Check for a vacuum leak on the intake after the maf. The parts you replaced affect specific cylinder misfires, not random ones. Low fuel pressure and/or a bad coil could also cause random misfires.

Ted

 

From what we checked I don't believe its low fuel pressure. I will check the coil and see if it has any problems. Thanks.

 

With a bad coil you would also have a rough running engine. The higher the rim the rougher it will run. I listed the vacuum leak first for a reason. If I remember correctly you have a vacuum operated fuel pressure regulator. A vacuum leak in that line is a double whammy.

Edit: Scratch the double whammy. The lower the vacuum on the regulator the higher the fuel pressure. A basic test for the regulator is to pull and plug the vacuum line to the regulator, which gives you maximum fuel pressure if the regulator is good.

Ted

 

What psi number would be maximum fuel pressure?

 

You say you checked the fuel pressure and thought it was low.
I'm not looking it up; but, if you have a TBI, you only need 13 lbs.

 

According to the manual the correct fuel pressure is between 55-65 psi.

 

Your fuel pressure need to be over 60 PSI before the engine starts. After it starts, it will fall into the 50s PSI range. Vortec engines are very sensitive to fuel pressure. Vortec engines are CPI (AKA spider injection) not TBI engines. It takes a lot of pressure to force the fuel through the popit valves. These intakes are known for vacuum leaks. An easy way to find a vacuum leak is to use an unlit propane torch and run it over all the vacuum lines and the intake manifold. You can also do this with a can of carb cleaner, but that is messy. This must be done with the engine running @ idle speed. If the engine speeds up when passing the propane bottle (slightly cracked open, BUT NOT LIGHT) the engine will suck in the propane, or butane into the intake and the engine will speed up. With all of that being said, you really need to do a live time code read on the truck. This will show what the sensors are doing with the engine running.

 

Thanks I will try that. So just use a propane torch that is unlit and open it up a little so some propane comes out. Go over all the vacuum lines. If the engine speeds up their is a leak there.
I will check tomorrow but I know if you turn the truck keys in the on position the gauge will read 60. Once it is started it reads between like 50-55 psi.

 

Yes on the propane torch. Also run it over the intake maniflod, (EG where it connects to the block, heads, where it splits apart ECT) Your fuel pressure is good. You can also remove the breather, from the top of the throttle body, then turn the key on so that the fuel pump comes on for a second to charges the fuel injectors. without starting the truck, open the throttle body and see if you can smell fuel inside the intake. You can also look inside to see if anything is wet in there. Nothing should be. If there is fuel in there, you have a leak, and it need to be fixed. Here is the video I used to jump my fuel pump relay. I removed my throttle body, got a small mirror that fit inside the hole where the throttle body was, and with a flash light I could see all my popet valve lines, the spider, and almost all of the areas inside of the intake to see fi I had a leak. I had no leak, but this works very well. By removing the fuel pump relay and jumping the connectors, you can make your fuel pump run while you look inside the intake.

 

Thanks I will try that. I will let you know if i need any other help. Thanks for the info.

 

Y W

 

You may also want to pull your spark plugs and read them. This will pinpoint where the issue is.
https://www.youtube.com/results?search_query=reading+your+spark+plugs

 

Yea I am going to do that this week. I'll let you know what I find.

 

Did you get this resolved? I just had to deal with this very code. I guess I should have looked at the maintenance schedule more closely, since my 97 C1500 Suburban 5.7L Vortech requires new spark plugs every 30,000 miles. Mine had 50,000 miles! The electrodes had eroded to a gap in excess of .070 that started out at .045 when new. The gap was so large that the spark had begun arcing through the plug wire insulation since that had become the path of least resistance. New plugs, wires, rotor, distributor cap, and air filter and my Suburban is now one happy truck again!

 

Yep we ended up changing all the same things that you just changed. All new rotors, dist cap, filter and wires. Truck is now running great. We also have a new fuel pump and fuel filter.