Forgive me if this has been covered before but I am having trouble with my 2004 silverado 2500hd 6.0. Im going to lay it all out here. So couple of months ago got the knock sensor code. I cleared it thinking maybe a fluke. Stayed away for a while but coming back. Also had an exhaust leak at manifold thought maybe it was the cause so took it inthey put on clamps that sealed it up (apparently a common practice). Worked great for the popping sound the leak made. Knock sensor code again. Also wanted to give it a tune up being it was getting ready to hi 200k... ordered some new coil packs plugs and wires that were red to go with the truck color. Immediately got multiple random misfire code. Bought newplugs and duralast wires thinking maybe just bad wires and possibly plugs. Still throws the same code. So I switch coils to the originals because I wasnt getting this misfire problem until I did the tune up. thought the aftermarket stuff was the problem... still getting the code along with knock sensor. One more thing... when i changed the plugs for the second time in about 200 miles 1 plug had the tip burned and corroded not wet but lots of build up around the end and the actual platinum tip was completely gone (from the misfire?) Can the knock sensors cause all of this? Was it the exhaust leak fix possibly linked to all this? This has all been done very recently and have had nothing but trouble. I am currently ripping into the intake to replace the knock sensors hoping it will be my fix all saving grace but having my doubts. Just dont understand the random multiple misfire all of a sudden. Any advice would be greatly appreciated.

 

The knock sensors are common failures on these engines - by now you’ve probably seen that oil and/or water can pool in the sensor bores which will eventually ruin them. If you haven’t already, I’d recommend getting the pigtail kit with the sensors as the wiring and connectors can cause just as much trouble. Also, it doesn’t hurt to spread a thin layer of RTV around the edge of the bore caps to prevent water from pooling in the future.

That said, it sounds like the tune up was overdue, but there are several possible causes of random misfires, so here’s some things to consider:

1) Have you checked fuel pressure? Key on, engine off it should be 55-62 PSI and hold for at least 5 minutes. If it doesn’t, the fuel pump, injectors or regulator (if equipped) could be at fault.

2) What was the quality of the gas you last filled up on? Was it a higher concentration ethanol blend? Did a tanker happen to be delivering fuel at the same time (this can stir up debris in the storage tanks that gets pumped into your tank if you happen to be fueling at the same time)?

3) Check engine vacuum once it’s up to temp. You want to see a steady needle between 18-22 in*Hg at idle. A fluctuating needle (3-5 in*Hg or more) or a low vacuum (account for altitude) indicates base engine (mechanical) problems as opposed electrical/control side problems.

4) Inspect the coil grounds on each bank, located at the rear of each cylinder head. These grounds are known for having problems with corrosion and weak crimps that can cause misfires. You can do a quick inspection by unplugging a coil and checking the resistance between the black wire and the engine block - anything more than 0.5 ohm is reason to suspect the grounds need attention.

5) Are you using any aftermarket devices? Remote start, OBD monitor/tuner, cell phone charger, etc? GM has issued bulletins for at least the past 10 years regarding aftermarket devices and how they can cause interference with normal vehicle operation. If so, unplug the device(s) and see if performance improves.

6) With the engine idling at operating temp, wiggle test the harness. If you can cause the engine to stumble or stall by wiggling the harness in a certain area, open the harness for inspection and repair as necessary (disconnect battery first).

Good luck.

 

I will give those a try. I did get the new harness and rtv gasket maker to build the dam. I do have a bullydog gas tuner installed but it has been there since i bought the truck 6 years ago. But still something i will look at.

 

So far so good. Changed the knock sensors, harness, and intake manifold gasket and no check engine light. Fingers crossed that the misfires were from the knock sensors. Going out of town with it and will be a good test I imagine. Thanks again!

 

Good to know it’s running well so far - I hope that took care of it. It may have also been a combination of the knock sensors and a leaking intake gasket - I don’t think they’re a problem for these engines specifically, but my experience is that plastic manifolds and aluminum heads just don’t stay together over time.

A tip on future misfire diagnosis - focus on what could be causing misfires on the affected cylinders. For a generic P0300, anything that feeds air, fuel, or spark could be the cause (methodology is essential). For specific misfires, such as 1 or 2 cylinders, or 1 bank only, focus on what could only affect those cylinders. In the case of only 1 bank misfiring, for example, the EVAP and PCV systems, EGR, MAF sensor, and fuel supply (supply only - not injectors) would be ruled out because those things would affect all cylinders.