99 silverado 4.8 Backfiring, Hesitation

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.

dude, i have a 93 5.7L sierra. i will tell you right now, the timng is off, b about 8-12 degrees. i wil bet a hundred doallars on it. i to atiming light to it and i was almost knoced over by how much it was off. for some reason the timing slips on chevys. i have to readjust every 10k miles. trust me, it's the timing.

How do I check the timing if I dont have a distributer? my guess is its done either by computer or by the crankshaft.

ok. you have a DIS (Distributorless Ignition System). you have to look up on mitchell (or any repair manual) on how to adjust your timing. i've never done it on a DIS but it has something to do with the timing light. do you know how to check the timing? my first suggestion is to buy a repair manual, cause you end up payin way too much for anything that you could spend just a few min. doin yourself.

I have a Haynes manual and the only thing I can find is how to change the timing chain. Do you know what else I could look it up under

look in the index under timing. it should be there. in my manual the timing adjustment was in chapter one.

could be a vacum leak . ls1 type moters are known to have a leak under the intake.

I don't think you can adjust the timing manually on a distributorless system, there is nothing mechanical to adjust. Take it to AutoZone for starters and have them scan-tool it for free, might head you in the right direction at least...

you cant change the timing!it done by the computor.i have the same problem.it ether a vacum leak or bad injector.mine going into the dealer today.1/30/04i will let you know.

oh one more thing u can check . some heasatation problems can be caused by dirt and roadgrime collecting in the mass air sensors temp and airflow pickups (ie little wires between the two metal posts ). those wires heat up to a certain temp and the airflow moving accross them cools the wires ,if dirt is built up on them they wont tell the computer to send more gass to the engine. try cleaning it with good carb spray and letting it dry out. then reinstall. do you have a k&n filter? oil from the filter can stick to it and cause problems to. this usually happens when u put a new one in or recharge the old one. hope this helps.

I looked at the threads where people were talking about removing the screen from the MAF and I did that but I will try using that carb cleaner on the wires anyway, and I dont have a K&N filter, I had one on my old truck and thought it was a pain in the ass.

Thanks though I'll try it.

P.S. Yeah John let us know what you find from the dealer if you would please.

i changed the coils and plugs.no help.pressured cleaned the injectors.no help.as per the dealer it a bad injector.they are going to do it on monday at a cost of $750.till i get it back we will see.i found out they are also having a problem with the driver for the injector in the ecm.till monday.

the problem was bad gas!!! they had to replace the #8injector and clean #7 at a cost of $804 she runs as good as new.

Was it that the bad gas caused the injector to go bad or was it they had to remove all the gas from the truck along with replacing and cleaning the injectors.

as per the dealer i got slug in my last file up of gas.it cloged #7 complete and #8 was so so.they did not remove the gas from the tank they just replaced #7 and cleaned #8 thats it .

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.