Cyl 3 Misfire at Idle - Out of Ideas
 

My tuner and I are stumped. My 2005 S2000 has a misfire at idle and only at idle. The rest of the rev range is smooth and solid.

The factory motor had 150k miles, but was showing low compression: Cyl 1 to Cyl 4 was 210/200/190/170. Leakdown was over 30%. We just installed a rebuilt cylinder head with OEM valves, bronze exhaust guides, Supertech titanium springs and retainers. Valves are adjusted at 0.009 on intake and 0.010 on exhaust. The ECU is an AEM series 2.

Compression is 210 in each cylinder and leakdown showed less than 10%. Coil packs and spark plugs had less than 1000 miles but we changed them anyways. Plugs look great. We swapped injector sets with no impact. He took fuel away on individual cylinders and confirmed the swing in AFRs but no audible change in the misfire (which is odd). He then added in fuel but AFRs didn't swing down (also odd). Finally, he changed the AFR across the board from 11.0 to 16.0 but again there was no impact to the misfire at idle.

Any thoughts on what would cause a single cylinder misfire like this?

My guess is injector driver on the circuit board. That or ground wire fault in harness. In for results good luck

Thanks for the ideas. We'll give them a shot and I'll report back if/when we get it worked out.

Try 0.012" on the exhaust valve lash.

Billman's rec.

Thanks for the recommendation. Since I've not an 05, non-drive by wire, the .010 should be fine. If all else fails, we may adjust it a but more though.

Can you try and run it on stock ecu?

That's a good idea. We can swap in factory injectors and see how it goes. That would rule out the ECU.

All right - I think we've got it solved. He pulled the valve cover and ran another leakdown and heard a hiss on the exhaust side of Cyl 3. Turns out a carbon deposit had lodged and was preventing a tight seal on the valve. He cleaned it out and everything is top notch.

Thanks for all the ideas!

That's interesting! Glad its solved now