Hi, I'm wondering if anyone can point me in right direction or offer some advice on my daughter'* 2008 Pontiac Grand Prix V6 3.8, 123,000 miles. New Temperature sensor, ECM, Ignition coil.


Some history: The car would not start on a cold night, battery cranked it over, so I attempted to put some Heet antifreeze in the gas tank. It finally started with a loud terrible boom, barely idling with the accelerator almost to the floor. At that time, I thought it was because of bad gas in it after running it low, so I drove it home, during the drive the engine light came on and started to blink. After driving it about 20 miles it did not get any better. Lesson learned, big mistake driving it home, it was misfiring horribly.


Updates: After looking at the spark plugs and wires, I noticed the ignition coil had a large crack and replaced it. After that, it still misfired. I noticed the spark was not present across the new coil, so after some testing, I replaced the ECM. That seemed to fix the misfire. The car now idles and starts great, but the engine light is still present and the engine surges while driving at any speed losing a lot of power. Also, the exhaust seems louder than before.


Question: The code shows up as a P0420, Catalyst System Efficiency Below Threshold. the question is whether the loud noise was the exhaust manifold cracking, causing air/fuel issues, or whether driving it 20 miles with a bad misfire caused the cat to go bad. At this point, I feel like just replacing the cat with a direct fit for the car, but not a fan of throwing parts at cars as it does not alway solve problems. While driving, the car hisses when accelerating and loses power to the point where you have to let off the accelerator or just floor it. Any advice would be appreciated.

If the exhaust manifold cracked, you'd have an ongoing undesirable noise.

My guess is that the catalytic converter was the problem all along and the guts of it have turned into a molten ball of catalyst and is plugging the outlet side of the catalytic converter.

It'* also possible that it was dumping fuel when you were trying to crank with no ignition, and when the ignition finally did spark one off, it also detonated the fuel mix in the exhaust. When that happens, BOOM.

A boom possibly big enough to break up the catalytic converter. And, now that the initial problem is fixed, you need to mop up the collateral damage.

I detonated the muffler on my mom'* Blazer this same way. I had the plug wires on the distributor wrong, and cranked on it for a bit, and it went boom too. The muffler was already hating life, but that finished it off.

And in your case, the catalytic converter may not have been happy before this happened either, but now it is definitely a problem. Also check to make sure there are not chunks of the converter in the muffler by banging it.

Raw fuel will ruin a cat in short order.

Well, I appreciate the advice from everyone and the knowledge, past experiences shared. I will try to to take it off today and see. Might be replacing the muffler too!!

Update with the result. After removing the catalytic converter, if shaken there was some movement, so the core was broken up. The catalytic core was still intact behind the screen, so none made it into the exhaust system. Replacing the cat with a direct fit and adding a short 4" section of exhaust pipe along with a new resonator fixed the engine surge, loss of power issues.

Thanks for letting us know how it turned out!