Hello. I have a 2002 F150 5.4 4 x 4. I recently changed the spark plugs and ignition coils at 205,000 miles. I now have a couple cylinders that are misfiring. The code is P0301 and P0303 for cylinder one and cylinder three. Since the repairs were done I’ve driven maybe 40 miles. I don’t suspect it’s a vacuum leak as the intake manifold was changed and didn’t notice any hissing sounds. I’m thinking it could possibly be the fuel injectors. Is there anything else that I can check that relates to a misfire? I didn’t have this issue until I changed out the spark plugs and ignition coils but I had to pull up my fuel rail in order to get some of the ignition coil‘s out.I also recently installed a new fuel filter and ran some fuel injector cleaner which I here can gum up your fuel injectors. What else can cause a misfire? Or am I on the right track?
Did you use OEM parts? Where did you buy them? There's lots of counterfeit "name brand" parts being sold by eBay and Amazon vendors. If you reinstall your old parts does the misfire go away?
Previous coils were an Amazon brand that went 75k and the plugs were motorcraft until it misfired. New coils are duralast with auto lite plugs and only reason I picked those up were because neither Napa, autozone or oriellys carried motorcraft coils or plugs and I needed something to get me back on the road as fast as possible. Some of the old plugs were destroyed so didn’t want to bother trying to reinstall them especially for them being cheap and having a bunch of miles. Was actually surprised they lasted as long as they did.
You may want to check that those coil connectors are locked in. On those 5.4's the locking tabs on the coil connectors get brittle and sometimes break. Sometimes they don't actually break all the way off, but they crack and no longer "lock" the connector onto the coil.
That will cause a misfire. Grab the connector, and without releasing the connector's locking tab, see if it pulls off.
If it does, and you can't get it to lock on, you'll have to replace the connector.
Since you were messing around with the injector rail, give those two cylinders' fuel injector connectors a little tug also just to be sure they're locked in.
Since the misfires didn't exist before you changed the coils and plugs, don't stray too far away from what you were working on.
Another thing to consider on that engine. It only has something like a 4 thread depth on the cylinder head for the spark plugs.
The torque spec on the spark plugs is something like 15 ft/lbs. But with that shallow thread depth, those engines are known to vibrate spark plugs loose and spit them out like an old man eating sunflower seeds.
So make sure they're still tight.
It's best to torque the plugs between 20-25 ft/lbs. (tighten down until the plug bottoms out, then rotate the ratchet another 1/8 turn if you don't have a torque wrench.)
If all that is OK, then you may want to "back pin" those coil connectors and using a test light, check for 12 volts on the power wire (key on) ((The test light will light steady)) and check to see that the Ground "signal" is getting to the coil connector on the other (Ground) wire. (the test light will flash during cranking for the ground).
The wires are 20 years old. They get brittle and sometimes moving them around can cause a short or a break.
Since you suspect a fuel injector issue (2 injectors failing at the same time is very unlikely) you may want to hook up a scanner and observe the Live Data fuel trims on Bank 1. If the injectors are acting up it will be reflected in the fuel trims on that cylinder bank with the 1 & 3 cylinders.
Since you did move around the injector rail, it may have resulted in old fuel injector wiring shorting out or breaking.
If you notice a high + or - fuel trim, you may want to buy a noid light and check those fuel injector connectors to be sure power and the injector ground pulse (from the PCM) is reaching those injectors.
One more thing. @chucktobias is right about OEM parts with that 5.4. Plugs and coils need to be Motorcraft or you're inviting problems
Connectors lock on just fine. Local shops didn’t have motorcraft plugs or coils so I got duralast coils and auto lite plugs. Tried changing the fuel injectors (all 8) and made the problem worse. Now it’s showing lean codes both banks 1 and 2 with negative fuel trims around -20% for so I’m going to clean the old ones and reinstall them then see if I can order some motorcraft coils and plugs if I can find them.
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with that kind of mileage, it's a good idea to change the injector o-rings too
So I fixed the lean codes by cleaning the old injectors and reinstalling them. Now I have a po316 and po303. Changed the camshaft position sensor and the crank shaft position sensor but code is still there. Don’t have a multi meter to do a voltage test on the new coil packs. Could a faulty coil pack cause the po316? Also fuel trims are looking a bit better but could still use work. They are STFT B1 4.6% LTFT B1 15.6% STFT B2 4.6% and LTFT B2 14.8%. Not sure why LTFT in both banks are so high but I’m guessing it has to do with the po316 code.