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CEL code P0304 - coincidence?

2006 Subaru Forester X 2.5L AT non-turbo

I changed the oil and oil filter; replaced the air filter (Fram), and replaced the alternator and a/c belts. I also got a full tank of gas at Sam’s Club, I was nearly empty. I’ve gotten gas plenty of times there, before.

Next day, the CEL comes on. It’s a P0304 code. (Cruise control light is flashing, too.) Engine stumbles and runs rough when cold; idles fine when warm, runs OK, but not the same, when warm. CEL is steady, which is “not bad” but I can make it flash if I hit the accelerator while the engine is cold. Resetting the computer by disconnecting the hot battery terminal didn’t work; CEL came on again two days later and symptoms remained.

I buy some MAF cleaner (CRC) and use it; MAF was pretty dirty on the upstream side, actually. No luck.

I’m going to change the plugs and wires (they need it, anyway) and I’ve ordered a new ignition coil, which I’ll also switch out.

I’ve also ordered the gasket for the throttle body, which I’ll take apart and clean once that comes in. Last step is to order replacement O2 sensors.

I’m 99% certain there’s nothing wrong with the engine, in terms of rings, valves, pistons, etc. I think I’m covering all the bases in addressing this myself but I’d certainly welcome any additional insights. I’m on a limited budget so I’m doing all the labor myself and ordering non-OEM parts on-line, after researching to make sure the parts have a good reputations, fit, etc. (Yes, that’s all a big FU to Subaru service for being over-priced but besides being relatively poor I also enjoy doing this stuff.)

So, any comments, suggestions, helpful tips?


A P0304 code indicates a misfire in cylinder #4

If you’re sure it’s not mechanical within the engine, then either spark or fuel injection is the problem.


“I’m going to change the plugs and wires (they need it, anyway) and I’ve ordered a new ignition coil, which I’ll also switch out.”

Since, as Tester noted, that code indicates a misfire in one of the cylinders, I think that the likely cause is that one of those admittedly overaged spark plugs is the source of the misfire. If new plugs and wires don’t cure the situation, then perhaps the coil will be the answer, but I think that it is premature to “throw” a coil at the situation until you know whether it is needed.

Plugs and wires first.

-sigh- Changed the plugs and wires this morning. Ran an errand, car ran beautifully, no stumbling or hesitation, plenty of power. Then, on the return trip, the stumbling began occurring at around 40mph @ 2250 rpm.

Plugs looked OK.

It’s time for a compression test.
Swapping fuel injectors is another good test.

First, thank you all for the advice. I will continue to follow up on it.

Second, is there any chance at all that any of the maintenance I did could have caused this problem? Or even a bad tank of gas?

OK, I’m not a complete idiot, but I could give one a run for his money, sometimes! After checking under the hood, I discovered one of the spark plug boots was loose, as in disconnected. I must not have pushed hard enough when I installed it. I fixed that and discovered the other three were also loose, although not completely disconnected. After making sure all four connected with a nice, satisfying click, I took the car for a test spin.

No stumbling, good acceleration, alles ist klar, Herr Kommisar!

Not often, but sometimes the solution is that easy. Congratulations! I have experienced boots that are so tight a good connection is difficult.

Good for you for sticking with it. Glad you are back on the road with a purring eninge. I’ve had spark plug wires on my truck that appear to seat properly, but don’t actually make a good contact w/the plug, b/c the little circular thing at the top of the boot on the inside had enlarged enough it wasn’t making a proper contact with the top terminal on the plug. Easy enough to fix, just use a pair of pliers to carefully squish it down to proper size. One diagnostic method I’ve used – now I’m not exactly recommending this since it might not be entirely safe – but a person could touch each spark plug wire (the insulation only, near the junction of the wire and boot) while the engine was running. If the connection to the plug is weak, you might feel a little tingle of a shock in that wire.

While it’s seldom ever done, I strongly suggest a valve lash inspection. If this is over your head mechanically then you may have to bit the bullet and pay to have it done.

It’s not difficult if you know what you’re doing. It can turn into an expensive faux pas if you make a mistake so you might consider biting the bullet and having it done if you feel your mechanical acumen is not up to the task.

And I’m also aware that if you have a Subaru dealer do this you may be given a few reasons why it’s not necessary. Those reasons are 100% BS.

I also ended up replacing the ignition coil and removing and cleaning the throttle body. Much more power, now! And still running fine.