My '97 F150, Std Cab Short bed, 4.6 Triton threw out a P0305 code several times, the first time it ran so badly that I took it home and looked at it. I reset the codes and waited. Rinse and Repeat. I put in a new set of plugs, reset the codes and drove 200 miles with an engine that barely ran. No codes. No nothing. The monitors even reset, except maybe the CAT. The point is, I was waiting for it to throw another code, but it never did.

The plugs on this engine are very difficult to replace, especially #4. I started in again on replacing plugs today (09/11) and began with Bank 1. I removed the fuse terminal on the firewall to get to the back of the engine, and found that I had not plugged the boot on very well. I continued on with plugs that was the same as the one I took out. I thought that I had put in Autolite plugs instead of Motorcraft, but the plug was already Motorcraft. I made sure to get the plug boot firmly in place and put everything back together. Cylinders 1 & 2 are pretty easy, and 3 is doable even with the fuse terminal installed, so I had little to loose (minimal chance that the fuse terminal would have to be removed again).

The engine runs perfectly smooth, so the plug wire not being installed well was the problem. The problem is, my truck ran very very poorly, and the Check Engine Light never came on. It popped up a few times, I installed new plugs and the engine ran remarkably worse -- burned valve, no compression, that kind of "worse". Stupid me, I thought the ECU would pitch a fit and tell me the cylinder that was under performing. Today, I only plugged in the #4 wire and instantly everything was good.

What are the parameters that determine MISFIRE? The ECU told me that #5 was misfiring and I seldom felt anything, but the code lit. After I put new plugs in, the engine ran like crap and the #4 plug was not plugged in, yet there was no code for it. I've read that a MISFIRE means that the expected work was not performed. When a plug fires, the ECU knows the load and throttle position and it expects the next cylinder to arrive at it's firing position within a certain amount of time. Everything was going along well, and P0305 came up. If there was no code, I don't think I would have known that the engine wasn't happy. When I put in new plugs, I did it so badly that I should not have been driving the truck, but there was no code.

 

Why did you jump to the plugs? The most probable thing is a coil.

There is a misfire count that has to be exceeded before you get a code for it. And the misfire monitor runs under certain conditions but not all. There's a low level of misfires that always occur... So the engine temp has to be in a certaon range, The fuel level has to be above a minimum amount, etc.

You may never feel some misfires that give you a code. Others may seem like they're going to shake the truck apart and it barely runs. Coils usually manifest themself where you can feel it at about 40 mph when you hit the accelerator

https://www.fordservicecontent.com/f...m973.pdf?div=f