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Dan, Technician
Category: Ford
Satisfied Customers: 24164
Experience:  Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
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2008 f150 5.4 3 valve with p0303 code fuel injector firing

Customer Question

2008 f150 5.4 3 valve with p0303 code fuel injector firing coil is firing has a marginally lower compression reading in #3 cylinder. Put new plugs in and the #3 plug still looks like it did when it came out of box, has seen no fuel or combustion process is not wet fouled. I thought maybe the intake valves were not opening directorate plug not getting fuel on it but ford dealer pulled valve cover said everything looked fine and said They need to pull the head. I've never seen a plug look new due to 140psi compression when spec was 165. Is it possible the intake tract is blocked on the #3 cyl or that the injector is not firing at rpm due to a PCM issue?
Submitted: 1 year ago.
Category: Ford
Expert:  Dan replied 1 year ago.

The coil is the most common thing to cause a engine misfire so you could start by swapping coil with another cylinder to see if misfire follows if so, replace the coil. If not you will need to use a spark tester to see if coil produces good spark. If it doesn't then you will need to test wiring between coil and pcm to see if problem is wiring or PCM. If good then use a led test light to check for injector pulse, if no pulse then check wiring between pcm. It would be more likely a coil issues because injectors would usually set a different code. Then last stem would be trying a cylinder leak down test to see if that could determine anything before pulling head but either way head would need to be pulled if it gets good spark and injector pulse. Let me know if you have more questions, thanks.

Customer: replied 1 year ago.
Tried new coil and swapping coil, tested peak voltage from PCM to coil checked injector with noid light. Pulled injector rail and ran brake cleaner with a hose, pressure tester and 12 volts to test injector it sprays correctly. Cranked engine with fuel railed pulled up and watched all 4 injectors on that bank mist equally. My main question is how is it that the plug looks new? I've drove 100+ miles on a dead cylinder with constant miss and I have a dry new spark plug. I'm a can am and yamaha master certified tech and have built plenty of bike engines, am very familiar with coil overs and modern fuel injection. I've never seen a dry spark plug unless there was a worn cam lobe, broken rocker, obstruction in the intake past the injector or no fuel being delivered. Ford dealer wants to pull the head but I'm going to pull the intake and visually inspect for obstruction. They already pulled valve cover and verified cam and followers are ok. If it was compression related I would have a wet fouled plug. I'm just hoping someone has seen this before.
Expert:  Dan replied 1 year ago.

If compression is too low for fuel to combust then the fuel would go right into the oil without igniting so the spark plug would not look any different. All of the things you mentioned would cause low compression so fuel can't ignite. At this point I'd do a cylinder leak down test which should tell you where compression is being lost. If you verified its getting good spark and fuel spray then mechanical is only possible problem left. Let me know how it goes or if you run into more questions, thanks.

Customer: replied 1 year ago.
Every internal combustion engine I've worked on with mechanical issue even with no compression has a wet fouled plug. If it has 140psi compression it should be running but inefficiently so. I'll keep you posted I'm going to put a noid light on the injector and observe the signal while driving. Thank you
Expert:  Dan replied 1 year ago.

Some engines work differently, rule of thumb is 100 psi should give you some type of combustion but I've seen it go much higher and not. Let me know how it goes, thanks.

Customer: replied 1 year ago.
Removed the intake and there was a broken joint going to the intake charge plates or secondary butterfly's . The drivers side bank was closed completely. There was also a vacuum line off on the bottom rear side of the intake. Power is restored and it's running good now.
Expert:  Dan replied 1 year ago.

Good to hear, I'm surprised only 1 cylinder was effected. Thanks for update and have a great night!

Customer: replied 1 year ago.
Well when the secondary's on the drivers side bank are closed but the throttle body is still opening to supply air to all 8 cylinders it would create a surge of air and an extreme lean condition to the 4 pass side cylinders. Considering how misfires are detected is by using the cam and crank sensor position when a miss is detected it's not designed to take into consideration an obstruction in the intake on cyl 5-8 so it's just a good guess that cyl 3 is actually causing a misfire as far as the PCM goes. Plugs on cyl 1-4 were white plugs on 5-8 were rich I zip tied all the secondary's wide open removed linkage, hooked up the disconnected vacuum line and put a new set of plugs in (covered by motorcraft warranty) it runs smooth and has no codes.
Expert:  Dan replied 1 year ago.

Nice find. Thanks again.