History: New engine, PRC heads, and procharger installed. Tuner was never able to find/fix a random misfire >4800 RPM. Tried new coil packs, new ECM, plugs, wires, checked springs... I eventually had to retrieve the car and I'm trying to diagnose the problem.

Issue: I noticed via my Aeroforce gauge that the misfire exactly matches a drop in the commanded AFR on my gauge. Commanded AFR sits at 14.6 at all RPM's up until ~4800RPM, at which point it quickly drops to ~9.0. At this point the car begins to misfire, loses power, etc.

The catch: The run logs I saw while he was tuning were as flat as could be... AFR never wavered. The fact that the measured AFR's didn't waver would make me think it doesn't really matter what the commanded AFR is... the actual AFR measured at the tail pipe was perfect. Have I potentially ID'd a problem, or is that illogical?

He tried MAF tuning, SD tuning, re-flashed back to stock and started from scratch... the car always started misfiring at about the same place. It misfires at WOT, and also if I very slowly creep up to those upper RPM's and just hold it there... it makes no difference.

 

Post your tune, config, and a log. What gap are you running in your spark plugs?

 

What plug, plug gap, and fuel are you running?? What injectors?

 

I will ask the tuner if he will e-mail me the tune and a run file. They changed the plugs and wires, so I will have to check on the model and gap. I'm running premium 93 octane, on siemens 60lb injectors.

I forgot to mention the misfire is present with meth turned on or off. With the misfire also happening when not at WOT, just when I'm slowly easing into those RPM's, does that rule out that it's caused by spark blow-out?

 

Has it been the same tuner all the time? Has anyone else looked at the tune or car? If you want to make the trip to College Station I'll take a look at it if dont get it figured out.

 

Thanks! I haven't gotten a file from the tuner, I've not gotten a reply from him. After blowing about 16 hrs on it this weekend I got really irritated and dropped it off at a second shop today out of desperation. Man I hope he finds something!

 

Pretty odd that commanded afr never wavers until then. Almost sounds like it is making an abrupt change into PE and dumping in fuel. Hard to say much with no log or tune file, but that is kind of what it sounds like. Good luck with it.

 

The plug and gap question is a good one too just incase that is about where boost starts to come in strong and it could be blowing spark out.

 

Absolutely. Had a Subaru I was tuning the other day and it was raising hell going into boost. Had him pull the plugs and found over 40 gaps.

 

OK, so out of desperation we unplugged every electrical terminal under the hood and re-snapped them together to make sure everything was snapped down good, loosened and re-tightened every hot and ground wire we could find... and it seems to be running fine.