TheSilentBeast

I have a 98 silverado k1500 z71 with 239000 miles that has a p0300 code. I have done a whole ignition tune up... new wire-set, plugs, distributor cap, ignition rotor, ignition coil. So basically it is like chasing a ghost. I need to get it fixed ASAP so I get it smoged to get get my tags. PLEASE HELP!!

Ronin1014

p0300 is a generic code for multiple/random misfires. Could be a failing coil pack, O2 sensor, EGR valve, cam positioning sensor, catalytic converter or failed exhaust valve. Those are the ones I know of aside from plugs and wires which you have tended to already. Start with those is all I can say from here.

jgray152

I had this issue too. I was chasing it down forever. I then looked at the spark wire ends connecting to the distributor, the electrode in the wire was getting pushed back into the boot when pushing them onto the distributor creating a large gap in the connection. They were cheap wires.

TheSilentBeast

I also forgot to put up this picture... This is when I did a I/M readiness test on it with my scan tool. But I dont know what it means. Was wondering if you do or someone else does?

Ronin1014

I believe red X means fail, circle with strike through it is an active monitor and green check means pass. My Scanner is way different so some one with the same scanner will have to confirm or deny my statement to be on the safe side.

TheSilentBeast

I looked in the manual and you are correct... But does that mean that I need to replace them or just clean them?

Ronin1014

Being a 98, I would go through and replace the O2 sensors first then check over the catalytic converter (Both if true dual system like mine) for cracks in or around it and see what happens from there.

jgray152

A misfire can set off emmisions codes such as O2 and cat. EGR and EVAP shouldn't be thrown though. Maybe your EGR is bad? EVAP is a different system which controls fuel vapors from the fuel tank and sends them to the engine to be burnt. O2 sensors shouldn't cause a misfire. If you really want to see if they are the cause though, simply unplug them and run the engine. ECU should fail safe that system and run engine normal.

o2 sensors can't be cleaned. BTW you can find good deals on amazon for same brand o2 sensors. well below the cost at a local parts dealer.

If you check the wires and all looks good, take some brake cleaner / carb cleaner and spray around the intake manifold and check for leaky manifold gaskets.

Also try poping out the brake boost vacuum check valve. Round plastic thing with a large vacuum hose connected to it. pull it out of the booster and put your finger over the end of it. If engine runs normal, booster is bad. If you take of the check valve and put your finger over the hose end, this will check for a bad check valve leak. Check all vacuum related sources such as cruse control if you have it.

TheSilentBeast

Could I use wd-40 instead to check for an manifold leak? Someone said that I can use wd-40 to do that just want to make sure...

Ronin1014

No, you should use brake or carb cleaner. WD-40 doesn't have the same pop to really change it's idle noticeable enough for this application, additionally it's a lubricant and could make any leaks worse and make trouble with replacement should it be necessary.