Code P0304

Hi,

Is it only cylinder four that has the misfire code? Interestingly, the A/C places a load on the engine and allows the idle air control valve to allow more air to enter the combustion chamber. I'm wondering if the fuel injector is starting to fail in cylinder four.

If it is only cylinder four, I don't feel it would be things like a vacuum leak or low/high fuel pressure because that would affect all cylinders and you would likely have a P0300. So, it makes me think about what I mentioned or even if the cylinder compression is low on that cylinder.

Tell me, have you removed the spark plug and ignition wire for inspection to cylinder four? If so, was there anything out of the ordinary found?

If not, take a look through these two links and let me know if you are comfortable performing them or if you have already done this. Also, if you don't have them, most parts stores will lend you a compression gauge and fuel pressure gauge free of charge.

https://www.2carpros.com/articles/how-to-test-engine-compression

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

Let me know. Also, let me know if you have checked fuel pressure and fuel pressure drop-off.

Take care,

Joe

I have replaced plug wires and coil pack. Cylinder four plug is dry and tannish brown in color. My fuel pump has been whining/whistling for some time now. I'll check compression and fuel pressure soon, in a day or two. You mentioned a possible valve problem; when started cold after sitting overnight tailpipe sound and feel is normal/smooth. But after warming up I can feel bursts and hear a phush, phush, phush sound. Could cylinder four valves be out of adjustment, too tight?

Hi,

They aren't adjustable on this vehicle. Also, you would hear a ticking sound if they were loose.

The sound you hear may indicate a possible issue with a plugging catalytic converter, but I don't feel that would cause only one cylinder to misfire.

Since it is specific to one cylinder, I feel that is where we should focus. If it was random or multi-misfire with different cylinders, then I would be looking at other possible issues.

Regardless, let me know what you find with the tests. Also, let me know if there is anything I can do to help.

Take care,

Joe

Hi Joe,

How do I perform a fuel pressure drop off test?

Hi,

First, connect the pressure gauge. Turn the key on so pressure rises. When you turn the key off, it should only drop about 10 PSI within five minutes.

Let me know if that helps.

Take care,

Joe

Hi Joe, pressure gage hose adapter won't thread on to fuel rail Shrader valve because the valve starts rotating in the end of the fuel rail. Is there a tiny open-end wrench for that Schrader valve? By the way, cylinder four dry compression: 90 PSI. Wet (4 squirts engine oil) compression: 126 PSI.

Hi,

The valve core in the fuel pressure port is similar to the valve core in a tire. There are tools made specifically for them. I attached a pic below, so you have an idea of what it looks like.

The compression is a concern. The idea that it is 90 dry sounds too low. Also, you went up 36 psi wet, so I suspect the compression rings are worn in that cylinder.

Interestingly, Ford doesn't provide a minimum compression required. Instead, they tell us between the lowest and highest cylinders, they need to be within 75% of each other. Basically, with that cylinder at 90 psi dry, there can't be a cylinder with more than 112 psi. Even 112 is somewhat low. If I recall (from memory) I used to see them around 150psi +.

You could check all of them to see what is found. However, my theory is low compression in the number 4 cylinder is causing the misfire.

Let me know.

See pic below.

Hi Joe,

My Schrader valve rotation problem was solved with a thin 7mm open end wrench to hold it while I use a 1/2" open end wrench to screw the adapter onto the Schrader valve. Haven't tried this yet because the car was hot after getting 7 mm wrench. Cylinders 4 and 5 injector resistance was 12.7 ohms for both. I don't know yet if I have a valve core problem (referencing your picture).

Hi,

The valve core should be tight. My concern is with the compression. Are you comfortable checking all of them?

Joe

Why do you mention the valve core? I can't screw the Ford adapter fitting on far enough to engage the two valve core pins (1 in the adapter and 1 in the fuel rail Schrader valve) and I'd like to know if my fuel pressure is leaking down.

I'm sure the valve core is tight because it's not leaking gasoline; just like a car tire would leak air with a loose valve core.

I'm sorry. I thought that is what was loose. I was wondering how you were able to tighten it with an open-ended wrench. LOL

Are you sure the adapter is the correct one? That doesn't make sense unless the threads are damaged on one or the other.

Joe

Threads on both look good.

The only thing I can say is it has to be the wrong adapter. Have you tried seeing if a local parts store to see if they have one?

Joe

The Ford Schrader valve adapter was broken (interior pins rattled when adapter was shaken) so O'Reiley's warranted the whole kit back to manufacturer and they only had the one kit, so I'll have to rent one from a different auto parts store. I believe I'll just drive the car until it quits.

Hi,

I can't believe how many faulty parts and tools such as this I am seeing anymore. As far as driving it, let's do our best to get it running the best we can.

I have a feeling the ultimate cause of the issues is compression. Let me know if you checked the other cylinders.

Take care,

Joe

I only checked cylinder 4 compression.

Okay. If you have the chance, you may consider testing all cylinders so we can compare. I have a feeling the fuel pressure isn't the issue.

Let me know.

Joe

Too difficult for me to check bank 1 compression, but I can check cylinders 5 and 6.

This morning I disconnected the negative battery cable for 1 1/2 hours then did the procedure from owner's manual to reset idle and fuel trim settings in the PCM. Also drove a 1/4 tank of gasoline with Berryman B 12 fuel system cleaner through the motor. Now idles 20 minutes without a P0304 code, but ambient air temperature. This morning was upper 60s F. Idle now seems smoother. Previous owner put K Seal head gasket repair in at 152,000 miles. Just before I bought the car. Be interesting to see cylinder 5 compression tests?

Also, 1 1/2 month ago O'Reilly's sold me a Standard (brand) IAC valve; my car needs the Delphi (brand) IAC valve which I bought and installed last Tuesday. You probably know the former is always open normally (no B+ applied) and the latter is normally closed. Resetting the PCM and having my car relearn the idle and fuel trim settings today has made a huge difference for the better: smoother idle, hot idle for 20 minutes without A/C on and no P0304 code, and it seems my car's response is better (quicker). I'm very happy with my car now, Joe.

Hi,

That's good news. I'm glad to hear it's running better. If you do have a chance to check the two other cylinders' compression, let me know what it is. However, if the light is off and it is running good, it may be better to leave well enough alone.

The concern I have is the head gasket sealer that was used. It works when used correctly, but it's not a permanent fix. At least that is what I have experienced.

Regardless, take good care of yourself, and please feel free to come back at any time in the future. You are always welcome here.

Joe

Hi Joe,

I'll likely check cylinders 5 and 6 compression by midweek next week and I'll let you know the results. Thank you for your advice and guidance Joe.

Best regards,
Jim