Just picked up a 99 RCSB with a 4.8. Tonight I started it and let it warm up(it was around 20 degrees). dropped it in drive and the idle jumped up and the truck took off on its own without me hitting the gas. I stopped on flat ground in D and let off the gas and it took of slowly like normal with no gas. Around 4 or 5 mph rpms jump up and the truck goes over 10 miles per hour on its own. I am going to take a look tomorrow afternoon just figured I could get some ideas on where to start looking. I cleaned the tb(didnt take it off just what I could get with intake off) yesterday and it drove fine after. Quickly sprayed carb clean on any possible leak spots on intake, with no luck. I will be looking more tomorrow, anyone have any other things to look at? Thanks.

 

How the idle when you start it up on cold mornings?

Might have leaking intake gaskets.

 

Mine does something similar as well. When you fire it up cold, it will sit and idle fine, but in gear it will keep giving itself throttle to about 10-15 miles and hour. Won't do it when its warmed up. I suspect it's something weird with the DBW setup, because it will settle back down if I tap on the gas.

 

I sprayed around the intake gasket as well with nothing, I suppose that doesn't mean anything for sure. Cold mornings it starts fine and idles high like normal for a bit then settles down to normal.

Slammed96, sounds like the same thing though mine is not DBW. I will try tapping the gas to see if that has any effect next time I drive it. I even notice when slowing down the downshifts seem super delayed like its staying in higher gear longer due to the higher idle.

 

Yours is most likely related to your mods and the way the tune is set up.

 

It could be leaking on the under side which is the issue I had on my 2000.

I Litteraly poured brake clean from a smalll container over the top and it didn't make a difference.

We buy it in 5 gallon bulk and put it in a reusable pressurised container but to get it from the 5 gallon to the Quart container I used to put it in a smaller 1 gallon container. Any way it is very dangerous and I strongly recomend NOT doing that!!!

If you have 100,000 miles and its never been apart I would just do it and install the newer style gasket's

 

I don't believe this will solve your issue, but its free. The problem you are describing sounds like to much un-metered air is getting in. Most likely from a vacum leak.

The problem below is usually not enough air and the passages get blocked and this is typically the fix. However it is remotely possible that to much air is getting passed. I would still recomend replaceing the Intake manifold gaskets.


Another thing you can try is cleaning the Throttle body. It has an air diverter valve on the bottom of a cable style throtle body that controlls the idle air flow. It is called an IAC = Idle Air Controle Valve.

Clean all the air passages and the valve off. Be carefull with the valve, it is sensative.

 

Thanks for the information, I'll get the truck in the shop when I'm back in school and check these things out.

 

Probably the TPS or worn out TPS connector, since you have an older truck with the cable TB. It will send erratic readings and cause the IAC to let more air in when it shouldn't. Just carefully grab the TPS wires and jiggle them and see if the truck revs up on its own.