Multiple misfire codes , what the heck ??
 

Check engine light came on while sitting at a stoplight , the truck was kinda lopeing off and on at idle , but took off just fine and seemed ok till i got home , one thing stood out was the trans would suddenly start downshifting / upshifting when going up very slight inclines - where as it never struggled before on the same roads , anyways i pulled the codes and got these ,, PO300 , PO301 , PO305 , and a pending PO303 code . I know that the po300 is a random multiple misfire code and the other codes are the cylinder #`s that are misfireing , #1 & #5 with #3 pending ,, Plugs , wires , cap and rotor are new and replaced back in august , and everything looks fine under the hood , anyone have any ideas as to what could be going on with it ?

My truck does this too only with P0300, 301, and 303.

I'm going to try and replace fuel filter and then pump if I need to.

Plugs, wires, rotor and cap are all new too.

That might be a little difficult for you as the filter is part of the fuel pump and not a separate replaceable item . I done some pressure checks last night and i get a spike of 50 psi on the gauge when i first turn on the ign , and when the relay clicks the pressure drops to about 48 psi , after 10 min of sitting the pressure drops to around 42 psi , after 20 min it drops to 32 psi , when i start the truck and let it idle , the needle on the gauge flutters like crazy for the first few minutes and then sorta slowly floats between 44 and 46 psi ,, im guessing that it`s probably the pressure regulator at fault and it too is part of the pump assembly and it`s looking like for me too that replacing the fuel pump is my only option ,, but first im going to pull the plugs for 1-3-5 cyl`s and look at them to see if anything out of the ordinary is going on ,, maybe it`s the injectors leaking down all of the sudden :icon_denk: ??

I have the same sodes identicaly now for the last two weeks and every winter they show up along with a p0420,the truck doesn't perform any differently and they only show up in the winter time for me,usualy in about another week they will dissapear on a cold start up,i know that my plugs,wires,cap and rotor and cat and o2 sensors are all good so my feeling and my mechanics are that the Ethenol in the fuel(winter months) is a stronger mix and that is what throws the codes,im sick of them myself and the light on the dash is even more annoying,but i delete the codes with the negative off the battery and the ignition and they just come back again,it's just in the winter so im used to it being a 2000 my truck is in top shape and i know my vehicle so i just keep up on the matienance and ignore the codes. i run Sunnoco 91 in every tank too

My truck was throwing all those same codes. After replacing all the sensors, tune-up, wires, ECU, fuel pump, etc., the cure turned out to be a thorough cleaning of the throttle body and the fuel injectors. Hope this helps to sort your problem.

UPDATE ; sort of , i hear you guys on the winter fuel mix causing the problem , i did pour a bottle of seafoam in the tank and ran that through , no changes that i can tell on the way it runs , it still has a noticeable shake at idle & in gear , and runs out fine when pulling away from a stop light . Today i pulled and inspected the spark plugs , all were good and clean , decided to pull the injectors and check for leaks ( read about doing this on another thread ) and im happy to report --- no leaking injectors when they are pressurized with the key on , but it still slowly lost fuel pressure after about 10 minutes of being pressurized , sooooooo it looks like a fuel pump R&R in the future for me :icon_doh: