P0300 Code. Here's my issue and question.
 

Okay, just bear with me guys because I want to give you as much info as I can so you can help me. Car has 112k and is all stock components except for in sig. Car started miss firing the other day. Never had an issue of any sort up to this point. Cold start, hot start, it will miss fire, almost to the point of dying. Idles fine though. Starts misfiring whenever you accelerate. It will do this for 4 or 5 minutes then will stop. Won't miss a beat until you kill the car. As soon as it stars up, idles fine and will stay missing again once you get going. After a few minutes it quits again. This cycle repeats its self every time I start the car. Is only been doing this for a few days now. When its running bad the check engine light comes on with a P0300 (Random multiple cylinder misfire). When it stats running good again after a few minutes the light will go out. Any ideas? Time for new plugs or something more serious?

That particular code can be triggered by almost anything that can cause a misfire, from a broken valve spring to a bad plug or plug wire. Have you done anything under the hood lately? Like oil the K&N?

No. I haven't done anything under the hood. All the plugs and wires are stock.

The most efficient way to solve this is to take to a shop with the Scan tool to monitor the misfire counters etc. and isolate the problem.

Another approach is to check/replace those things that due anyway. At 112k miles, for example, if the plugs, plug wires, fuel filter, or front O2 sensors are original they are at their useful life anyway.

It might be clogged cats. I no someone who had a misfire on a ford explorer and he replaced the cat and the misfire never came back. i no it was a ford but that might be a possible solution.

Well I just drove home from work and here's the latest. Got in the car, started up fine. missed under a load for quiet some time. Also to note, while it's missing, if I put the pedal WOT, the missing completely stops and the car starts to pull like normal. Get on the freeway and the missing clears up and goes away. Get off the freeway 15 miles later and is runninng fine. Pull into gas station to get gas, still running fine. After getting gas, start the car up, instantly running like poop again. Ideas??

Everything that comes to mind should be triggering another trouble code in addition to the P0300. Engines that run lean work better once warmed up - but a lean condition should trigger another code. The O2 sensors and catylitic converters also like to be warmed up, but either would be triggering codes.

I'm just as confused. I've got a set of ACDelco plugs, MSD wires, fuel filter, pcv valve, 2 Delphi o2 sensors, new accessory/ac belts, an oil change and a Magnaflow catback (To replace the FM currently on it) just sitting in my garage because I have no time to put them on. I think it's time. I may also run a bottle of fuel injector cleaner through the this tank of gas.

I wish it was a lean condition. If anything I would think it's running rich. I cruise between 60-65 mph on the freeway in 6th gear and I have yet to get more than 17-18 mpg. I'm averaging about 180-200 miles per tank.

If your luck is like mine, all those new parts will trigger some new code or problem.

Good luck.

When you shut the engine off do you get any run-on or dieseling?

No I don't. It starts up and shuts down just like it should. It's only when I first get started down the road. It's weird because it will be running just fine, and I can turn the key off, then instantly start the car right back up and it will start missing again...

It could be something like a lifter that takes awhile to pump up and that collapses quickly on shut down. I have seen folks chase this code (P0300) for weeks because it can include so many different things. A scan tool would at least tell you which cylinder, or cylinders, are involved.

I've had a lifter do that before in a different car and it made alot of noise. Mine is quiet aside from the noise associated with the misfire. It doesn't sound as if it would be a lifter.

Maybe a sticking injector or possibly a vacuum leak that seals itself after the engine is warm.

So I took a look under the hood just looking for anything obvious. The only thing I found was the line running from the pcv to the intake. The rubber has given away and that line is completely collapsed. Whenever I grab the line and pull on it it causes the engine to surge. Could this be the issue?

To my knowledge that could definitely cause a misfire. I remember my girlfriends car had the same problem, it ran like shit until I replaced it. It also had a bad plug though.

that's def a problem...vaccum leak allowing air into the intake that the MAF isn't registering. Definitely replace that line....heater hose lines work fine.

The hose isn't broke open. Its collapsed closed.

Now that I've been under the hood messing with that line my car runs even worse. I just pulled the line off then put it back on and now the car misses just idling which it wasn't doing before...