How to: Engine misfire procedure

[2000StreetRod]

Background

Associated diagnostic trouble codes (DTCs):

P0300 Random/Multiple Cylinder Misfire Detected
P0301 Cylinder 1 Misfire Detected
P0302 Cylinder 2 Misfire Detected
P0303 Cylinder 3 Misfire Detected
P0304 Cylinder 4 Misfire Detected
P0305 Cylinder 5 Misfire Detected
P0306 Cylinder 6 Misfire Detected
P0307 Cylinder 7 Misfire Detected
P0308 Cylinder 8 Misfire Detected

The PCM computes the power of each cylinder and when one is low reports a misfire for that specific cylinder. Typical misfire sources are ignition, fuel injection, low compression, or a defective evaporative emission system.

Ignition problems could be due to a bad plug, wire, coil or crankshaft position (CKP) sensor. Sudden ignition related random or multiple misfires on more than one bank are probably due to a bad coil or CKP sensor. Ignition caused misfires are the most likely to occur and (except for the CKP sensor) the easiest to isolate and eliminate.

Fuel injection problems could be due to a clogged fuel filter, a failing fuel pump, a defective fuel pressure regulator, or a bad fuel injector. The only problem that is cylinder specific is a bad injector. Misfire DTCs in conjunction with too lean or too rich DTCs may be due to the abnormal air/fuel ratio. Mixture related DTCs should be eliminated before pursuing misfire DTCs. See Lean condition procedure A common cause of fuel related misfires is a clogged fuel filter. See Why change fuel filter? Another common cause is a failing fuel pump which is frequently caused by a clogged fuel filter. In 1999 Ford switched from a fuel return system with a fuel pressure regulator mounted at the fuel rail to a fuel returnless system with a pressure damper mounted at the fuel rail and the fuel pressure regulator mounted in the fuel tank. See Ford Explorer Ranger Fuel pressure test procedure A clogged fuel injector can sometimes be corrected by adding fuel injector cleaner to the fuel tank. If the fuel filter is soon to be replaced (every 30,000 miles), wait until the replacement is accomplished and then add the fuel injector cleaner.

Low compression could be due to worn/broken rings, worn or sticking valves, blown head gasket or cracked head. Low compression on all cylinders in one bank indicate a slipped timing chain for that bank.

 

[greasemanicure]

I'm going around in circles over a P0304 code on our 96 4.0 OHV ex. In this order and over a span of maybe 4 months, I changed the plugs, changed the wires, changed the coil pack, changed injector #4 (including all disturbed gaskets, O rings, etc.), and the EGR tube seal to the intake. Still have a rough idle and the P0304 code after I jump on it to pass or climb a hill. No other codes. Swapped the #3 and #4 wires and the trouble did not clear or change. Overall it acts like #4 is not there. Rotten mileage and performance. Checked compression and it is around 150 as are the others. The connector for injector #4 is so buried as to not be able to get a noid light on it without major disassembly. Can I check for signal to #4 injector at the massive plug just above plug #6? Any diagram to show what signal is on what wire in that connector? Maybe the PCM is not talking to injector #4? Time for a shot of Rebel Yell and some thinking.
Joe

 

[2000StreetRod]

Injector #4

Injector #4 is connected to Pin 100 of the PCM via a brown/light blue stripe wire.

Injector #6 is connected to Pin 99 of the PCM via a light green/orange wire.

Injector #2 is connected to Pin 101 of the PCM via a white wire.

The power side for injectors #4, #6 and #2 are connected to Pin 97 of the PCM via a red wire.

You might try disconnecting the PCM connector and measuring the resistance between PCM Pin 97 and Pin 100. Then measure the resistance between PCM Pin 97 and Pin 99. The readings should be approximately the same and verifies a complete path to the PCM connector. Then make sure that Pin 100 does not get pushed back when the PCM connector is reinstalled.