P0303 Fault Cy D MSFR Cy 3
 

Hi all,
I'm back after a busy summer season. My '96 F150 is picking up a P0303 code and is running rough at idle and during normal driving operation. It doesn't seem to run as rough if you rev the motor while in park or neutral, but it does miss or have bumps in the sound of the motor to a lesser degree than with it in drive and going down the road. As a reminder to those who may remember my truck and to those who don't know my truck yet, my '96 F150 is OBDII mass air, originally a 5.0L now a 5.8L stroked to a 393. I have a SCT Xcal programmer that is used to flash the ECM for the increased size of the motor and fine tuning. The truck is set up for emissions legal operation, so EGR etc... is connected and operational (or at least should be).

I have eliminated the possibilities of spark plug, spark plug wire, and coil failures by swapping in new plug and wire and another coil I had sitting around. I am running a Jacobs Ignitions off road ignition module and coil, but when I swapped my spare coil in, I eliminated the Jacobs ignition for that test, so with or without the Jacobs Ignition I still have the same problem. I'm starting to think I have a bad distributor or stock ignition module which is still used in conjunction with the Jacobs module.

Alot of the posts that are about the same code of P0303 are from newer trucks, which point to a coil pack, plug, or wire. Or there are a number of them that point to the EGR system. I don't see how the EGR system of my '96 could pin point to a single cylinder, but what do you guys think?

I'm leaning hard on a distributor or module, by the way, this truck has a remote mounted module on the driver's side fender. Any help is much appreciated, I'd like to get my truck running smooth and proper!

Well, I guess I need to check cylinder 3 for a mechanical failure ie... vavle or valvetrain problem, piston rings, etc... before I start changing electronic parts. I did verify spark at the spark plug even though I just did a complete tune up on the ignition. The problem is definitely with cylinder 3 and not any other cylinder. I will also check on fuel delivery as well.

What exhaust is on the motor? If it's anything except stock then the computer may mis-identify a misfire.. it's all about the timing of the exhaust pulse in relation to the stock O2 sensor location.

It is not stock exhaust. It has shorty headers on a stock Y-pipe. I mounted the O2 sensors about 3 inches from the header to Y-pipe flange, as close the the header I could get and still have room to get to them. The Y-pipe is originally from a late '80s early '90s Bronco with a 351W so it had just one O2 sensor down at the Y. I've been running this motor about 4 years and it always seemed to run properly until now. I always did have a noise on that side of the engine that I just figured was a noisy lifter (flat tappet). But now, I'm wondering if it was some kind of misfire the whole time. Just enough to make a noise but not enough to make it run real rough?? I also thought the noise could have been a valve. I always had good oil pressure and seemed to have plenty of power so I never worried to much about it. I guess maybe now I'm paying for it.

I forgot to ask, what is the proper way to test that theory out? Can I disconnect the O2 plug and run it for a short period of time to see if it smooths out. I know it will throw an O2 code, but if it doesn't throw a misfire code on cy 3, that would be the answer to the problem, right? I do have the capability to reset error codes so that is not an issue.

Update on troubleshooting the problem. Apparently, the original problem was a cracked ceramic insulator on cylinder 3 spark plug (I looked at the old one and the ceramic was cracked) and when I changed the plugs and wires I happened to crack the plug on the exact same cylinder during the installation, what are the chances?!? I did a compression test on cylinder 3 and also cylinders 1 and 6 just for comparison and when I removed the plug I noticed the ceramic was cracked! I swapped the cracked plug to cylinder 1 just to confirm and the misfire followed the plug as I figured it would. I replaced the cracked plug with an older plug for now until I can get to the store and the truck runs smooth and strong now. By the way, I had 135psi on cy 3 and 155psi on both cy 1 and 6. That means cy 3 differs from cy 1 and 6 by almost 13%. I know I haven't checked all 8 cylinders, but should I be concerned about cy 3?