New to the forum, just bought a 2007 F150 XLT 5.4L 4X4 a couple days ago. The CEL light was on when I bought it and it has a pretty bad shudder, especially when driving up hills and even more so driving up hills while pulling a trailer. The idle is very smooth, no misfire concerns there.

The previous owner said his mechanic told him it was due to a bad catalytic converter. My brother has a cheap OBDII scanner and while driving it yesterday it showed that cylinders 1-4 all had a significant number of misfires while cylinders 5-8 only had maybe 5 misfires total. After seeing that we are thinking the place to start will be by replacing the bank 1 cat.

The codes were P0420 catalytic converter, P0300 random misfire, and P0303 cylinder 3 misfire.

Does anyone have any advice or experience with similar issues? Any help is appreciated. Thanks!

 

The P0420 can be caused by a very wide amount of issues. It can be a bad converter, an exhaust leak, bad O2 sensors, and even a bad temp sensor. What it's basically telling you is the 02 sensor behind the converter is getting an unexpected reading. Before replacing the converter I'd have an exhaust shop take a look at it with a laser temp probe. You'll need to have the exhaust system up to normal operating temp for them to read it. I doubt the misfire is related. Considering you just bought it, I'd suggest a double shot of Techron in a half tank of gas and drive it around to let the Techron work. This will do 2 things. It will clean any fouled injector and it will remove combustion deposits that makes the spark plug removal difficult. The spark plugs are a muliti-piece design and are notorious for breaking off in the head. Removal by the Lisle Tool is required for broken plugs. I'd change spark plugs and COPs using ONLY Motorcraft parts. These engines are finicky about the parts you use so stick with Motorcraft parts or expect issues. After that, I'd change the oil and filter to unload all of the junk the Techron washed into the oil. If the mileage is over 150,000 miles, your engine can benefit from moving up to the next viscosity like a Xw-30 oil. The Pennzoil conventional 10w-30 has the best detergent package out there as of today and is 3 times more heat tolerant per ASTM D-5800 than Mobil One. Wix, NAPA Gold, and Purolator are all decent filters. Might also consider an ATF change too. If the truck has a T-Lok axle under it, it's supposed to have the fluid changed every 35,000 miles and it takes a friction modifier for the T-Lok mech. It is alcohol based and will absorb water out of the air which is why it needs to be changed. Check the air vent on the rear axle and insure that it is not broken and runs up into the fender or water will get into the axle.

 

Your bank 1 cat is plugged, CAUSED by the misfires on that bank. Replace or gut the cat and change ALL spark plugs. If it still misses, start replacing coils.

 

This was a big concern of mine once I first noticed my random misfires at low speeds in OD, misfires mean fuel is going into exhaust, and cats don't like raw fuel. Back in pre-cat days, you could live with a miss for awhile, but that was then and now is now.

 

^^x2 on the cats.