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350 mag mpi strange noise after change spark plugs


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Hi everybody!!!

I have a 2002 malibu response running with a 2011 350 mag mpi.

As the engine was running rich (smoke and gas smell) I decided to change the spark plugs and rotor cap !!

The I first changed the rotor cap, tried to start, everything ok!!

Then I changed the spark plugs, I previously had NGK bpr6efs, and I put NGK iridium itr4a15. But since I changed, the engine emits a strange noise with some smoke going out arrount the flame arrestor, is harder to start, lack of power (max 2500 rpm), gas smell and smoke are still here.


I tried to test the cables disconnecting it, putting a plug, ans it seems I have sparks.. When I was doing that I heard a "boom" at a moment, I guess some gas excess was in the cylinders?

I dont know if its due to that, but I have now 2 iridium plug broken :


What could it be ?? The model of plugs is recommanded on marineengines.

I tried to put back the previous plugs but the problem is still there !

What could It be ?? What should I do ??

I was thinking of buying a new set of brp6efs...


Thanks for your help!!



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Do you have the firing order correct?  You might have switched plug wires which would create the backfire issue.  Google small block Chevrolet for correct firing order.  

Make sure the plugs have the same projection in to the combustion chamber and the new ones are not longer than what you took out.

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I agree that it sounds like you switched plug wires.  Or rotated the distributor.

Since the problem still exists with the original plugs, I would double, triple check the plug wires.  If that all seems OK, go back to finding TDC on #1 cylinder, and start from there.

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