Last year I put Bosch Platinum +4 plugs in the car, hoping they would live up to the advertisement. (Silly I NOW know.) They worked great for awhile, but 3000 miles later, it started to not run as well. Biggest indicator, 19 mpg highway driving. I then replaced them with Autolite Double Platinum plugs. No difference at all. On the last tank, driving around town, it actually got worse, 10 MPG! Now, I admit it's winter, and it idled a lot, but seriously!? 10 MPG!? Should I put Motorcraft plugs back into? It is higher mileaged, 111000, but I am wondering if I didn't cause damage to the car running the Bosch plugs in it for thousands of miles. I also recently took it to an exhaust shop, (hunting down this problem) and he said that at least one of the catalytic converters are not working right, no codes however. Rear cats specifically. I am almost ready to trade the car off, and I would not be able to get another Mustang, and that wouldn't be any good.

 

If it is a plugged converter, you will quite often see decreased gas mileage.

 

Replacing the spark plugs right before the issue occurring is probably coincidence, especially if you've tried two different sets.

 

It isn't the plugs

 

banana in the tail pipe?

 

Thats what I was afraid of. Only problem is I don't know which ones, and can't afford to replace all of them. Especially since the computer won't throw a code. Well, time to start saving I guess! Thank you all very much!