Auto Stop/Start won't always start and 2nd-3rd gear misfires
 

Hi all,

First off, car is 2017, 15k miles.

JB4, green wire connected, 5 gallons E85 with 11 gallons 91 (w/ 10% ethanol) -- gives me E33 and conservatively about 94 effective octane.

I have an issue where sometimes the auto start/stop will stop, then when I release break/accelerate it turns over a bit but doesn't quite start. I have to quickly hit the start button a few times until the car starts.

The car will also occasionally misfire when changing from 2nd-3rd when WOT and go limp mode. A quick restart fixes it. This happens in either Map 2 or Map 5. Logs show great timing, upwards slope from 14ish o 20/21 at change, no drops. It also ONLY happens going from 2nd gear to 3rd, right at the change, every time. I think it may have started after the green wire was hooked up (green wire helped trims substantially). Trims now rarely exceed 30/35.

Originally I thought the misfire was possibly a bad spark plug as I know that can cause both these issues. It tends to only happen in a certain bank, but it has happened on both sides of the engine (obviously can't isolate the cylinder). It's also happened without E85 using boostane and a 95 octane mix.

Any thoughts?

Sounds like we need @BMS here to provide some input.

Well, I think basic troubleshooting 101 would be to pull your JB4 and see if you can replicate one of these problems as they could be completely unrelated. You have two distinct issues, and Eco Stop/Start problem and a misfire. While the mis-fire is probably related to the JB4 and ethanol mix the eco start/stop is most likely not.

Similar eco start/stop issues have been reported across platforms A/C/E...in mostly stock vehicles on this forum. Most successful interventions required dealer intervention.

You're probably right Mr. J. I figured the auto start was probably more likely to do with the ethanol mix since the engine has some pretty specific engineering to improve its quick start, so the fuel mix might be throwing it off. I couldn't find any auto-start threads, but also only searched C43, I'll look around. And the misfire is just odd because it is ONLY from 2nd-3rd, and it happened with a non-ethanol mix too. That's why I suspected spark plugs. Gonna run Map 0 and run the tank down too and try and recreate it, would prefer not to take it to the dealer.

Thanks for the help.

Boostane could've mucked something up. Did this happen before using it for the first time? Definitely map 0 or pull the JB unit to troubleshoot.

Happened before Boostane the first time. It's actually why I tried swapping to Boostane. Other than spark plug longevity my understanding is Boostane shouldn't a problem for an engine (some claim it can help?). I got better timing and its more convenient for me to just run an E mix though.

I might try pulling the green plug. Not sure why that would cause it, but it was one of the few variables that changed before it happened the first time.