4.8 Backfiring whenever buying gas

Silvy-99 · March 13, 2004

I have a 99 silv. 4.8 with 94,000 miles and Ive had misfire codes on multiple cylinders and service engine light coming on so I took it to the Dealer and they said it was the fuel pump for ($600) and I said Ill do myself. Anyway I put new fuel pump in, fuel filter, bosch platinum plugs and wires and problem still persists, but its only when you first fire the truck up, after it runs for about 5 min of driving it will run pretty much fine as in no backfiring but the hesitation is still there. I cant even burn out, if you hammer the gas it will lag then kick in, back and forth until about 15-20mph then it hauls ass. But cant figure it out.

Any help would be Appreciated.

citydriver · March 13, 2004

94k miles,oxygen sensors may not be reading right till it gets hot..coolant temp sensor might be misreading..dirty throttle body...have the injectors been cleaned lately...Just some things to check out

Silvy-99 · March 14, 2004

I have had the injectors cleaned by the dealer, I haven cleaned the tb... Any idea on which of the four oxygen sensors it could be maybe before or after the cat conv.

Everytime I get gas, If its 5 bucks or a fillup, right when I pull out of the station it backfires and wont go until you slowly step on the gas and wait for it to get up to about 20mph mark then it kicks in and runs fine.

Thanks

GM GIMP · March 14, 2004

I backfire after I get gas too! SORRY

citydriver · March 15, 2004

If it is 1 of the oxygen sensors,it may be impossible to tell w/o checking the crosscounts on a scanner...But the interesting thing is it happens after you gas up..

Try checking the plumbing of the emmissions/vapor system...you know the system that turns the CHECK ENGINE light on when you leave your gas cap off

I'm not sure exactly how to do that w/o checking the service manuals

Silvy-99 · April 13, 2004

I got to reading in the manual and the oxygen sensors dont control anything until the engine warms up to a certain temp., so that kinda kills that idea. I might have forgot to mention its not just when you get gas its also whenever the truck sits a long time or I put some injector cleaner in it then it gets worse.

I cleaned the Throttle body and it was no help. And I noticed the other day that Shell Gas makes it worse also.

454ss · April 13, 2004

if you get this figured out please post what the problem was. i have a 1997 k3500 with a 454 that does the exact same thing, thanks.

apache sierra · April 16, 2004

you might check the gas cap. They sometimes plug up and don't vent properly causing the symptoms you are describing. If the cap is original it may be plugged.

Butch 02 Denali · April 16, 2004

Many good ideas! I'm not sure what it is, but there are 100's of possibilities. I do know why it does it only after filling the gas tank. Every time you put at least 6 gallons of gas in, the computer will monitor the knock sensor for a time afterward. If it detects a sustained knock it will put the computer in a default, much reduced timing mode. The factory has these two spark table built into the programming, and this is the basis for Wester's dual octane tuning, he just raises the two tables up to the level you choose, like 87/91, where the factory is something like 80/87 (That's a guess for comparison, it's actually degrees of retard the factory uses)

My first thoughts of the fix would be a bad knock sensor or a corrupted computer program.

Try putting a half a tank of 93 in next time it's near empty and see if it makes a difference.

Butch 02 Sierra Denali

The Dude · April 19, 2004

I backfire after I get gas too! SORRY

A little Gas-X or Beano should fix that problem

Rookie · April 19, 2004

it's only a thought but try the filter. it may be causing a lean condition causing the backfire. i know it will do it on a Carb not sure about fuel injection. cant hurt its only $10.00.. good luck

2001 HD · April 19, 2004

Other things to check.

Fuel pressure regulator ( Wetness inside vaccum hose-should be dry).

Fuel filter orientation (check flow arrow on filter).

Defective purge solenoid on engine.

Any codes?

Silvy-99 · April 19, 2004

Im not getting any codes on it. Not since the initial time I took it to the dealer which was oct/nov of last year. Ive changed the fuel filter, and have checked the pressure regulator. As far the the purge solenoid on it, I dont know. I think I might try the knock sensors, but I really hate to have to tear the top of the motor down to get to them.

Thanks