I'm getting a DTC code PO300 misfire condition. I just replaced the wires, plugs, added headers and a new y-pipe with o2 sims. A real slight change in rpm can be heard; the SES light actually flashes when revving at a constant 3k then stops flashing when the rpm's are back at idle. I made sure when I attached the wires to the plugs, opti and coil that I heard them "snap" on. Maybe I should have checked the resistance of each wire before installing them. Iheard that Taylor's were pretty good wires.
I turned off all the lights in the garage and didn't see any sparking anywhere under the hood, from above or below. Just for the heck of it I checlked the resistance across the #7 spark plug (NGK TR55) and it was around 5k; I checked 2 of the original AC's and they were 2k.

Any opinions on what I should look for or do next??

Much appreciated........

 

ttt

 

ttt

 

what are the plugs gapped at? did you replace the coil wire?

 

.050; yes I replaced the coil wire.

 

just took a shot in the dark there...it has happened before around here...

 

it could be possible that you have one defective plug, though not likely. or you might have one bad injector. just possible ideas...

 

when was the last time you changed the fuel filter? are there any vacuum hoses that were left disconnected?

 

Vaccuum hoses, are the rear AIR/EGR holes closed, fuel filter, have you had the car tuned to remove the AIR/EGR?

 

A bad O2 can cause a misfire condition .... have you ever replaced them?

When I installed my random tech Y, I replaced both front O2s and started getting that code. Turns out one of the new O2s was bad.

Thomas.

 

he said that, he put in o2 sims. I guess it is possible 1 of those are bad...I have never used them not sure.

 

"A bad O2 can cause a misfire condition .... have you ever replaced them?

When I installed my random tech Y, I replaced both front O2s and started getting that code. Turns out one of the new O2s was bad."

I didn't replace the 02's upstream of the cats. Is there any way to check if their bad? I replaced the downstream with 02 sims; they have a red light that's blinking on them so I consider them to be working.

Stoopalini, could you hear a slight miss after you installed the ypipe and found one of your 02's was bad?

The fuel filter has never been changed but I also didn't have this problem before the changes to the engine.

 

I did a resistance check on the coil wire and it looked good. I checked the resistance of the windings on the coil and all three pins were around 6k.

I took the code out last night and ran the engine in the garage today for about 20 minutes; no code showed up but I still can hear a slight miss; it just doesn't sound right to me. I can't take the car for a ride because it's up on jack stands due to the installation of the BMR rear end components and the weather here being cold and crappy.

I was able to snag onto a Hypertech power scanner from a friend and this was the readout I got:

RPM- 0
Idleld- 3188 RPM
EGR d/c 0%
Bat 12.1
spark 0btdc
Retard 0 deg
IAPwb1 0.0 ms
IAPwb2 0.0 ms
stftb1 =1% 128
stftB2= 1% 128
LIFTB1 +1% 128
LIFTB2 +% 128
f1 TRIM C - 0
hO251B1 400 mv
hO252B1 varied from 396-624 mv
hO251B2 635 mv first check; 118 mv second check
hO251B2 varied from 388-620 mv
MAF 0.0 gls
Loop= open
TP (%) 0%
TP (v) 0.53 v
map 99 kPa
baro 100 kPa
IAC 101 counts
ECT 169 deg
IAT 81 deg F
EGR load 0%
AFR 12.0 1

I'm just learning some of this stuff so bear with me; what does this data tell me?

Thanks in advance.....

 

ttt

 

bad downstream o2s should only throw a constant SES light for emissions and will not cause a misfire (which causes the SES light to flash). however bad upstream o2s will cause a mis, i had a bad upstream o2 after a header install and got a multiple cylinder misfire on one side.

if your car seems to run fine untill it gets into closed loop at which point it starts to mis, then this might be the cause. if it does only mis in closed loop and you checked the upstream o2s, i would still lean towards something electrical or sensor related.


good luck, hope this helps