My 2003 Mustang GT keeps on throwing up P0303 and P0304. I changed the ignition coils and the plugs. The car also has a smell of unburnt fuel. The EGR valve and solenoid has been replaced and I have used a multi meter to test the fuel injectors to see if they were in spec, which they were. Any ideas out there of what could keep tripping these codes???

 

wiring, pcm, fuel injectors, bad connections at the coil or injector, low compression on those cylinders. since you changed the coils and plugs, i would start with a compression test to verify that the issue isnt internal to the engine. good luck

 

Agree with the above.

I would use a noid tester first to verify that the injectors at 3 and 4 are getting proper voltage, if that checks out then send them out for cleaning, and if the issue persists then perform a compression test.

When you replaced plugs was there anything unusual about the ones at 3 and 4 such as oil fouling? If not then although not 100% but still a decent chance you can rule out a compression related cause to your problem.

 

As stated above, compression test first. If the compression is off, take a scope and take a look around. Otherwise, move on if the compression checks out. Coils and plugs were changed, but was the wiring? I would also test fuel pressure, as an issue may be fuel pump and/or filter.

In the worse case scenario, check out my latest project car: https://mustangforums.com/forum/4-6l...-5k-rpm-s.html
Started throwing a misfire code, and weeks later started hesitating. Failed a compression test, and took a scope to find out a chunk of the piston was missing.

If anything, as Z28KLR said, check the plugs for abnormalities. For example, my plug with the misfire had no gap -- and probably very little gap before the hesitating but during the code tossing.

 

Update: I changed my fuel injectors and the smell of unburnt fuel went away. I also did a compression test on all cylinders and they all came out to about 180. Fuel pressure test came out within specs. Wiring is fine, and fuel injectors are getting proper voltage. I am still having the cylinder 3 and 4 misfires though. Now they are coming up separate instead of at the same time. Any other suggestions to test?

 

assuming spark/fuel/compression are correct.....my bet is the intake gaskets going bad. What are the fuel trims (long term and short term) while running? Any p0171 and/or p0174 codes?

 

What heat range spark plugs are you using? After my TVS install I kept getting misfires with cooler plugs. They seemed completely random.

 

I just had to replace all the connections from intake to head covers to keep air pressure. I was having rough idle and miss firing at high rpm replace every hose car runs like new. they rot out and leak your psi from your intake and maf runs nuts, when they get wet they will swell up and it runs even worse... sound right?.. when I replaced my hoses the guy thought they looked like 1 inch elbows and hoses they where meant to be 1/2 inch.

 

^ im not sure what he is saying exactly, but he has the right idea. ^ check for vacuum leaks, and my starting point would be intake

 

sorry brain surgery I try my best to not sound retarted but I always do.... but most the time you can get the idea.... the rubber worn hoses that go from valve covers to intake replace all the hoses/elbows maybe 20 bucks