There are 8 fuel injectors on your 4.6L or 5.4L Ford (or Mercury or Lincoln) vehicle and they ALL need to be spraying fuel into their respective cylinders. Every now and then, one or more fuel injectors won't because they are not either getting power or the activation signal from the PCM...
...And when this happens, you'll get a bona-fide Misfire Condition, rough idle and Misfire Codes (P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, or P0308) lighting up the check engine light.
This article will help you to get to the bottom of the issue by explaining how to do a Noid Light Test on your Ford vehicle. Also, this article supplements the information in the article: How to Diagnose Misfire Codes P0300-P0308 (Ford 4.6L, 5.4L).
Here are the contents of this article at a quick glance:
Read the following test steps first before attempting the tests.
At the bottom you'll find a section that'll help you to interpret your Noid Light test results on your 4.6L or 5.4L Ford (or Mercury or Lincoln) car, pick up or SUV.
Locate the fuel injector that you're gonna' test and disconnect it from its electrical connector. If you don't know which one to test first, I suggest you test them all.
Once you have unplugged the fuel injector connector from the fuel injector... insert the Noid Light into the connector. Use the correct Noid Light for the Injector connector on your specific vehicle. Noid Light Sets will contain several different types of Noid Lights. Using the wrong one will damage the female terminals inside the fuel injector electrical connector (and this is something you WANT to avoid).
Once everything is set up, stand a safe distance from the engine, but within eye-view of the Noid Light and have your assistant crank the engine. Be careful, the engine may or may not start.
You're gonna' get one of three results: a) The Noid light flashed On and Off the whole time the engine was cranking or running, or b) the Noid Light did not flash On and Off at all when the engine was cranked or was running, or c) the Noid Light came On and stayed brightly lit the whole time the engine was cranking or running.
Repeat the above 4 test steps on the other 7 fuel injector connectors. This is a key step, since you'll be able to compare a BAD Noid Light test result with a GOOD test result in this manner.
OK, in this section, we're gonna' explore in depth the three Noid Light test results. Choose one of the following CASES:
CASE 1: The Noid light flashed On and Off: You need to compare this flashing on and off to another fuel injector connector. Both flashing on and off results should be identical.
If it's so, then... this indicates that the fuel injector is receiving both Power (12 Volts) and the injector pulse signal from the PCM (Powertrain Control Module=Fuel Injection Computer) and everything is good.
If after testing all of the fuel injector connectors and the results indicate everything is normal, the next step would be to test the fuel injectors themselves. Go to the end of this article and you'll find the link to that test article.
CASE 2: The Noid Light DID NOT flash On and Off: If this result is obtained from just one fuel injector connector, then it indicates that either power or the fuel injector pulse signal, from the PCM, is missing.
If all of the fuel injector connectors did not flash On and Off, when the engine was cranked, this usually indicates one of several things: a) blown fuel injector fuse, or b) a bad Crankshaft Position Sensor, or c) an ‘open’ short in the wiring, or d) a BAD PCM, although this is rare.
CASE 3: The Noid Light was lit brightly but did not flash On and Off: This result is usually a dead giveaway that the PCM is fried. In some rare cases... the circuit that feeds the fuel injector connector with the On/Off signal from the PCM is shorted to ground.
“A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.”