Exhaust Post Shutoff Detonation

[Durango]

Don't see something like this in a Search...
Has anyone had their 3.6 on a JL detonate fairly loudly after shutting the engine off? Mine has done it about every third or fourth drive - I shut off the engine, get out, and just as I get a few steps away, BANG! - I jump and look back to make sure it isn't on fire.
No CELs, no problems driving including getting decent mileage. But I sure hate to see if go up in an explosion!

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[Jelllo]

Don't see something like this in a Search...
Has anyone had their 3.6 on a JL detonate

Could be crappy fuel...are you using Top Tier gas?

 

[Durango]

No - use Chevron gas almost exclusively. Maybe a tank or two of Valero has run through it. I wonder if maybe I should put a can of the fuel stabilizer or similar in it, in case I did get a bad batch traveling...

 

[rubileon]

Don't see something like this in a Search...
Has anyone had their 3.6 on a JL detonate fairly loudly after shutting the engine off? Mine has done it about every third or fourth drive - I shut off the engine, get out, and just as I get a few steps away, BANG! - I jump and look back to make sure it isn't on fire.
No CELs, no problems driving including getting decent mileage. But I sure hate to see if go up in an explosion!

Detonation means the fuel inside the cylinder detonating under compression before a spark from the spark plug. This kind of detonation takes place when the engine is running. Not when the engine stopped and you have gotten off the vehicle. Are you sure this is what you mean?

From the bang that you mention, sounds to me like unburnt fuel in the exhaust igniting. Even then if you have a stock exhaust, you shouldn't hear it unless a large amount of unburnt fuel makes it past the catalytic converter somehow. Externally examine your cats, resonator and muffler to see if there are signs of bulging from such explosions.

The other thing that could happen is a true back fire. The brilliant Internet population refers to the "pops" coming from the exhaust as "back fire" but in a real backfire, you can have fire coming out of the intake. Check to see if there are signs of flames or smells of fuel in your intake... preferably when your engine is cold... you don't want that happening in your face.

 

[rubileon]

One more thing... if it happens 25-30% of the time, there's no excuse not to record it and show to a dealership a few examples.

Maybe there's a faulty injector leaking fuel under pressure right after turn off. Could lead to a fire.

 

[Durango]

Detonation means the fuel inside the cylinder detonating under compression before a spark from the spark plug. This kind of detonation takes place when the engine is running. Not when the engine stopped and you have gotten off the vehicle. Are you sure this is what you mean?
From the bang that you mention, sounds to me like unburnt fuel in the exhaust igniting. Even then if you have a stock exhaust, you shouldn't hear it unless a large amount of unburnt fuel makes it past the catalytic converter somehow. Externally examine your cats, resonator and muffler to see if there are signs of bulging from such explosions.
The other thing that could happen is a true back fire. The brilliant Internet population refers to the "pops" coming from the exhaust as "back fire" but in a real backfire, you can have fire coming out of the intake. Check to see if there are signs of flames or smells of fuel in your intake... preferably when your engine is cold... you don't want that happening in your face.

Sorry - I was intending to use the term detonation in the traditional sense, but rather for its more literal sense, where something detonates. And that was what was happening - something past the engine was detonating after - and not directly after - shutting the engine off and beginning to walk away. (in other words, not immediately on shutting it off)
The interesting thing? It has gone away as fast as it appeared. So now I am more inclined to think it was a fuel issue - although I only buy name-brand gas, maybe one of those stations had something else in it? It didn't seem to last more than the one tank full - although I'm only on about a tank and a half later, so who knows?

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