RPM's jumping around

CLTX11 · October 27, 2006

I searched the forum and found a couple of items under "hesitate" and have some of the same problems as well but not enough info.....

For a 98 5.7 liter Silverado Z71

Whenever I am going at a constant speed or gradually accelerating the truck feels like it is hesitant and the RPM's jump around with the hesitation. If I accelerate faster than normal it will not hesitate to go. When I was entering the freeway and gave it some gas and when it switches gears near the redline, it feels like it doesnt know which gear to go in.

I also noticed recently that the "Check Gauges" light will flash for a split second when accelerating. All the gauges are fine except for the more than common problem with the gas gauge not working properly (read about that so I dont need help with the gas gauge.)

As many others I have performed a full tune up.

Thanks for any help.

Also is the an online "database" for the engine diagram and help on "how to's?"

Ie. I seen a post about the problem might be the MAF sensor, how would I go about checking it out etc.

c10ls01 · February 7, 2007

Sounds to me like a faulty throttle position sensor or a bad "stepper motor". This motor controls the idle and light "off-idle" behavior of your engine. It often gets sticky due to carbon build-up.

I had about the same problem on my 96 s10. My rpm would move up and down if I ran the truck at 1500rmp anything
above that or below seemed to be ok. I finialy found out what was causing the up and down rpm. It was the computer

the output of the computer to the throttle position sensor should be a constant 5vdc. Mine was fluxuating up and down

by 1/2 volt. this made the computer think I was playing with the pettle. The dealer changed the computer out and truck

runs fine now.

superbee · October 28, 2006

Check your plug wires, might be loose or be arcing. Do the "midnight check"

CLTX11 · October 28, 2006

Well it had this problem before I did the tune up and I made sure all the wires were snug so I dont see a problem there.

Update on the check gauge's light. I was stopped and the check gauge light was on, I then noticed the oil pressure gauge fluctuating when stopping and going. Ill have to look at it tomorrow morning, its not time for a oil change or low but I will go ahead and change it tomorrow.

CLTX11 · October 31, 2006

My gas gauge doesnt work anymore. I was going to wait til the pump went out before I fix it.

CLTX11 · October 28, 2006

Any more ideas on the hesitation and the RPMs jumping around?

90GMCZ71 · October 31, 2006

Any more ideas on the hesitation and the RPMs jumping around?

<{POST_SNAPBACK}>

How does the inside of the dist look? The vents can become blocked which can cause the cap and rotor to corrode.

If the check engine light came on, you should get it scanned to see what codes are stored.

//90GMCZ71

CLTX11 · October 31, 2006

It was doing this before I replaced cap, rotor, PVC valve, wires and plugs. It is still "jumping."

When I am gradually accelerating or going at a constant speed say 40 MPH, it feels like what would be a bad O2 sensor would cause. It feels to be bogging down and the RPM will jump up and down til I either let off the gas or give it more than normal acceleration.

I found out the check engine light was from the oil pressure gauge bobbling around. Checked oil, it was fine but went ahead and changed it out and it fixed the check engine light problem.

90GMCZ71 · October 31, 2006

Does it do it at idle or only at constant speeds? How is the fuel pressure? Does the pressure bleed down quickly when the engine is turned off. Pressure should be 55-65 PSI at idle.

//90GMCZ71

CLTX11 · October 31, 2006

It will only do it at constant speeds. Seems to be fine when at idle.

Pressure for the oil? It is fine now that I changed the oil, I have no idea why it was having that problem.

90GMCZ71 · October 31, 2006

I was refering to fuel pressure. If it only happens at speed you may be having TCC shudder. One way to test this is to lightly press the brake pedal when the problem occurs. Press the pedal enough to turn the brake lights on but not enough to slow you down.

//90GMCZ71