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(Subaru) Intermittent Starting Problem - Mechanics "Have No Idea"

I have a 2003 Subaru Forester that has been having starting/misfire issues for about 8-10 months at this point.

Last November, after making several short trips, I attempted to start the car. It started, but poorly, rumbling and shaking excessively with (what was later diagnosed as) a misfiring problem. I had it towed to a mechanic because it was undriveable at the time, but come morning the mechanic could find nothing wrong with it and could not duplicate the problem.

Fast forward a few months to February/March, it begins acting up again, and the Check Engine light has come on and stays on, reading a P0420 code. It begins having issues shifting gears, so I take it to the mechanic again, and they tell me that a lowered efficiency of even 5-10% in the catalytic converter can cause the P0420 to read and refer me to a transmission shop. I take my car to the transmission shop, and they fix a couple minor leaks, but say there is no real problem with the transmission. Instead, they say there is a misfiring problem. I drive it back to a mechanic -again-, and they diagnose a failed ignition coil and replace it. It’s fixed, finally, correct?

Incorrect. It is now mid-August, and my car has had three separate issues with starting over the past week. The first two times, the engine will crank and appear to start, but it won’t catch. The third time, it repeated the condition that initially happened last November: after several short trips, the engine cranks, tries to start, but fails, and instead rumbles and shakes excessively as if it’s misfiring. Fearing the worst, (the first time this happened, I tried to drive it home, and a small fire started under the car; I was not looking for a repeat) I had it towed again to a mechanic, and they spent two days with it and -once again- could not recreate the problem.

All I know so far: the starting trouble is intermittent. The battery and starter both test well and check out fine. The trouble comes most frequently after making several short trips (about 10-15 minutes, max, each time). It has no trouble starting first thing in the morning. The P0420 code is still there. Any suggestions?

Makes me think of a leaking/dirty injector(s) causing a flooded situation , after sitting over night the fuel evaporates

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You can try this, works with most cars

Try Clear Flooded Engine routine for warm starts to see if it makes a difference.

Turn on key
Press gas pedal down to the floor all the way and hold it down>>>you are now in Clear Flooded Engine mode, computer shuts OFF fuel injector pulses but Spark is ON

Crank engine, it should NOT fire or start, if it does then fuel is present, it has leaked in when engine was off
Release gas pedal after 5 seconds or so and see if engine starts and runs smoother, like cold start

If engine fires or RPMs get above 400 computer will exit Clear Flooded engine
If you release gas pedal computer will exit Clear Flooded engine

I’ve got news for you.

When a fuel injector leaks, the fuel ends up in a cylinder. But instead of the fuel evaporating, it leaks past the piston rings into the oil pan, contaminating the oil.


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I actually did some (albeit under-informed) research on common causes for a problem like this and ran across the cracked/leaky fuel injector. When I’d brought my car to the mechanic most recently, I asked him if he thought the fuel injectors might be the problem, and he said that if that were the case, my car would be overall running much more poorly than it has been. He said overall it tested too well for him to think the injectors were the root of the problem.

Any thoughts on the fuel pump or fuel pump relay being the cause?

That’s true but I don’t think it’s relevant to the OP’s question. The OP said this happens typically after short trips of 10-15 minutes but starts fine after sitting overnight. That would be consistent with leaky injectors. They leak as long as the fuel system has pressure in it and that would be the case when trying to restart after a short period of time (depending on the severity of the leak(s)). As it sits overnight, the pressure drops, the injectors stop leaking, and all of that fuel that leaked in, as you said, ends up in the oil. Now the cylinders are clear of any unburned fuel and when they go out in the morning, it starts right up albeit it’s running rich (it’s always running rich) and causing unburned fuel to enter the exhaust, contaminating the cat and there’s your P0420 code. I’m with It_s_me.

That certainly sounds like it makes sense! Is there any chance you have an estimated cost on repairs? I’ve read replacing fuel injectors can be incredibly pricy.

The other thought I had is that because this vehicle has a horizontally opposed (boxer) engine, leaking injectors would make this engine run worse after sitting for short periods of time because gravity won’t cause the fuel to run down past the piston rings and into the crankcase but it will stay in the combustion chamber. Maybe someone that’s more of a Subie aficionado can correct me if I’m off on that.

Cost to replace injectors? I see the parts listed between $25-$100 per injector (you have 4) and the difference is the “quality”. My wife has a Crosstrek (essentially same engine configuration) and the injectors are easily accessible so I can’t imagine labor being terrible but maybe someone here has a labor guide and can give you a better estimate. Or call a good independent shop, or a dealer, or ??? and ask. Good luck.

Thank you! I have an appointment to take the car into “The Subie Guy” on the 27th, and I’ll definitely be sharing this info. :slight_smile:

Suggest to determine for certain whether there’s a cat problem or not. A clogged cat will cause hard-starting and misfiring. In some cases the innards of the cat will break loose and shift around as you go over bumps, causing intermittent act-up problems.

A shop w/the proper scan tool (for example a dealership) should compare the pre-cat o2 sensor signal to the post-cat o2 signal, and from that comparison they will probably know if the cat is bad or not. It’s possible to get some idea of a clogged cat just from the intake manifold vacuum reading also, but the scan tool method is the best way.

I agree that there might be a need for looking at the catalytic converter, but I’d rather fix the problem -causing- the potentially malfunctioning cat first before replacing it only to have a new one ruined as well. :slight_smile:

How many miles on this vehicle?

110,000 miles

Ask the shop on the upcoming visit in their experience how many miles the cats typically last on the forester. The one on my corolla is still going strong at 200K miles, so 110K would seem an unlikely time for the cat to fail from my perspective. But the robustness of the cat design varies vehicle to vehicle.

I’ll definitely ask. I don’t know a whole lot about the catalytic converter, but I can imagine having leaky fuel injectors since possibly even last November could greatly decrease its overall functionality.

Yes, leaky injectors can cause poor drivability and damage the cat. But from the posts we get here, leaky fuel injector problems are much rarer than cat problems. Usually when something damages the cat beyond plain miles driven or it was faulty when manufactured, it is a problem w/the ignition system.

The ignition coil was very poorly malfunctioning a couple of months ago–do you think that it might have damaged the cat to the point that it’s causing the starting problems I’m seeing now?

If the catalytic converter were restricted to the point that the engine were difficult to start there wouldn’t be enough engine power to drive the vehicle. Does your engine lack in performance?

Depends on how long it was malfunctioning. The experts here may have an idea. My guess is that it would take some time, weeks — but that is a guess. Could be only hours.

If the catalytic converter were restricted to the point that the engine were difficult to start there wouldn’t be enough engine power to drive the vehicle. Does your engine lack in performance?

According to the mechanics I’ve taken it to, no. It starts most of the time, and once it has started, it runs really well.