Subaru misfire

2002 Subaru Impreza RS 78,800 miles

Codes: P0304 and P0302. Many times these have occurred. Have replaced plugs/wires couple times, front o2 sensor twice, and had tune-ups. Changed valve cover gasket?due to leakage.

Codes: P0328, P1518, P0172, P0302/P0304, P1398, and P1134. For these, the Knock Sensor was replaced.

I have had an ongoing misfire problem with my car for years now. The mechanics are failing to fix the problem.

The majority of misfires happen when starting from stop in 1st gear, and when shifting from 3rd to 4th gear. I have also noticed a lack of acceleration at times. Sometimes, upon initial start-up, the car idles extremely rough and can idle out (shut off).

I do know the clutch is showing signs of slippage (at least I think). The dealership tried to readjust my valves somehow once, but I know very little of that.

What do you all think?

The lack of acceleration can be explained like this.

In a manual transmission, I was previously able to ride in second gear and step on the gas and be thrown back in my seat. NOW, I do that and the car seems to bog down and accelerate at a slow rate of speed. Once I hit about 3.5k RPM then it tends to pick up normal acceleration. Doesn’t happen but 45% of the time or so.


A recent talk with a mechanic indicated a rare problem. He said it could be the valve guide. What is a valve guide? Is this relevant?

JMHO, but any time a vehicle is in a shop for an engine performance problem and the spark plugs are out a compression test is a given. At 78k miles a mechanical problem should not be present but certainly can be.
This would be especially true based on the comment about “tried to readjust valves” and the valve guide comment. I don’t see a valve guide being at fault but a tight valve lash adjustment could be.

The easiest thing to do right now (and far easier than pulling the plugs and running a compression test) would be to simply connect a vacuum gauge to the intake manifold and see what’s going on. If the guage shows an oddball reading then a compression test should follow.
Unfortunately, it seems many mechanics do not use a vacuum gauge or even care but it’s a great place to start and if you do have a problem with compression due to valve lash the vac. gauge will show it.

There’s a number of things it could be but a vacuum gauge and/or comp. test should be step 1. It’s easier to weed problems with the mechanical end of things out before throwing cases of replacement parts at it.

(As I mentioned, 78k miles it young for a mechanical problem but we had a Subaru in the shop once in which both cylinder heads were trashed, and not even repairable, due to tight valve lash. And this car had a whopping 7k miles on it.)

You are in need of a good mechanic. One who knows how to troubleshoot a problem, instead of throwing parts at the problem. Ask around for such a mechanic. There are a few available.
Besides the vacuum gauge checks, I see a need to look at the performance of several sensors, and control valves, with a quality scan tool. In particular, the tps (throttle position sensor), iac (idle air control) valve, and the MAF (Mass Air Flow) sensor need examining.
Let’s not forget the performance of the fuel pump, after a fresh fuel filter has been installed.

I am suprised Subaru engines still require manual valve adjustments? Not on the leading edge of technology.

It’s not just Subaru. There are others; the Honda CRV for example.
The issue I always have with mechanical lifter engines is not the design but the recommendations by the factories that valve lash can be ignored for 100k miles plus before even inspecting the lash. It’s certainly not the car owners fault if a problem develops prematurely.

The majority of car owners will get lucky and survive this but there’s going to be a percentage of them who are going to suffer valve and cylinder head damage (too tight) or camshaft/lifter/cam follower damage (too loose).
With the 7000 mile Subaru I mentioned the valve lash was too tight on 3 out of 4 cylinders and this led to such severe valve seat burning the heads were nothing more than scrap aluminum.


Find a qualified mechanic to do a leak down test, compression test, and coolant “exhaust gas” test. I suspect, like other Subaru engines of this age group, you have a failing head gasket. This condition will cause symptom codes such as a misfire. I recently spent quite a bit of time troubleshooting this with OBD scanners and test equipment on my sister-in-law’s Subaru and discovered a leaking head gasket. (To the outside thankfully). Find a different dealer. Some Subarus are actually covered under a “silent warranty” fix-- if they determine that you didn’t abuse the car and it was maintained under their service campaigns. Good luck.


Thank you for the tip. I will let the mechanic know.

I have it at a good mechanic now, and I can’t get the thing to misfire right now. Some days it is really bad, others it runs quite well. It is realy frustrating.

Sometimes this kind of problem has been solved by replacing the cam angle sensor.

Well, I had the valves adjusted…and it fixed my hesitation problem.

The car ran great for about 2 weeks, now it is acting up again. I am wondering if it can be 3 things(through my own research)?

  1. Catalytic Converter
  2. Timing
  3. The actual computer of the car.

Also, I know I need a new clutch (due to some slippage), but I don’t think this has anything to do w/ misfires.

I have been battling misfire codes on random cylinders since I replaced the long block, 2 1/2 years ago. The only reason that I am working solving it is for emissions. If I can keep the light off long enough to pass emission, then I am done. In the meantime, I have a new coil, new plugs that I gapped myslf, new Spark plug wires are coming, and I replaced the Cat and o2 sensors two years ago, a month after the engine rebuild. I cannot find mechanic with common sense to get to the heart of the problem, and I am no longer willing to drop a bunch of dollars on my 2003 Impreza Outback 5 speed. I have kept it because it has a manual transmission, and a Six Star engine. I will never buy another Subaru, not even new. Too many stupid design choices, Cowardly supplier management and it is too noisy. I just need to get 30 more miles on, so everything resets, and I can pass emissions. Two years from now, it will be gone. I will send it to an enemy.

I think you may be a bit too harsh on Subarus. You say you replaced the long block 2.5 years ago.
When this is done valve lash should be checked at that time AND again after a short run-in period. Say a few thousand miles. Who did the work and was this lash procedure done or not?

Read my above posts on valve lash issues. I suggest you buy a vacuum gauge. They can be had cheap at Harbor Freight for 15 bucks or 25 at Auto Zone. Easy to connect and will tell you something instantly. At idle the gauge should show a reading of 20" of mercury give or take a bit. Unless you live at high altitude. The gauge needle should be rock steady. If there is a compression issue/valve lash issue it will be noticeable on the gauge.

Random misfire codes P0300? I would look at the crankshaft position sensor and the belt/chain tensioner. If you are randomly getting misfire codes for specific cylinders P0301-P0306, then I’d certainly check those valves.

@NYBo It was flagged and I agreed, because it didn’t add anything to the discussion other than to jab the new poster for adding a new post to an old thread. It’s come up a bunch of times before (I’m recalling your posting the Holy Thread Resurrection meme) and Discourse doesn’t have the feature to close threads, with the idea that people * should * come back and keep discussions alive.

The new poster came cack after 7 years of inactivity to flame a manufacturer. Not exactly a useful contribution to an ancient topic.

I agree with NYBo. It was also bashing a car manufacturer over an engine that the car maker (Subaru) had nothing to do with during this episode.

If it were me I’d run a compression test (dry and wet if need be) as a first step depending upon what a vacuum gauge showed. If the vacuum gauge and/or compression test shows a problem then the complaint should be against the people who rebuilt that engine which was Six Star.
Subaru did not do the rebuild and their fingerprints are nowhere on it,

Right now this is akin to someone’s LG refrigerator breaking down, being repaired by a local independent shop, and then blaming LG for any subsequent problems.

You complained about the age of the thread, offered nothing to solve the misfire problem and did not defend the manufactures problems. How was you post helpful to anyone?