Code P0300

'02 TB w/ code P0300 / Random misfire. Started the other day when it was very damp out, FWIW. Feels like ur driving on rumble strips during acceleration. Rough idle also. It does not do this continually though as after a while (minute) it clears up and runs smooth for a while then runs rough & light will again for a while. I seafoamed it today since it been about 60k since last seafoam. I thought that might have helped untill I drove about 20 mile and it went rough again. I also cleaned the oil out of the cam sensor elect connector, since that caused rough running before, although it didn't throw that code. It seems to be very intermitant. '02 TB, 185k miles, orig plugs changed at 125k. Is it actually just going in & out of a limp-mode? Thoughts??? O2 since it's random ???

Thanks, dmince

Sounds like an ign misfire, bad plug or coil would be the first thing I would look into, anything but AC Plats or Iridiums will not hold up in that engine.

I agree...usually injector or secondary ignition.....but when an injector goes it's constant....

Plugs were changed 60 K ago(how long is that time wise?), but how about the cap, rotor and wires?

Remember no cap rotor and wires on the 4.2 TB.

Pulled the coils and plugs today. No water in the holes just small amount of oil in three. Plugs were all basiclly the same brown. I pulled 1-4 one at a time and put a spare plug in the coil and held to ground, all 4 had spark. With all the plugs dark so far I installed all new plugs. Did not chk the 5&6 coil due to all the plugs looking similar brown. reassembled all and it seems rougher than before. I started it three times and left it run for a short time 3-4 min. the third time it ran smooth for a while. I did pick up one coil on the parts run, I'll try again tomorrow morn swapping out one coil at a time. FWIW, my son had the exact same symptoms on his honda (only after warmed up) and swapped out the O2 & fixed it. ??? Although this started after a rainy night, I'm not sure this is the typical TB wet cowl condition I've been reading about.
Thanks, dmince

Probably 3-4 years since last the plug change. Yes, AC #41-103 Iridium used last time and today.

What scan tool are you using to get the P0300 code?

Actron. Cp9235. Just ur basic scan/reader

O2 will not cause a miss, after the ign. is checked test the resistance of the injectors, look for one with a big difference from the others.

Max, once again you are on the ball.......I was thinking Blazer, 4.3, not Trail Blazer, 4.2......

Actron. Cp9235. Just ur basic scan/reader

Is that capable of giving you the TRUE code(s) or is it giving you generic OBD II substitutes?

Is that capable of giving you the TRUE code(s) or is it giving you generic OBD II substitutes?

Just gives code and description

Fixed.. bad coil on 5. Smoothed right out.
Thanks for all ur input & help.

Fixed.. bad coil on 5. Smoothed right out.
Thanks for all ur input & help.
That's kinda my point. The REAL code delivered by a REAL scan-tool using professional-grade software would have been P0305--misfire on #5 cylinder. You'd have zeroed-in on cylinder #5 immediately.

The generic OBD II code pulled off by a consumer-grade code reader is P0300--"random misfire". You were led to dick with cylinders that had no functional problems--because your code-reader isn't capable of delivering the TRUE code. Your tool lied to you, told you the misfire was "random" when it was specific to #5.

Folks wonder why I crusade against junk consumer-grade code readers. I used the Actron web site search function, the 9235 isn't even listed.

Not everybody can afford a Tech II scanner for an occasional car problem. If they could they would probably just take it somewhere to get it fixed. Most of the time scanners just point you in a direction and the technician still has to use his expertise to core down to the exact problem. It may have took him longer but he seems to have a head on his shoulders and perservered to a solution. Way to go dmince!

A basic Actron like I use for quickie diagnostics pinpoints cylinder misfire, 9185, no one is saying a Tech11 scanner is required here. Just a working one.

That's kinda my point. The REAL code delivered by a REAL scan-tool using professional-grade software would have been P0305--misfire on #5 cylinder. You'd have zeroed-in on cylinder #5 immediately.

The generic OBD II code pulled off by a consumer-grade code reader is P0300--"random misfire". You were led to dick with cylinders that had no functional problems--because your code-reader isn't capable of delivering the TRUE code. Your tool lied to you, told you the misfire was "random" when it was specific to #5.

Folks wonder why I crusade against junk consumer-grade code readers. I used the Actron web site search function, the 9235 isn't even listed.

Thats exactly what he's sayin.

Your right on the scan tool not zeroing in the the exact cylinder but it's better than nothing. I think it was a bill a few years ago and has saved me that many times as a Trailblazer owner :). I just carry it with me all the time. From what I've read the P0300 was common with other posters and if they got a P0301 and swapped coils it gave the P0300 and did not follow the bad coil. I assume they weren't all using cheap readers also???
It was time for a new set of plugs anyway. Probably should pick up a spare coil for insurance with 186k miles on the OD.
All's good....and the Trailblazer is back in commission...Thanks to all the help here as usual.

Thanks, dmince