P0302 code with a miss only when idle

I have a 2002 Subaru Forester L with 178,000 miles on it. I just had the valves adjusted. I have replaced the coil pack, #2 injector, plugs, wires, and throttle position sensor. I have also cleaned the idle air controller. The car only shows sign of a miss when it idles. It gets great gas mileage otherwise. I am at a loss on what to do next as the car doesn’t have a MAF sensor, I can’t do anything with it. I am open to any suggestions.


Good that you had the valves adjusted

May I ask if you had the valves adjusted because they were noisey, routine maintenance, low compression, etc.?

Or did you have them adjusted, because of the misfire?

Is #2 injector being pulsed?

Hooking up a noid light would give you that answer quickly

Check the resistance of #2 injector . . . same as all the others?

Does #2 cylinder have good compression?

Same as the other cylinders?

What are the numbers, for all cylinders?

the compression was 180+ lbs on all cylinders and will check the resistance. my mechanic said they were noisy and they could have been causing the missfire. it was a straw that was grasped at.

So the valves were adjusted to try to cure the same misfire, but the misfire remains? hmmm … well, one good thing, the misfire is only occurring on cylinder 2, that should make diagnosis easier. It’s unlikely to be something that affects all the cylinders like a faulty fuel filter or fuel pressure or engine air filter, cat, MAP sensor, etc.

Since the compression seems good in number 2 as well as the rest, I think I’d try swapping things between cyl 2 and 3, and watch to see if the misfire moves to cyl 3. If it does, for example after switching spark plugs, then you know the cause is the spark plug. Coil packs, spark plug wires, injectors.

If you can’t get it to move to cyl 3 by swapping, it would have to be something wrong with some part that isn’t swappable, like the engine computer or the wiring harness. Checking the injector’s electrical pulse as suggested above is a good idea toward this end.

I remember a car several years ago . . . there was a misfire on only one specific cylinder

Fuel was good
Compression was good
injectors all checked out good
ignition was ruled out as a possible cause

That misfiring cylinder failed the running compression test, though

The mechanic . . . not me, it was a colleague . . . pulled the head, as per shop foreman

The valve spring for the offending cylinder was broken

Replaced the valve spring, and the problem was resolved

Replace the injector…I bet if you disconnect the #2 injector at idle, there will be no difference…

“I have replaced the coil pack, #2 injector,”

Caddyman stole the words out of my mouth. Its at least a good test to try…ez and will lead to resolution… Then try swapping the injector… Of course you can also verify if the injector is working with a noid light and or a mechanics Stethoscope…or a long thin screwdriver…whichever you have handy. See if the injector is pulsing. Then proceed according to findings. I also like the broken valve spring solution…but you have several things to do before damning the mechanicals at this point… There is a proper line of troubleshooting to adhere to…and you havent gone thru all the available steps yet… Let us know what you find.


@db4690 writes …

Compression was good .... That misfiring cylinder failed the running compression test, though

I’ve never heard that term. What is a “running compression test”?

Maybe the compression issue needs to be revisited and forget all about that running compression test. It’s not needed.

I wonder a bit about the line of thinking by a mechanic who thinks that a misfire will be caused by excessive valve lash. The odds of that are about zero unless the lash is beyond obscenely loose.
In a case like that a valve lifter should be noticeable from 3 blocks away.
Double the valve lash and a misfire will not occur because of it.

My suggestion is to connect a vacuum gauge and see what’s going on. I have seen a few Subarus that developed a miss on one side or the other due to an air leak. Depending upon the location of a leak it may affect one side only due to the Boxer engine design. The vacuum gauge will also reveal if there’s any compression issue. A miss at idle can also be caused by a weak valve spring; something else that should show up on a vacuum gauge.

“forget all about that running compression test. It’s not needed.”

Maybe not in this case

But it is a useful test

I just consider the running compression test a colossal waste of time and have never in my lifetime found something like that to ever be needed.

If the compression really is 180 on that cylinder and there are no issues with weak valve springs or air leaks then what needs to be determined is whether the injector is operating properly.
That cylinder has a new injector but that doesn’t mean there are no issues with the wiring, wiring connector, or ECM.

It should be a simple matter to listen for an injector clicking.

Thanks @jesmed1 for the link, good info there.

If a static compression test is good, but the running compression test fails miserably, perhaps you’d better think about those valve springs again

“colossal waste of time” is relative

So far you’ve not needed it, but others have

You can speak for yourself, but not everybody else


I would try dumping a can SeaFoam into the gas tank.

If the #2 intake valve has excessive carbon built-up on it’s back-side, it can act like a sponge and absorb some of the fuel that’s supposed to go into the cylinder causing a lean condition and a misfire.


If there’s a valve spring problem that can be determined inside of a minute with a vacuum gauge; far faster than any kind of compression test.

If it misses only at idle I would be looking for a leaking intake gasket.

Regarding the running compression test (which I maintain is needless and time consuming) I might pose this.
Changing plugs and/or running a compression test can be labor heavy as it may involve removing injectors and fuel rail, intake manifold, etc, etc, etc.

What is the customer reaction going to be when told of the need for a running compression test at 100 dollars per flat rate hour X repeated R & R of major components along with part costs such as an injector seal kit, intake manifold gasket set, etc?

I don’t see any customer approving of or accepting calmly a 1000-1500 dollar charge for running that test.

well, no gaskets leak, told from a smoke test. everything ohm’d out and is fine. so the only other thing it could be is the ecm or the wiring.