[FIXED] 2003 Toyota Corolla 1.8L 1ZZ-FE throws P0304 misfire

Hello all new here. Last time I worked on a car it was with a timing light and a dwell meter so a bit out of date. I have been watching a lot of videos and trying to learn as much as I can however I'm at a stall with this car right now. Here goes...

My son's 2003 Corolla with 185K miles keeps throwing P0304 "misfire cylinder #4". Idles a little rough and weak on power when pushing the accelerator down. Blinking CEL on higher RPMs. Spark plug #4 looks black and sooty the others look white and "clean". I swapped plug #2 with plug #4 and all that did was foul up plug #2 in about 10 miles of driving. Still got the P0304. Replaced spark plug #4 still get P0304 (actually I replaced all the plugs). Swapped coils with #2 still get P0304. I verified I am getting good spark from the coil using a Lisle coil on plug spark tester. No signs of oil leaks around #4. Did a compression test and got 167psi on #4 and #2 did not check the others. Not sure if there are other ignition tests I should perform.

Looking at fuel injector #4 visually its much dirtier than the others and when I unplug it even the inside of the connector is dirty and looks wet. Outside of injector looks wet where it meets the engine, the others appear dry. I wiped the inside of the connector with a Q-Tip and it looks oily. Clean oil. Sprayed it out with contact cleaner, same with the connector. Both wires on the connector have a small bare spot just before where the wire goes into the connector. Covered the wires with a little electrical tape as a temp fix. Not sure if that has any effect. Used a Noid light and checked fuel injector #4 and saw the flashing light. Got similar reading from #2 (flashing light). Checked all fuel injectors with an Ohm meter got same readings of 16.5 ohms.

Runs a little lean with LTFT about 15% and STFT about 0%. Varies a little but not much at idle. Gets much closer to 0 for both when RPM above 2000. So there might be a small vacuum leak however IMHO not enough to cause a P0304 although I could be wrong. I have not done a thorough vacuum leak check however a quick look and spray of carb cleaner did not show anything.

So my thinking is that fuel injector #4 is failing. Is there a way to test it before I replace it? And if I have to "drain the fuel" already in the fuel injector manifold thing how do I do that so it's not leaking fuel all over and not so messy?

Incidentally, before I decided to put my own time and effort into this, we took it to a shop and they replaced one fuel injector (not sure which one they all look the same right now) and the also did a "clean fuel injectors by running the engine on pressurized injector detergent". So not sure if all the injectors are failing, clogging, etc. I have not done much research on reviving fuel injectors however I will if consensus is that I should either replace or repair fuel injector(s).

Thanks in advance for all the collective help!
Michael in San Diego

Hi everyone,

Is it available to scope pulse width (comparison) in order to rule out injector driver?

Fuel trims are screaming vacuum leak, but could be a false one. What are your O2 readings?

kind of tricky this one.

What are Map and Maf telling to PCM while fuel trims are that high?

Can you read IAC position and traget rpm vs actual rpm while fuel trims are that high?

Unfortunately resistance measurements on solenoids isn't much use in this case, as a bad injector can show a good static resistance reading but go high resistance as the circuit starts to operate.

The two most direct methods to check would require some tools, either a scope with an amp clamp or a fuel pressure gauge and an injector pulse tester to check injector contribution.

One way that could help if the injector is bad enough and you have a clear line of sight with it would be to get a long screwdriver to use as a stethoscope. Put it on a good injector and listen to it operate then move over to the suspect cylinder and see if there is any difference with how loud the ticking is. Like I said, probably won't be able to hear any difference, but it's worth trying if it doesn't mean tearing anything down.

You could try the ohm meter on the suspect injector and compare it to the known good ones. Can be tricky to probe the pins.

They should all be about the same resistance. Even if they are all the same, it doesn't prove the injector is good. But if the suspect injector reads open (OL mOhms) or excessively high resistance it is definitely bad.
The methods mentioned above with a scope or injector pulser may still be necessary.

One other thing that comes to mind is that an engine can have a mechanical fault and still have "good compression" if the exhaust valve(s?) don't open.

I suppose depending on the shape of the intake that you could have a runner specific vacuum leak that could cause a single cylinder misfire, but I don't think it would cause the engine run as poorly as you describe at higher rpm.

Thanks for all the help guys much appreciated! The car is my son's daily driver so I only see it once or twice a week so its taken some time to figure this one out.

I ended up swapping the #4 fuel injector with the #2 fuel injector and the misfire followed the injector so I replaced it. Problem solved!

Best wishes and stay safe everyone!
Michael

Thank you for letting us know what fixed the problem.