My big orange 2006 Silverado 2500HD (known now as Donald Truck) is suffering from a P0300 code, which means rando misfire.
I've done plugs and wires, which well well overdue, cleaned the maf and put a can of sea foam in the tank.
Still it persists.
Looking at the fuel trims for evidence of vacuum or air leaks and the numbers are <5% at most and often <3%. This doesn't suggest air leak to me.
I have a new OBD scanner coming tomorrow that should let me see cylinder misfire counts, but I'm currently stymied.
What should I check next in the list?
Other symptoms - very slight miss while idling. Pulls like a bastard still. Misfire wakes the CEL light up around 50mph and around 2,200rpm. If I keep under that the ECM never recognizes anything is wrong. Fires right up from dead cold without any hesitation at all.
Intake manifold gaskets get hard and suck air when the engine is cold. Spray some water or carb cleaner at the base of the manifold right after starting it cold. Should feel a stumble if they are bad
Easy Saturday job, no coolant in the manifold. Careful not to drop pebbles and crap down the intake ports, and its helpful to have a spare set of hands while its going out and
back in.
I swear I typed this same reply a couple of weeks ago...
I read your question over again. Higher rpm suggests the intake isn't the trouble. Is your scanner capable of showing the misfires as they count? Really helps to narrow it down. Fuel trims stay good when it starts to miss?
Light acceleration miss at cruise really suggests weak ignition somewhere. No acceleration miss suggests lean.
Hd trucks won't always code for misfire, depending on the emissions rule pack you live with. Misfire counter should still be there, somewhere.
Exact symptoms my truck has and it has been diagnosed to be the intake manifold.
Going to take care of it once things get a little warmer.
One of your coils may be on it's way out, if your scanner can identify which cylinder is mis firing
The truck doesn't divulge individual cylinder misfires sadly.
I hooked up the scanner and drove around last night watching fuel trims. Under load I could get it missing and it was adding up to 10.9% fuel trim to bank 2 and 8% to bank one. It won't do that unless it's under load.
Right now I'm thinking air leak on the intake side. I'm making a smoke leak tester to see what I can find - intake manifold gasket is a pretty prime culprit for this though.
If you can induce the misfire under load, I would recheck all the ignition. Dropped a plug and closed the gap? Cracked it with the socket? Greasy fingerprint caused a carbon track? New wire faulty out of the box? One of the coils is soft?
10% fuel trim is on the edge, but not badly out of line.
Coil could be soft. Smoke machine is a cheap thing to build and I can eliminate a whole slate of things by using it.
I have never, ever, seen GM truck coilpacks fail. Not saying that it's impossible, but it's very far down the list of things to check.
I have seen many wiresets fail. The plugs also start to fail around 60-70k, sooner on more heavily loaded vehicles.
There's also an F.U. ground behind the right cylinder head that causes all sorts of weird issues.
There's also the possibility that your fuel pump is crapping out if the miss happens mainly under high load. GM made awesome coils but the electrics on these trucks SUCK, the only ground path from the battery/engine/cab to the frame is through a braided ground near the idler arm/AC compressor, that corrodes to nothing. The fuel pump grounds to the frame. When that ground goes bad, everything works but the fuel pump and a couple other things, because almost everything grounds to the cab or the engine.
In reply to Knurled: that is great information. I think I am going to be adding some new grounds to my truck as soon as the weather gets better.
Hay Dave if I came by could you hook the smoke tester to my truck?
In reply to dean1484:
Sure, or you can borrow it. Just needs an air compressor and 12v power to work.
The truck has hydroboost brakes so I pulled the PCV line out of the top of the manifold to introduce the smoke. Turn the smoke generator on and the only smoke is coming out the PCV line I just pulled out. Plugged the PCV line and got no smoke from anywhere else and the glove I had over the throttle body didn't inflate.
How boned am I?
Have you pulled any plugs yet and looked at them?
Not since I replaced the old ones 30 miles ago
Did the old ones have any scorch marks on the outside porcelain?
DaveEstey wrote:
That is not the important end of spark plugs anymore. With modern ignition systems, almost anything will fire inside the cylinder.
Look at the porcelain. Are there any black stripes? That is a carbon track, and it will make the spark jump to the head instead of the ground electrode.
Streetwiseguy wrote:
That is not the important end of spark plugs anymore. With modern ignition systems, almost anything will fire inside the cylinder.
Not really. They're more tolerant of abuse, and modern fuel control and combustion chamber shapes are much better at presenting a good environment for the sparkplug to light off, but when the gaps open up from wear and they get fouled and glazed with crud, they don't work so good no mo'. And then the higher stress on the coils takes them out. Just ask any Ford truck owner about overstressed ignition coils 
And you still need to look at the business end of the plug to get an idea of what is going on in the cylinders. That will never change.
In reply to Knurled:
I bet I haven't seen a misfire caused by an "inside the engine" spark plug problem in 20 years. On the other hand, its pretty rare to have a week that doesn't need a coil or boot or wire. And, I don't consider an oil fouled plug to be the fault of the plug.
Fuel trims tell me more than looking at a plug any day.
And Ford just builds crap coils.
The ceramics show no signs of carbon tracing.
Found another vacuum line that was diverting smoke into the crank case. Plugged it and could find no leaks so I'm counting intake manifold out.
Ran a compression test. 165+psi on all cylinders. Plugs that came out looked good.
Pulled the injectors and they all ohmed OK.
Getting frustrated.
Crank position sensor going bad?
