I have a 96 formula. Just bought it. Wasn't running when purchased. Sat for 4 years. Fixed a bad opti and fuel pump. Runs pretty well now. Love the 6 speed. It's got what seem like a few small issues. It's throwing p0420 and p1416. At anything other than idle it runs real strong, no hesitation, stumble, knock etc. However it idles pretty rough and has a slight exhaust backfire. It does pop on decel but it has about a 1" hole it the muffler(its a flowmaster or something but the rest of the exhaust is stock). Sometimes it will pop once or twice as you turn it over while starting, again not a big backfire, just a pop. It is a little tough to start sometimes but I have a used fuel pump in there that has good pressure but the check valve is shot so I have to prime it a couple of times to get the fuel to the rail/injectors. The motor is bone stock. I pulled a plug and it looked new. Havent checked the others yet. I see the p0420 is o2/cat related. From other posts I've gathered that p0420 shouldn't cause driveability issues. It also seems to be running rich which I'm guessing could be causing the popping at idle. The o2s are being real stubborn. I'm trying to swap them side to side but they are not cooperating. As for p1416 I checked the plumbing to my pass. air stuff and the check valve and they are ok. Are either of these codes the likely cause of my idle/pop issue or should I turn my attention elsewhere?

 

P0420 tells you the cat is not working correctly, or the O2 sensor behind it is not working correctly. Figure out a way to swap the two after cat sensors side-to-side. If the problem follows the sensor, its a sensor problem. If the problem stays on Bank 1, its a cat problem.

P1416 tells you there's a problem with the AIR flow to Bank 2. Could be a melted hose, stuck check valve, etc.

If you're getting excessive unburned HC from a faulty cat on one side, and you have a leaky AIR check valve allowing excess air into the other bank's exhaust system, it going to make that bank run rich. Now you have excess air and high HC's in the exhaust, and they may be igniting.

 

After removing all the spark plugs I noticed the cyl. 6's plug was fouled black. I believe this to be the root of my problems. I put a new plug in and it began to get blackened after just a few trips. I swapped plug wires and it still sounds like I have a miss at idle. I used a spark tester to verify good spark to the plug. It looked steady and strong. I also found that the intake manifold bolts were very loose, at least most of them. I tightened them but the problem is still there. I have also swapped fuel injectors so I know they're not an issue. Could the leaky intake situation have burned a valve? What else that's not spark related would cause the misfire? I replaced all the plugs. The motor seems ok but you can tell it's running rich. You can accelerate fine, the problem really shows itself on deceleration or at idle. Then idle is too high, around 1100 rpm, and you can hear the motor popping gently through the exhaust. If you try to decelerate by using engine braking the motor pops and snarls sometimes pretty loudly. I'm guessing both of the popping/backfiring is coming from the unburnt fuel in the exhaust from cyl. 6. I'm going to take the valve cover off when I get home and check the pushrods for bending and rocker arm condition/adjustment. If that all seems ok I guess Im looking at pulling the head to look for burnt valves etc. Does anyone have any suggestions as to problems that would cause only one particular cylinder to misfire besides spark related issues. I'd feel silly if I tore the engine apart and the fix was something simple.

 

Compression and leakdown test before dis-assembly.