Random Missfire on 99 Sierra 1500
 

Hello all, I'm hoping I can get some help from the group because a local shop and now the dealer service team have failed to. I apologize for the novel below but this has been kicking my butt for several weeks now.
Over a period of a couple of short drives, my truck went from running perfect to barely running at all. The rough running (like running on 6 cylinders) would start immediately when cranked cold and would never change characteristic regardless of conditions. The computer would only log P0300 "Random Misfire" codes.
After doing all of the normal maintenance items (replace plugs & wires & fuel filter, removing and cleaning throttle body and MAF sensor & KN filter) the problem didn't change at all.
I took it to a local muffler shop to see if the catalytic converter was the culprit and instead they hooked the truck to are larger diagnostic computer that read the spark misses per cylinder and when under very heavy load the #7 would miss heavily and #5 about half as bad. This led me to replacing both of the coils which also had zero impact on the problem.
I took it to the dealer and after $500 of labor they said there was contaminated fuel and they cleaned the fuel rail and injectors. In retrospect I think the technician ignored the fact that I told them I had a can of sea foam in that tank. I couldn't afford the additional $400 they wanted to drain the tank so I had the truck towed home so I wouldn't suck in potentially bad gas into a clean injection system.
I dropped the tank and fuel looked perfect but I disposed of it and cleaned the tank and refilled and replaced the fuel filter again. When I started the truck it ran perfect for my about 10 mile celebratory ride. It started perfect this morning and after 1 mile I felt the miss come back.
Two potential related side notes here;
1) The bolt has been broken off in the manifold closest to the firewall (#7 cylinder) so I've been listening to that hissing when cold for several months.
2) The fuel pump has been whining for about 2 months and very noticeably wasn’t during my victory drive last night but I heard it again today after the problem came back. I've been told that a pump either does or doesn't work and wouldn't cause this problem but now I'm not confident of that.
Any advice would be greatly appreciated because this truck is fantastic and it's the first mechanical problem I’ve had with for 8 years since I’ve owned it.

Considering the truck ran OK after the injectors and tank were cleaned and you put new gas in it, it appears to be fuel related. I would replace the fuel pump. Normally the pump will make some noise when the tank gets close to empty but is pretty quiet when say above 1/4 tank or more. If you can spend a few days checking the relationship between the pump noise and how the truck runs that would pinpoint it for sure.

The factory spider injection system with poppets has failed. Needs to be replaced with updated system.

Yeah I have to agree with RUFFNECK4LYFE. Replace the spider injector system and see how it does. With the GM tech wanting to clean your injectors, he was trying to see if he could avoid replacing the spider. Problem is, those little poppet injectors wear out, and there's not much you can do with them. Best solution is to replace the spider, and check your fuel pressure. Rather, check your fuel pressure first, then replace the spider.

Your poppet injectors work off of fuel pressure, as your injectors fire, they pressurize small tubes with pressure nozzles on the ends of them. Those nozzles are the "poppets" I referred to. They're little pressure switches that open and close when the injector pumps more fuel into the tube. So it's a mechanical device that just plain wears out.

Sierra- Non poppet style injectors.

Fuel rail probably has water still in it. The way the fuel rail is designed #7 is the first to get contaminated fuel. I would pull the fuel rail and drain the fuel into a container. Then take the #7 injector out of the rail and drain rest of fuel from the rail out were the injector was. If you let the gas sit you should see water in the bottom of container. Put it back together and see how it runs. These early trucks had a problem with canister vent valves that would let water into the fuel sytem thru the evap system in high rain areas.