Replaced engine, misfire still

[Skylar 827_7]

Where to start, I had an 08 4x4 Tahoe come in the shop, check engine, p0300 random misfire with cylinder 6 reading prominent with the tech 2, operator ran it past scheduled mx intervals and no oil was on the dipstick, This is a cop truck and lets just say air force cops aren't the best at treating their vehicles properly. So I threw some 15w40 in it to thicken it up. It also had a loud knock. Bad enough it wasn't gonna pass qc check. In the government vehicles safe and serviceable is sometimes just what you get. But I couldn't get the knock and the valve tick to go away, I changed the oil and seafoamed the oil, pulled all rockers and rods, couldn't find a visible prognosis for noise. Timing chain was good. As for the misfire code, I swapped plugs, wires, coils, injectors, and could not get the misfire to migrate cylinders. Oil pressure and fuel press is within spec and I did every test the tech 2 will do. So we ordered an engine for it figuring its got some internal damage. Threw that in and well, wouldn't you know still a misfire on cylinder 6 and the new engine runs worse then the old one that had the knock. But this new one isn't knocking thankfully. Tomorrow I guess I'm gonna be pinpoint checking my harness for anything I can find or if I have a bad ecm i'm not sure. Anybody got any ideas? I'm desperate here maybe someone has more experience than me.

 

[sumo]

I think your right in the next step of diag. Harness. Check your connectors for the injectors and coil. Could be a bent pin or corroded wire. You can pick up a coil spark tester and see if that coil is firing. You can try unplugging #6 and see if the idle changes.
You said you replaced the engine. Did you swap the entire engine or did you transfer intake plenum over to the new one? Did you happen to insect the condition of your intake gaskets?

 

[Skylar 827_7]

I think your right in the next step of diag. Harness. Check your connectors for the injectors and coil. Could be a bent pin or corroded wire. You can pick up a coil spark tester and see if that coil is firing. You can try unplugging #6 and see if the idle changes.
You said you replaced the engine. Did you swap the entire engine or did you transfer intake plenum over to the new one? Did you happen to insect the condition of your intake gaskets?

Hey thanks for the reply, I did actually unplug #6 injector, idle changes but it isn't repeatable, it seems like the entire time no matter what injector you unplug it idles rough, then it sounds okay and then keeps switching back and forth. As to your other question, I just put the old intake back on with all new gaskets for everything, I ordered the set with every gasket I needed in it. And I didn't have a coil spark tester do I unplugged a wire from the plug and got another plug and stuck it on it and grounded it out and looks like my coils are firing fine but I didn't check them all. I will also mention this engine has new plugs and plug wires but I elected not to replace coils because of price and the problem didn't move cylinders when I switched them.

 

[Brian Wilson]

I would start with a simple coil test. Buy a very cheap voltage checker. Lysel make a decent one. Plug the number 6 spark plug wire into it and see if it can hit 30k volts. If not, change the coil. Any stock ignition should hit 30k easy. If that isn't the problem, look for a clogged injector. I see you have a tech 2. So just do a simple injector test on it. If it sticks or leaks it is bad. Change it.