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p0300 misfire problem

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poorkid

New member
ok guys. i have read the p0300 section with no answers. seems like us p0300 victims have the same problems but yet to hear what it was. this is my problem. my sister has a 1999 3.8 gtp. one day drove car to her sons house, came out and started her car to realize it was missing bad. almost like if you would take a plug wire off at idle. that kind of miss. with the CEL blinking. took it home. was scared. called me i told her to take it to auto zone to check code. it was #5 miss fire. her boyfriend put plugs and wires. still missing bad. i live 30min away so i told her to bring it over. she said that while driving it didn't seem to be missing but at an idle bad miss. i checked it with a code reader. i got p0300 random miss fire. i have a spark checker and checked spark from each coil from one to six one by one and all have spark coming out the coils to the wire and did the same at the plugs with all having spark as well. thought it might be the cat. disconnected at the motor right after the 02 sensor still missing bad. car was getting hot. temp gauge in the car showed normal but on the code reader showed hotter then the car. it got dark so i looked under the car and said theirs my problem. we have the cat,,,-short piece of pipe with the other 02 sensor,,, and then the resonator,,, then a Y back to the mufflers. well, right after the cat at that short piece of pipe with the02 sensor it was glowing. i said OK. took the flange apart at the resonator and STILL missing bad. i didn't think it was the cat because it was glowing after the cat, not before or at the cat only after. at this point i think since the CEL is flashing i think its sending raw fuel down the cat breaking it apart and maybe clogging the resonator. so i left it apart for now. today i checked the fuel injectors wire with a noid light with all having juice to them and removed the injectors just to see if any of them had bad carbon on them which they didn't. then check for fuel to see if they are shooting gas out and all shoot gas out of them. i mean I'm 100% old school. and this new **** is kicking my butt big-time. i haven't replaced any sensors because of my sister not having a lot of money. that's why I'm trying to check as much stuff as i can. i mean, you guys would know a hell of a lot more then i would when it comes to new technology. i mean, would a 02 sensor,-TPS,-MAP sensor,-MAF sensor,-cam ps,-crank ps make the engine run like if its running on five cylinders??? i also checked the fuel pressure switch on. and waited to see if it would drop after like 5 min or more. and held the pressure that is recommended. man. can some one please help me out here.... thank you in advance.
 


I didn't read all of that because of the huge wall of text but what plugs were used? Have you changed the coils or ICM yet?
 
not sure what plugs she bought and no, did not change the icm. as for the plugs. problem was their before the plugs were changed.
 


Misfire on 5, replaced all wires and plugs and it went to p0300. I would replace the wires and plugs with the originals and see if it went to one cylinder again. If it did then swap coil, plug, wire, injector to different cylinder and see which one it follows. Adding new wires and plugs just makes it way harder to diagnose.
 
Kyne said:
Misfire on 5, replaced all wires and plugs and it went to p0300. I would replace the wires and plugs with the originals and see if it went to one cylinder again. If it did then swap coil, plug, wire, injector to different cylinder and see which one it follows. Adding new wires and plugs just makes it way harder to diagnose.
Click to expand...

i have the old wires and plug. will reinstall them back on the engine to see what i get.. thank for the tip.
 


had the old wires but not the plugs. the wires are the same as the new ones. looks like at one time she had replaced the wires. but.
ok, went out today even though i had checked the spark coming out of the coils with my spark tester. i took a pair of pliers installed some rubber hose on the handles and checked each plug wire coming out of the coils and this is what i found. --> yes working--> no not working. take notice how the #1 plug acted.

DSCN5118_zpsa0a2e250.jpg
 
found out that a valve spring was broken in #5 cylinder. went to change the spring but i can't hold air in that cylinder to keep valve up ....anyone?
 


you try pulling the valve up while the air is on?

seen vids where people have put string in the cylinder, then turned the piston up, this holds the valves up.
 
around 90lb of psi should keep it up and you just have to make sure your compressor is big enough. Also keep in mind that when you use a engine compression tester..you have to remove the schradar valve (might be calling it the wrong thing) so it allows the constant air pressure to keep the valve up.

Worse case scenario you can always just pull the head.
 
yea, i tried pulling the valve up while shooting air in. and my compressor is big enough. i can hear the air on the opposite side hissing out somewhere in the motor. tried to see if i seen anything but nothing. ether the other valve in that cylinder bent, a hole in the pistion, or blown gasket. what do you guys think. and if any. what you guys think i missed? thanks.
 
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