P0303 Misfire Only New Engine
 

I finally had to join the forms because I think I’m the only person to ever have this problem. 2000 Ford Ranger 3.0 5 speed with 129k. Truck has a P0303 code that would not go away. Truck didn’t run bad, a little under power in my opinion but I live at 6000ft. Water pump cover failed and mixed oil and coolant together. Fast forward to today. Just installed a new long block from Dahmer. They install all wear items brand new including valves, rings, pistions, etc. in their long blocks. I have new spark plugs, cam sensor and synchro, coil pack, injector on the bad cylinder. Checked the plug wires which show good. Truck started runs good, but after the first 10miles P0303 cylinder 3 misfire came up. The only thing I have not changed to new is the computer, crank sensor, Mass airflow sensor. I know it’s not a valve issue because it’s brand new. Does anyone have an idea what can be causing this? Truck idle is normal speed, small shake at low RPM, intake was washed at the machine shop and we both didn’t notice anything that looked damaged. I personally don’t think the crank sensor or MAF would cause a cylinder 3 miss fire with no other codes. Truck does run a little rudder now than it did on initial start. Can the computer really make just one cylinder not fire properly. Please help 😑😑😑

Welcome to the forum

You say you don't notice a misfire?
They are pretty obvious especially going up a hill

So computer could be the problem
CEL(check engine light will flash) during an actual misfire
And you can feel it

Misfire code is set by the Cam sensor, when a cylinder fires it adds spin to the crank and cam, when it doesn't fire then no power is added
The crank and cam sensors read the rotation speed of the crank and Cam, cam spins slower so has better "resolution" of the 6 cylinders firing and adding power
So if #3 consistently doesn't add power then P0303 is set

Computer also has coil misfire codes and injector misfire codes, P0303 is a rotational misfire code, less power added to rotation of engine

So if you don't actually have a misfire then best guess is the computer circuit is misreading the data

Thanks for the very detailed response. On the old motor I spent two weeks changing spark plugs around, injectors, etc, etc before it blew.. Just to amuse myself today before I ordered a computer, I decided to try and move the injectors one last time. The injector worked with the test light. Showed good pulse and verified it by listening to the injector with an extension. Ticked when plugged in and stopped ticking when unplugged. I can’t explain why but I was able to get the problem to move from cylinder 3 to cylinder 2 on the new motor by swapping injectors. I verified this by RPM drop when disconnecting the injector wiring. Bad injector did not change and the good injector caused RPM to drop. This did not happen on the old motor. The brand new injector I had bought for the old motor that didn’t solve the issue must have been bad. I ordered a new OE injector and will see if it works tomorrow. I have high hopes.

@ RonD, could this be a flex fuel vs. non flex fuel injector problem ?

If moving an injector moved the misfire then you have an actual misfire

So get a new injector


As far as the larger(flex) or smaller(gas only) injector, its possible running one mis-sized injector could cause misfire code