So, I have a P0304 code, cylinder 4 misfire that will come up only when I first start the vehicle, like after sitting overnight. I will start the car, the SES light will start to flash and the truck runs pretty rough, but then it clears up within 10 to 15 seconds. SES light goes back to steady and the engine idles like there is nothing wrong. I have replaced the plugs to Bosch, wires to Magnecor, all 4 O2 sensors and I just replaced the #4 injector thinking that the old one was clogged or leaking. The P0304 code is the only one that is being thrown. Any one have any ideas? Was thinking it could be the Coil or a vaccum leak but it seems like those would last more than 15 seconds after start up. Thanks

 

NOPE, my money is on a leaking head gasket, leaking coolant into that cylinder over night and then running rough as it tries to burn it off on a cold start.

 

Sorry, but bad news maybe your future. Go to autozone and find the kit that test for combustion gases in your coolant. If it test positive, then its a coolant leak. Do you have to top off your coolant frequently? Let the truck sit for a few days and pull the plug. If uou answer yes to any of these questions, then you have headgasket repairs in your future.
If no, its possible you have a bad lead. But its hoghly unlikely with the symptoms you have described

 

I had a little different thought. I agree with the possibility (actually probability) of a head gasket leaking coolant into the cylinder fouling the plug until it clears out. The other thought that came to mind was a leaking injector fuel fouling that cylinder until it clears out after starting, I initially thought this unlikely because that intake valve would have to be open for it to foul the cylinder, but maybe it's pooling on top of the intake valve and then when you attempt to start it it floods the cylinder when you're trying to start it. I think one way to be sure would be to pull the plugs (after it sits overnight), disable the ignition, and then crank it over and see if any spray squirts out the spark plug hole (don't be looking at the hole when doing this, I did this on a XKE years ago and the resulting spray of coolant/water took out a shop light 15 feet above the car, glad I wasn't staring down the hole) Again, I think the previous posters are probably correct, but there is another possibility.

 

D2 injectors rarely leak and if it was it would burn off in 2 or 3 strokes.
As for the test, don't spend money on a exhaust test, if anything a coolant pressure test is what would be called for.

 

Thanks guys, thats sort of what I was starting to think it could be as well since it seemingly runs fine after a few minutes. Figured something was in the chamber that had to be burned off which is why I replaced the injector, but that didn't fix the issue. Head gasket replacement isnt too bad from what I have experienced in the past on my 02 Disco and from my 04 Silverado. These Discoveries are by far the easiest cars I have ever worked on. Dont really have to add coolant much at all, but I just did the coolant flush less than 4k ago so it just may not have had time to drop that low yet. I will get a bottle of that coolant in the combustion chamber testing mess and try that out. in the meantime, going to start putting together the parts list. Would it be prudent to get the heads decked and cleaned up at this point? Or not a good idea since the block wont be getting decked?

 

Its all on you. Personally, every headgasket gets the head decked. Because you'll be pissed if you put it back together and it's warped.

 

Not trying to be a dick but 10 to 15 seconds of fuel is alot of fuel.

 

Not trying to argue either, but I'm thinking you've (since you are a tech) seen gas fouled plugs that wouldn't clear for quite a while since the gas was bridging the gap so the plug wouldn't spark until it got blown out (or you have to take the plug out and clean it manually) I know I've seen this a number of times over the years. I did agree that it was probably the head gasket. I think I averaged doing 3 sets a week during the time I worked at a LR dealership (nice job security )

 

Curious, I was looking at the head gaskets and was trying to figure how coolant would leak in to cylinder 4? Seems like the water jackets are only on the far ends of the heads? Just was thinking it would be on one of the end cylinders first?