p0300

[jeggers]

The previous owner says the cap plugs and wires were replaced. it does mis from time to time. Where should I start?

[HenryJ]

Do all the basics, check the cap and rotor. New does not always mean good. make sure the housing is not cracked from the loctite on the hold down screws. Check each spark plug looking for their condition. Replace it in doubt. Note differences.
Fuel filter, throttle body clean, MAFS clean, air cleaner, EGR valve clean, O2 sensors and wiring. Check all connections and look for damaged wiring both above and below the engine.
If those don't clear it up, check the fuel pressure and cylinder compression. You might consider the need to replace the intake gaskets and have a look under the valve covers at this point too.

Searching P0300 will yield lots of avenues. Sorry there is no clear cut answer.

[ApproachMedium]

Do you keep resetting the codes or have you been driving it around and its holding the same code? Does your check engine light ever flash while idling or while accelerating from a stop? My truck will often throw this code but a day or two later it throws the P0303 code after reset which is misfire on #3. Mine is unfixable without rebuilding the motor thought because I have lost compression on #3 and there is a valve problem on that same cylinder . We think it got that way because many wont change that plug since they think its not easy to get to. Dont make that same mistake the previous owner of my truck did if you want the plugs changed do it yourself. One bolt detaches the steering shaft, you pull it back and you can get right to the #3 as easy as any other.

Brule has some good advice above, I would definitely check it out esp if you just got the truck. Keep driving it around and check the codes. If you know someone that has an advanced car diagnostic computer or software have them plug it in and monitor the engine running at idle. Thats how we found that my engine does not fire at all on #3 at idle but as soon as it hits about 1,000RPM it fires like it should. Needless to say it gets excellent fuel economy on the highway, and terrible fuel economy sitting in traffic.

Your motor shouldnt crap out though from something like this. Mines been doing it since 200k and it just hit 250k the other day.

[jeggers]

Yes it would flash when driving, I would reset iit and it would only come back while driving, never while sitting and idleing. It woulfd only through the p0300 code. I found a mess of issues with the truck. The transfercase switch had been bad for some time and the entire vacuum system has atf in it. I replaced many componets and flushed it.. to resolve all the vacuum leaks. the distributer had the screws stripped out and the cap was bounceing when driving. I got it to run well but woulld still through the code after some time. Unfortunatly, the atf in the vacuum system was mostlikely feeding into the engine for some time. I was offered what i paid for the truck and sold it. I explained the issuess to the new owner and I am pretty sure he was going to drive it like that until it died. So I traided my car in on a Silverado 2wd wt. Thanks for all the info and help.