I've been encountering a weird problem that I just can't put my finger on. I've got a DB8 GSR that randomly seems to misfire. It does this while just cruising down the road. I've gone ahead and ordered new plug wires and plugs, just waiting for them to come in. Hopefully that will correct the problem, but I'm afraid that isn't the issue. BTW, I'm running 310's on a PowerFC.
I finally noticed that when it misfires, the injector duty cycle is spiking according to the PowerFC Commander, would this happen when there is a misfire issue? What else could be causing the problem? It seems to me that something is causing the PowerFC to increase the injector pulse, right?

 

replace the igniter too. I (almost)guarantee that's the culprit(most likely). You'll know for sure if it just cuts off on you while driving one day or it wont start.

 

That happened this summer, so it's already been replaced.

 

I kind of have to agree with B16CRXT. Whatever it is, is probably ignition related. The reason that the a/f ratios are probably spiking is because there is no ignition event in the cylinder(s). This in turn causes all that unburnt fuel to travel into the header and detonate inside the header and it may sound just like detonation. And this also explains the a/f ratios spiking since not all of the fuel is burnt in the header. If this is the case then you should also feel a sputter or kind of like a brief and quick pulsing ignition cut.

I presume you already checked your spark plugs and that they are gaped correctly per your application.

 

It's not the a/f spiking, its the injector duty cycle.

 

That's a fail on my part. How did I skip that detail?!

If your duty cycles are spiking (depending on the severity), it can actually foul the plug and prevent ignition which in turn feels like a misfire since there isn't any spark..

Awkward. Is this the first time on PowerFC and cold weather since you got tuned? All the electrical grounds in place?

 

It isn't spiking that severely. I'll look tomorrow to be sure, but I think I'm at like 3-5% while cruising and it spikes to about 15%.
The car was tuned 2.5 years ago on the PowerFC, nothing has changed in the setup. It can't be the cold because it did it a few times before back in the summer/fall. All electrical grounds should be fine, but I'll try to check just to be sure.

 

Happened to me this summer too...on my way in to work. I was soo pissed. Luckily I had spare parts in the car that day.

 

Update:
General tune up seemed to do the trick. Once I put new plugs, wires, rotor cap and button it hasn't done it again since. I also had an issue with my A/C tensioner, removed the belt and it seems to be back to normal.