I bought a 97 Jimmy a few months back fromits second owner. She just moved nearby and drove it for 800 miles before I bought it. SUPOSSEDLY, her mechanic told her (before making the trip) that it needed a set of plugs and wires because it was running rough and the P0300 code was coming up. It idles pretty rough and sometimes it takes a while to start. While driving it, you can feel that there is something wrong but not as bad as when it's idling. I replaced the plug, wires, air filter,PCV valve and clean the cap and rotor. I also pulled the EGR valve and cleaned it. I figured she lied to me because it looked like a tune up was recently done. It has goodspark on all the plugs but seems like cylinders 5 & 6 (the ones close to the firewall) are not working. They get wet with fuel but you feel no difference when you pull either wireoff the plug. I have checked for anything obvious but I haven't got any luck. I'm pretty mechanically inclined but I really don't want to keep spending time and money on parts that won't fix the problem. I thinkmy next step isto check the compression to make sure I don't have any problems on the heads. Any ideas? Please help!

 

if you're pulling plug wires 5&6 with no improvement... then that's more than likely your issue.. first.. ohm out your wires... see if you have continuity... then check the plug in those cylinders.... clean... carboned.. wet... there could be a crack in the porcelin tht is causing it to arc before it hits the electrode... try swapping the plug from 5 to 1 (3 is a PITA) see if the condition moves with the plug.. same with plug 6 to 4 see what happens.. then swap plug wires 5 to 3 (it wont reach 1 and the wire part ofitisn't so bad) and 6 to 4 that will help you eliminate the plugs/wires.. if the issue moves when you swap stuff... if that doesnt work and it stays at 5&6.. could also take your ignition control module to advanced auto parts have them test it... and do an ohms test on your coil...

HEI
primary...0.2-1.5Ohms
secondary...5,000-25,000

EDI
Primary...0.2-0.5
secondary...5,000-25,000

all this stuff is free... and if none of that works.. you could do the compression test since its free... but I'd be looking for a spark issue... and replacing cap and rotor


another possibility is your spider... I had a similar issue where i had a p0300 it was the actual injector itself.. and a couple of poppets were stuck open....hence the fuel coating the cylinders....

let us know what you find

 

i just done a job on a 2001 p/u 4.3 po300 test all ignition no luck, i did a test with the snap on modis went to data and then misfire well shows me 2 4 6 misfirethis is why i did not get a single cylinder misfire,the last test was a gauge to test back pressure on right bank......pegged the gauge,,my cat was clogged just like that. put new Y pipe on and runs fine. min. back pressure is 1 1/4 lb....,,,,if you do not have the $$$200 gauge loosen the exhaust at the headers and try it.