last year I got a new fuel pump, because it would not start consistently. It is a 5 speed v6 vortex.

My present problem is that when you let off the gas, the engine bucks and jumps. This is after you reach cruise speed and you wish to just maintain current speed. No codes are set. I had a mechanic look at it and he was unable to find a problem with it after he sprayed the plug wires down to looking for arcing. The truck has ran fine the remainder of today (when I took it to the garage).

Can anyone help me?

[trey_dogg3]

My girlfriends V6 was kind of doing the same thing except it was when she pressed the brakes in. after 2 weeks in the shop we found the problem the ignition wire broke inside of the insulation. So when the brakes were appled the movement would cut the power to the distributer.

[RexNfx400]

It really sounds like secondary ignition missfire. AKA plugs, wires, cap and rotor. That would be the easiest. Or low fuel pressure. Pickup in distributor. Fuel injector cleaning(MotorVac)

Stay cool

[amac209]

1999 has no cap or rotor. there should be misfire codes if it's misfiriring. look for a a vacuum leak around the plastic intake plenum they have a tendency to warp. i assume your mechanic checked the egr, tps, iac, map, O2 , maf, iat and all the other sensors. what happens with the vacuum leak is vacuum is high at part throttle so air bypasses the maf. have you replaced your plugs and fuel filter recently?? if not do it now they may or may not be contributing to the problem but are part of the whole preventitive maintenence deal. get your buddy to have another try with his scan tool just because the CEL isn't on doesn't mean you don't have codes. good luck. oh right one more thing to check for is carbon deposits on the throttle valve and body. good luck!!

[trey_dogg3]

the v6 has a cap and rotor

[amac209]

sorry about that i didn't even notice the v6 part just assumed it was a 4.8, 5.3,or 6.0 v6 W-code, gotta be the last gm motor that still has one. thats probably why it doesn't have any codes the thing's a damn dinosaur. please disregard my earlier post well most of it anyway that was for the new GM small block familly.. maybe the points are dirty or the carberator power valve is leaking. okay i'm signing off now, sorry about the confusion

[Curth]

Thanks for the replys. Have bought cap, rotor, plugs, air filter, and fuel filter to replace the existing parts. The plug wires are less than 6 months old and, although not OEM wires, they were Bosch. I also put a new battery in. No good.

To maybe be a little more clear with defining the problem than I was the first time

When I accelerate everything seems fine, but when I try to maintain speed the bucking soon begins. It is a 5 speed and at a speed in any gear prior to the shift light coming on, it begins to bucking except if you continue to accelerate through the gears more rapidly. Maintaining a speed prior to the shift light coming on results in bucking. Also, in 5th gear it does it when you let off the gas a little. It also has had two instances where it refused to start as though it were flooded

Could my problem be the fuel regulator valve? Could it be the coil (bad ground)? Restricted cat converter?

Any help would be appreciated. Both mechanics and myself stumpt.

[jt-stl]

My 92 did the same exact thing...I changed the distributor rotor and that took care of it!

[RexNfx400]

Also a sweep test on the TPS should be done with oscilloscope. See if any glitches at cruise throttle opening.

[Curth]

Originally Posted by RexNfx400

Also a sweep test on the TPS should be done with oscilloscope. See if any glitches at cruise throttle opening.

I do not know how to perform that test but I am a technician by trade and am familiar with the oscilloscope. How should I perform the test and what will I be looking for.

Thanks

[RexNfx400]

I use the snap-on Vantage myself. There are so many different scopes tho. With key on. With harness hooked to sensor you hook up one lead(from scope) to the signal wire(of sensor) and the other wire from scope to ground. I don't have the voltage readings in front of me but, basically your sensor puts out .5 volts at idle 4.5 volts at WOT(wide open throttle). This signal cannot be interupted. It must be smooooth from top to bottom. You can use a (good) Fluke meter on volts can be hard to catch. You want no drop outs or glitches as you move the throttle from idle to WOT.
The sensor can glitch so fast it won't even set a code also. But you'll feel it in the powertrain.

Stay cool.

[nick072877]

This is the exact same problem I'm having.... did you ever find the solution???