P0300 & P0308

[Yeret]

Ah, yes, the dreaded P0300, the one code that I would have died happy to never see again in my truck. It also brought a close friend, P0308.

Brief background. Last summer, I noticed that every once in a while when the engine was under load in third gear, mainly while going up a hill, the "Check Engine" would start flashing at me. Didn't do it every time, just once in a while. Never got any set codes and the engine certainly wasn't acting like it was misfiring, so I just kinda shrugged it off as a glitch.

Today was the first day that I had the truck out in a couple weeks (finally got the Taurus' brakes sorted and have been driving that) and while leaving Wally World, I decided for the hell of it to kick off the overdrive while turning from the on-ramp onto the highway. Sure as hell, "Check Engine" starts flashing at me, but this time, it persisted well after I had let off the throttle, whereas before it turned off as soon as I had eased up. After a short bit, the light stop flashing but stayed on constant this time. Ah, maybe NOW I've got something to go by!

Got home, plugged in the scanner and got those two codes, which intrigued me. Back when my plenum was leaking and I was constantly fouling plugs, I was constantly harassed by misfire codes and boy, did the engine feel the part. Problem is now, the engine feels just fine. No hesitation, stumbling or any "misfire" related stuff. Power and smoothness-wise, the engine feels no different than it ever has ever since my plenum was sorted.

I checked a couple of spark plugs, including #8, and they looked perfect. Some light tan deposits, mostly clean insulator, gap still at .040." Ohm'd the coil, primary was 1.4, secondary was ~14,000. Near recommended limits, not sure yet what to think about it yet. Fuel injectors ohm'd fine, fuel pressure while running was 50 PSI on the nuts. Used my scanner's live data function and noticed nothing odd about the sensor or fuel trim readings. Didn't check cylinder compression...yet, but as strong as the engine runs, it doesn't strike me as a factor...

I'm tempted to say that the computer's getting a wonky reading from the crank sensor, which I had replaced a few years ago. I'm just not really sure what to think. As far as I can tell, the engine's running perfect, but the computer seems to disagree.

Anyone got any suggestions on what to check next?

 

[Yeret]

Pulled out the compression tester this morning and tested cylinder 8. Disabled ignition and fuel system and unplugged the injector. Engine was cold and I recorded 148 PSI. So, unless that would be considered TOO high, I definitely don't think cylinder pressure is causing my problem!

 

[Yeret]

Okay, I purchased a new ignition coil (NGK). Arrived today and I decided to ohm it before bolting it in. Recorded 1.6 at the primary and 12,800 in the secondary. Interestingly, my old coil ohmed 13,300 at the secondary, a fair bit less than the last time. The coil was still a tad warm when I first tested it and I know resistance changes with temperature, but shouldn't the resistance DROP as the temperature increases since electricity flows easier through hot conductors? Maybe the old coil's resistance increased enough when warm to trigger a misfire under load.

Anyway, I bolted in the new coil and driving conditions that were triggering the "flasher" did not do it this time. So, as far as I can tell, the issue seems resolved. As a bonus, the engine does feel a tad sharper than before, especially when the tranny kicks down a gear, and does seem a little stronger while approaching 3,000 RPMs. We shall see what happens in the future...

 

[SportRam00]

Could have been the coil... How did the wire look on that cylinder? Could it have been the cross talk from another plug wire?

 

[Yeret]

Wire looked fine. I routed the wires as per the TSB a couple years back when I installed them and the wires themselves are supposed to be RFI suppressed.