One-off engine misfire in an 2003 Subaru Forester?

Hey folks!

I have an older and well-loved 2003 Subaru Forester, manual transmission, about 180,000 miles. I bought the car used about 2 years ago. Since then, I have had extensive work done on the car, which is maybe to be expected for an older, getting-up-there-in-mileage car. In the past two years I’ve had the brakes done, head gasket replacement, new clutch, new exhaust manifold, and new catalytic converter.

A few days ago, I got up early to go to the gym before work. Car started fine, drove the short distance to/from the gym, and then about 45 minutes later went out to drive to work. When I started my car this time, the engine started rough, sounded loud/rumbly, the car was shaking, and the check engine light was blinking. The check engine light is usually always on (it comes back an evap code), but it has never blinked before. I turned it off, waited a few minutes, started it again, and the same thing happened. I left my car, got a ride to work, and called my mechanic and explained what was going on. We had gotten a heavy, wet snow the night before and he suggested it was probably the spark plug wires and that I let it dry out and then drive it into the shop tomorrow. I figured this made sense, especially because my car was parked in a closed garage overnight so it didn’t get snowed on, but then sat outside in the snow/wet while I was in the gym and then I parked outside again before leaving for work. I got home and the car started and ran fine and dropped it off at the mechanic’s the next day.

When I went to pick it up, he said that there were no misfire codes from the check engine light, only the evap code, and that he sprayed down the spark plugs and wires in the engine and didn’t get a misfire or see anything out of the ordinary so he said to just drive it and it should be okay, but if it happens again I may have a coil problem. I drive 45 minutes to/from work every day as well as out of town pretty regularly, so “waiting to see what happens” isn’t an ideal fix for me as I don’t want to be broken down on the side of the road, and my experience has been when things go wrong with my car they will definitely go wrong again-it’s not like getting a cold where it just goes away on it’s own! I like and trust my mechanic and he’s familiar with Subarus and does good work. I trust his diagnostic abilities but would also like a second opinion if any of y’all have any insight! Thanks!

Just my 2 cents, but when an engine performance problem exists on a car and especially one with high mileage I always run a compression test.

It sounds like this guy didn’t even bother to pull a spark plug out. Spraying the wires/plugs down with water is not a scientific way of determining whether or not a misfire exists although it can create one.

Regarding the compression test. If that shows a problem in a specific cylinder with an abnormally low reading then one should suspect a valve lash problem. Lash should be inspected and adjusted every 30k miles although this is seldom ever done.
There is no diagnostic code for low compression or a valve lash problem. Hope that helps and yes, I’m very familiar with Subarus.

It’s definitely frustrating when a driver has intermittent problems like that. You have 3 options:

Option 1: I can guarantee that the shop will work on it diligently 8 hours a day if you want them to do that, and you are willing to pay them their hourly fee up front. The problem is that might prove very expensive, likely more than the car is worth if you sold it today.

Option 2: Keep driving it, and wait until it happens again, and tow the car to the shop immediately. (Don’t run the engine if the CEL is flashing as that can damage the cat).

Option 3: Let the shop hold the car on site and have one of their staff drive it as their daily driver. Eventually the problem will happen and they can figure it out then.

Here’s a little anecdote: I owned a VW Rabbit years ago which developed a problem where it would just stop running , stall out, if I drove over a puddle of water. Even a small puddle. I’d coast to the side of the road & wait for 15-30 minutes, then it would start back up again and run fine, like nothing happened. Until I ran over another puddle.

One day the next weekend I decided to figure out why. So I started the engine in the driveway and sprayed it with the garden hose from every possible direction. From the top, from the right, from the left, from the back, from the front, from the bottom. Well,you get the idea, it would never stall. Purred like a kitten all the time. Damn! I figured maybe it somehow fixed itself. Sorry, next day, ran over a puddle, stall out. Then I decided to get serious. I figured it had to be in the high voltage section, so I removed all the spark plugs, spark plug wires, dist cap, rotor and inspected them all on the bench under good lighting. No problems with any of those. Next I removed the coil bolted on the firewall. Still couldn’t see any problem. But I wasn’t sure, so I cleaned it off in the kitchen sink – to spouse’s dismay – and got out my Sherlock Holmes magnifying glass. Sure enough, the underside of the coil had a very tiny crack in the ceramic housing. Replaced the coil, could run over puddles all day long, no more stalls.

Thanks for the input so far!

An update: I drove fine all day today (to/from work and around town) and then met some friends for dinner. When I left, around 9:30PM, when I started my car the CEL wasn’t blinking but it had a very rough idle, low RPMs, and a feeling of low power while driving. When I got home I let it idle in my garage for a little bit to see if it would clear up. It didn’t, and there was also a very distinct, acrid burning smell coming from the car. I didn’t see any smoke while I was driving, the engine didn’t even warm up in the time it took me to get home, and I was too tired to go and start it again and see if there was smoke coming from anywhere. But there’s obviously a problem somewhere that’s not going away.

If the CEL was blinking, that means there is an urgent problem and code will always be stored for that. Unless the code got erased (either by a scan tool or by disconnecting the battery) evap code must be what is turning on and flashing the CEL light. Suggest to tell us what that evap code is, and all diagnostic codes in ecm memory, current and pending. I’ve never heard of an evap problem causing a flashing CEL light, usually that is caused by serious misfires, but I suppose such a thing is possible. Not all evap codes are only for emissions problems. There are some evap system codes that can cause drivability problem; e.g. a faulty purge valve.

If the climate where you live has high humidity it’s possible to have an intermittent miss caused by a combination of aged spark plugs and humidity.
You did not state if the plugs have been changed.

Take a can of WD-40 and liberally spray the entire secondary ignition, meaning the plug wires, coils, etc. and see what happens then. WD can prevent ignition misses due to high humidity and so on.

In theory spraying the wires can show up a problem like this. In practice, not always.


Misfires must occur on two consecutive trips to store a fault. It is a common occurrence for a customer to experience an intermittent misfire and have no faults stored in the PCM.

Ignition system maintenance (spark plugs and ignition wires) may be over due on this vehicle.

Even a flashing CEL sometimes doesn’t store a diagnostic code? Good info to know, but surprising to me.