Backfiring??

I took off my Cat and replaced it with the Cat pipe. I didn't do anything else to it.
But last night when I was racing around, after I got up to 6k RPM, and then let off the gas to shift, the car backfired. BOOM.
I tried the same thing later, and it did the same thing, one big BOOM just after the RPMS started to drop!
Have I finally abused my engine now and bent something??


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Kurt B
1984 911 Carrera Cabriolet
75 914 1.8

Yeah. It's probably a bent crankshaft.

Seriously though, I have learned over the years to look for real simple causes to automotive problems. This is because I have had a tendency to assume the worst. Don't do this. Internal engine parts don't bend (with just a few exceptions). At least 50% of the time, automotive problems are very simple and require no new parts to fix.

Backfiring always makes me suspect an ignition problem. Look at and clean all ignition electrical contacts. Measure the resistance of the spark plug wires. Test the CDI system. Put it on a "scope" if you have to. If the ignition system were igniting the gasoline in the cylinders properly, that gas woudl have no reason to explode in your test pipe.

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'83 SC

Bastard.

Now that you mention that...it could be the plug connections. The plug wires are new, but they're not a real tight fit over the plugs since I didn't actually pull the pins out of the old wires and put them in the new ones. So the fit makes contact, but doesn't have a hard "pop" on the plug. They could be working loose from time to time, something I need to check out.

This happened on my TransAm. I'm don't remember exactly the deal but I'm pretty sure it had something to do with the backpressure and running a little rich and at high rpm when I let off it would pop. Anyway, without getting into a huge argument about how and why it happens, my point is that you may spend money and time, like me, trying to fix something that was normal ops considering the circumstances. I leaned out my computer to get the optimum mix and it significantly diminished (I understand different car etc...). Put the cat back on and see if it goes away before you spend any money. If it does, ask the smart guys here what you'll need to do to to compensate for the lost cat. Just my two pennies.
rj

I used to have a 67 XKE with a leak in the exhaust system between the 1st muffler and the header's. Back in those days I went for the cheep fix. I always new exactly when the patch let go because it would Back Fire while downshifting.

Leaky exhaust seems likely. However, it hasn't done it since I pushed all my plug wires snugly back on the plugs. As with any part you put on your Porsche not made in Germany, it didn't fit Exactly right. Yeah, it was clearly designed to fit, but rather than put holes where the bolts go in, they put...cut out holes. I guess so you can move it around a bit and compensate for their less and German Perfect machining. To be expect though...we can't all be German Machinists.