i have a 1999 chevy lumina 3.1 v6...i had blown head gaskets and it was running fine when i started replacing head gaskets,had heads machined,new spark plugs,all new gaskets,everything cleaned up..the haynes manual doesnt say anything about setting no.1 cylinder at tdc before starting...i did have to take the upper manifold and fuel injector rails off again to position some wires...but when all put back together now it starts up just fines but dies immediately and it will stay running if i keep the gas pedal mashed down some but runs fairly smooth except for some mild popping-backfires....could it be out of time ...i cant find any info on setting timing marks on the crankshaft...i never removed anything to do with the timing chain ...or could some o-rings have fell off when i had to pull the fuel injectors out again to reposition some wires that went under...i,m at a total loss on this...on 4 cylinder motors i,ve put new head gaskets on i always lined up camshaft timing marks and crankshaft marks...could i have hooked up the fuel lines to the fuel injector rails backwards or what...just trying to give all the info i know...appreciate any help...thanks

What you have done, has nothing to do with the timing....so if it ran ok before the work was done, then the timing should b e ok...

Hopefully you torqued everything correctly.....

Installed the pushrods right?...the exhaust push rods are longer....

Hooked up everything electrically correct?

You bring up the issue of o-rings on the injectors.....it'* quite possible they may have popped off and are creating a vac leak...when you have someone start the car, spray carb cleaner under the upper plenum, and see if it affects the running of the engine.......if it does, you are going to have to pull the upper plenum and remove the fuel rail again, to make sure the 0-rings are in place....did you apply petroleum jelly to the o-rings for easier installation?

It'* impossible to hook up the fuel lines incorrectly...

yes torqued everything...i assume the wiring plugs only fit to certain items ..it ran fine when i started just starting to get hot...i used the o-rings that came in the gasket kit..they fitted on easily no jelly...i,m gonna pull the fuel rail and put on o-rings from napa in a few days...i could have hooked the fuel lines up to the wrong fuel rail inlets could that have anything to do with it?...i did have a small tear of the upper intake gasket at one end when i repulled it but tryed to seal with gasket maker...but i,m gonna pull it off again and buy new gaskets again for the top manifold-plenum...i had moved crankshaft to clean all the pistons...didnt know if that would affect timing or not

Like I said, you can't hook up the fuel lines wrong......

So the car is ok, starting cold? How long before it starts to act up?

And rotating the crank, will not affect timing....

oh yeah i kept all the pushrods in their right positions

it starts fine cold then dies unless you give it a little gas to idle it up...it wont run long enough on its own to warm up...

Sounds like miswired spark plugs maybe. Have you reverified each spark plug wire from coil pack to plug for correct firing order?

yes plug wires and the coil packs are numbered for each cylinder

Have you inspected the MAF, there should be like 3 thermistors that should be fairly clean, I'd inspect them for damage, make sure they did not come loose, and if you have MAF Cleaner give them a clean.
You can see the 3 thermistors in this pic.

no havent checked the maf...i just took the whole duct work off as one piece...but i did have trouble with the map sensor that bolts to the upper plenum...dropped one of the screws and couldnt find it...is only on with one screw if that could make a difference...i,ve had to put it on the back burner for now until i get some free time