Hi Scotty and Co., I have a 2005 Pontiac Grand Prix Naturally Aspirated 3800 with 98k miles. Car was running great then started misfiring like crazy out of the blue and check engine light started flashing. Drove it a few miles home stopped driving it from then on and pulled code P0300 multiple misfire. Replaced coils myself with aftermarket new ones and didn't fix the problem. Replaced the ignition module with a new aftermarket one and that did fix the problem, car ran great again. However check engine light came on with P0420 Inefficient Cat. Reset it twice and it still comes back after driving 30-50 miles.
I have the following data after fixing the misfire and driving maybe 150 miles:
-At Idle Long Term Fuel Trim -18%, Short -3.9%, Mass Air Flow 3.93 g/s
-2500 RPM Long -16.4%, Short 0%, Mass 12.53 g/s
-3500 RPM Long -16.4%, Short +3.0%, Mass 19.78 g/s
A few months ago I watched one of your videos about the junky Jetta burning oil that a guy payed $7500 for. Out of curiosity I put my finger down the tail pipe and while it wasn't oily I noticed it was fairly sooty compared to my other two vehicles. But as the car ran so smoothly (and still does after I fixed the misfire) and burned less than a quart of oil every 5000 miles didn't think much of it at the time. Only had the car less than a year so don't know its full history.
Looking for some advice, is my cat shot? Only drove it 5 miles when it started misfiring badly and maybe idled it for 10 minutes trying to diagnose it. Was thinking of putting some lacquer thinner in the gas tank to clean the cat as you showed in one of your older videos.
Thanks for any help
driving too long with a misfire will ruin your catalytic converter with too much unburned fuel.
https://www.youtube.com/watch?v=3nmzq7gZJ20
No longer have a misfire, the misfire was fixed.
I understand, but the damage is already done
Driving it 5 miles and 10 minutes of idling is enough to destroy the cat? I didn't even drive the car a dozen miles when it started misfiring
it would have contributed
Do you have any leak in your exhaust system?
Hello, shortly after I bought the car last August it had a hole in the resonator which my local muffler shop replaced, they didn't see any other holes in the exhaust system. The car runs quiet.
Scotty has a video on cleaning the cat with detergent alongside with other stuff to do. Follow that one first.
I think I'll try the lacquer thinner and driving it a bit more and resetting the code a few more times before having to pull off the cat. At that point might as well swap it out with a universal one from eBay. Don't have to do emissions testing where I live so not in too much of a rush to fix it. Thanks
If you don’t have to pass emission test, why bother fixing the issue?
To get rid of the check engine light if I ever want to sell it. Plus my folks had a '95 Mercury Mystique some 15 years ago and probably the Cat got plugged and the car lost most of its power and smelled like crap. Would prefer if it didn't get to that point.
In the case of clogging, some people just remove the cat.