Your Ford (or Mercury) 4.6L or 5.4L equipped vehicle has 8 fuel injectors and when one or more go BAD, they will cause a rough idle / misfire condition that will set the dreaded misfire codes: P0300, P0301, P0302, P303, P0304, P0305, P0306, P0307, P0308 and fuel injector Codes:
This article will help you to diagnose a BAD fuel injector with a simple and easy resistance test of the fuel injector. This article is a part of a series of misfire condition troubleshooting articles for your 4.6L or 5.4L Ford V8 equipped car, or SUV or pick up. The starting point of this testing series is: How to Diagnose Misfire Codes P0300-P0308 (Ford 4.6L, 5.4L).
Here are the contents of this article at a quick glance:
You should perform this test with the engine warmed up, although it should not be HOT (normal operating temperature). Be careful, the engine can get very hot! If you have just turned off the engine, from an extended run time, let it cool down about 1 hour. If you have a scan tool, check that the engine coolant temperature sensor is reading around 100° F.
Starting out with a warmed up engine is not an absolute must... but it helps ensure a more accurate test result.
To be able to accommodate all types of experience levels, and to keep this test as simple as possible, the following steps assume that you'll be testing all 8 fuel injectors.
Disconnect the fuel injector from its electrical connector. Be careful, the fuel injector connectors are very delicate and the plastic locking tabs break very easily.
Once you have unplugged the fuel injector connector from the fuel injector... place your multimeter in OHMS mode and probe the fuel injector male spade terminals.
Since the multimeter has Red and Black test leads (and in case you're wondering), it doesn't matter what color test lead you use to test either of the two fuel injector male spade terminals.
After verifying the resistance value, write it down on a piece of paper. Don't forget to write down what cylinder that reading belongs to.
The average reading, for each fuel injector, should be about 11-18 Ohms. This will vary just a bit in your specific Ford or Mercury or Lincoln 4.6L or 5.4L equipped car, pick up or SUV. Now, repeat test steps 1 through 4 on the remaining 7 fuel injectors.
OK, let's interpret the results of your fuel injector resistance test. Read each CASE carefully, and choose the one that applies to your specific testing result:
CASE 1: The resistance value of all 8 fuel injectors is the same: This result indicates that the fuel injectors are not fried. The cause of the misfire condition or misfire codes or rough idle condition lies somewhere else. Go to the last part of this section for more testing options.
CASE 2: One (or more) fuel injector value is totally different from the rest.: Retest the fuel injector that was not within the stated specification. If the resistance value is still the same and drastically different from the normal resistance value (or different from the rest of the fuel injectors), that fuel injector is BAD. Replace the fuel injector.
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