99 4.8 Backfiring after gas fill-up

[Bill-SILV]

Well I have a update from my last thread.

http://www.automotiveforums.com/vbul...d.php?t=177017

And after reading that one I took the truck back to chevy and got the mechanic to go for a ride with me and the truck ran fine, so I asked if we could get some gas to see if that does it because sometimes that would do it. So we got gas and right after pulling out of station I could give gas (Flooring IT) and would go nowhere, the truck wouldnt even rev the engine and it was backfiring. The mechanic was stunned! And we got back and they discussed it and said I should run a Fuel Pressure guage on it and tape it to the windshield and recreate the fill-up and see if it drops below 55-60psi and if it does, its the fuel pump which Ive already replaced!! Anyway 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 (warmed up or something Im guessin) then it kicks in and runs fine. Im at a loss and the chevy Dealer is REALLY PISSIN ME OFF!

P.S. The mechanic and I tried the pulling the vaccum line off the pressure regulator and no gas came out of it.

[Bubba14]

Bill, I may be just quessing but here is a freebe you can try.
Maybe it's a vacuum leak?
I had a vacuum leak on my 1999, 4.8L Silverado.
Spray some starting fluid around the egr intake line where it goes into the intake manifold, then spray it all around the intake manifold gasket seam.
Do this when the engine is first started before it warms up. The gasket on mine was sealing off when the engine warmed up and made it difficult to find the leak. The chevt service dept wasn't much help. I found the leak with help from guys on this website and had to tell the chevy dealer to change my intake manifold gasket. They wanted to argue and I said just do it. Now my truck runs just fine.
Anyhow back to your truck. Try the starting fliud trick. It's only about a buck fifty a can. Real cheap way to search for a vacuum leak. If you do find a leak don't over spray the area you could damage something.Good Luck

[Bill-SILV]

What do you look for when spraying the starting fluid? Its not going to bubble is it? I tried it but didnt know what to look for, as in finding the leak.

[Bubba14]

If you have a vacuum leak you will not see anything.
However, you should hear the engine rpm's change. The engine rpm's should increase slightly.
On mine the engine stopped misfiring and ran smooth when the fluid was sprayed around the leak.
Use of the starting fluid is only an aid to help find a possible vacuum leak.
You may not have a vacuum leak. It may very well be something else.
I read your post from Jan 16 and it sounded similar to mine.
Multiple mis fires, service engine light etc. The codes on mine were 171 and 174 too lean banks 1 and 2.
The chevy service dept wanted to change fuel injectors 5 and 8, I had to convince them to change the intake manifold gasket after I located the leak. After the gasket was changed there were no more misfires.

[Bill-SILV]

Since the last time, I checked for the vaccum leak and found nothing. I also cleaned the throttle body and no help. If I run injector cleaner or Shell Gasoline it makes it worse. I looked into oxygen sensors but they dont respond to the computer until the engine is a certain temp. I havent tried the fuel pressure gauge either because I cant find one to purchase anywhere, but I dont think that will tell me anything.

[Bill-SILV]

I finally decided to take mine to a different chevy dealer in a different city. they had for 5-6 hours and found that my (Mass Air Flow Sensor) was very dirty! but they only charged me $82, I am very impressed and would recommend them since the dealer in my city told me over $1000 to fix it. Im totally happy for what I paid for- I can burnout again but not like it did when I first got it but Im happy, it doesnt backfire or hesitate at all.