Back fire at when sitting after drive


Master Mechanic
May 6, 2015
1987 Monte Carlo 4.3 all original drivetrain no mods, 130,000 miles.

This started middle of last summer. Only after driving the car an getting it up to operating temperature this will occurred. If I hit the thronal while in park the car will rev up but back fire a few times. Rev-pop-pop-rev then stay at the RPM I have the pedal at and back fire no more until after I return to idle. The back fire is only as it climbs in RPMs and only once or twice. It does not do this while driving. In fact it has no drivability issues, only this backfire when hitting the throttle in park after being fully warmed up. Is this a sign of issues to come? What can cause this? Only other issue is that after this occurs there is the slight smell of gas in the air cleaner. I have changed the plugs and wires since this started and that did not help.

Car has the proper amount of power and pickup and there do not seem to be any drivability issues. I use non-ethanol gas (90 octane in my area). When I changed the plugs they did have some build up on them but noting that I would have considered out of the norm for how long they had been used. I suspect the injectors may be original to the car but have no way to prove this. I have tried running fuel injector cleaner through it, do this every year when it comes out of storage. It has gone through a full tank since being stored for the winter. I fill it once every 1-2 weeks regardles of how much is left so there is always fresh gas and I never run below half a tank unless I cannot avoid it.

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