Coyote Gen III Backfire on Cold Start

Hi All,

I have a loud backfire on cold start of my Gen III, and I think it's the drivers side. I had my MKIV dyno tuned at about 1000 miles (last summer), and there's just over 3000 miles now. I don't recall whether I noticed the backfire before the tune or not. My first Coyote engine, so not sure what was "normal." I still have to check for exhaust leaks. I've read that a vacuum leak might pull in air during the exhaust pulses, mess-up the air-fuel mixture or reading at the O2 sensor, and cause unburnt fuel to be left somewhere (where this unburnt fuel sits before the next start... not sure, in the cylinder or in the exhaust system?). I also read that if the tune is too lean, it may cause the ECM to try and compensate by delivering extra fuel at start-up. I don't hear any backfire on hard acceleration. On gearing down under load (burbling), I'll have to pay closer attention next time I'm out to see if I hear any popping (if there is any, it's definitely not as severe and noticeable as the cold-start gunshot noise).

Anyone else have a similar issue, either with or without tuning? Any advice would be much appreciated. The car runs fantastic otherwise.

Thanks,

32J

These are just thoughts,,,, the ECU is probably in closed loop, using pre set richness, idle speed, etc while cold.
Be sure to pause after key on, pump running to get fuel pressure. Normally this takes, just a second in properly set up systems.
You might have an injector leaking into a cylinder or into the exhaust.
I would try the simplest first. Let the fuel pump run a second or two before firing it off.
Good luck, let me know what you find.
Running a Gen 1, no issues.

Okay, I'll give that try. I just finished going over the all the exhaust bolts (except at the block). There were some bolts that were definitely loose at the ball joints. I had to re-tap one of the nuts because it would not tighten any further, while the bolt was still spinning. I also added a couple of spring washers, so it's real snug now. The 4 to 4 gaskets were mostly tight, but I snugged them up a bit.

Thanks,

32J

I'm still getting backfire at cold start. I've checked the tightness of the exhaust and side pipes. And, I've been letting the fuel pump run for several seconds before ignition. Looking for suggestions on what I may try next. Something off with the tuning, maybe???