2000 Legacy B4 RSK engine stumble

Hello all,

I recently purchased the above-mentioned car, and it is having some issues.

When cold the car starts no prob, idles fine, and drives great too, pulling nicely without any stumbling. However, after the car is all warmed up, and has been driven for a good 10 minutes (give or take) when the throttle position is anything from 1/4-full, the car builds boost nicely and starts to pull hard on the first turbo, but consistently around 3000 RPM it stumbles HARD and it feels like the engine cuts timing or fuel or something to prevent damage to itself. It jerks on and off even if I keep my foot as still as humanly possible (not letting the car jerk my foot forward and back). This same issue occurs higher in the rev range too, i.e. if I slowly let the car pass 3000rpm on very light throttle to just about anything above it and gun it, then the same thing happens.

See, I would guess I have a boost leak or bad knock sensor or something along those lines given what I've explained so far, but the peculiar thing is that if the car is completely warmed up, then shut off for a few minutes (like when I'm filling up) then driven again, the car pulls hard under WOT without a single stumble all the way up the rev range (5000 is redline in my mind until I change the oil). I know for a fact it is making boost on the second turbo, and the car is already warmed up completely so I would guess any splits in boost lines or piping would leak boost if that is the problem. But after a good pull and a minute or two, the car stumbles just as it did before.

Some things to note;

-The car has a Mines ECU that has been sent to them and reflashed (has the Mines plaque on it so it is legit)

-"new" ignition coils and plugs (I say this because I am completely prepared to replace both

-the gas cap, when removed is under pressure and hisses as it's unscrewed (with a final puff when it's completely off)

-No CEL (this is subject of discussion as well b/c it is a JDM car, and the scanners I've tried don't speak SSM (a special Subaru ECU language) but this can be solved when I find the proper adapter.

-car has 80000 km, so hoses are unlikely to be leaky.

-The car was sitting for about a year before I bought it, and it had a full tank of gas the whole time, which I have to drive home on and this is when the stumbling started (somehow never happened during the test drive) but fresh 94 and injector cleaner made it much better, but far from fixed.

So my question is, did the bad gas really do this all to my engine? If so, why does it still run as I described?

I'm going to look for clogged breather lines for the fuel tank, and post an update.

-kaelan

Changed spark plugs, they are gapped correctly. The car runs smoother but still stumbles when on heavier throttle.

The old plugs had rust on the top, not on the electrode and I’m sure this wasn’t helping the performance but it still didn’t solve the problem. The coil packs looked to be in good shape, as they are brand new aftermarket ones.

I’m thinking the bad gas had some impact on the fuel system, so checking injectors now might be a good idea. I did use a fuel injector cleaner so I assumed that would hopefully be rid of any fuel problems.

Another update;

After a few tanks of gas, it seems to run very well up until the secondary boost kicks in around 4000-4500 rpm. I'm not sure if it's actually a boost problem, or if it is just because I'm at near to, or at full throttle when I'm in this rev range.

This was all before I found a thread:

https://uk.subaruownersclub.com/forums/topic/2366-manually-reading-ecu-codes/

This explains how to "reset" the ECU, or in my case show me a code that just the diagnostic mode (also explained) didn't. This thread also explains how to read the morse-like code given by the CEL.

When just the black connectors were plugged together, I was given code 23 for MAF sensor

When I attempted the ECU reset function, instead of the confirmation that it was completed successfully, indicated by a consistent flash of the CEL, I was given code 32 which is faulty O2 sensor!

However, I'm also reading that the O2 sensor doesnt really impact the high rev-range. I also unplugged the front O2 sensor plug, and the exact same higher-revs hesitation occurs. So does this mean that the O2 sensor isnt the prob?

have you looked at your injectors? something happening to them as the engine warms up?

I have deduced that it is a fuel problem, and probably a failing fuel pump as the problem is never consistent, and it doesn't seem to give the engine enough gas at high load high rpms. If it ends up not being the pump, then regulator and injectors will be next.

Turns out it was a fuel pump, I read a guide on uklegacy;

http://www.uklegacy.com/forums/index.php?/topic/106366-fit-hrc-255-lph-340-fuel-pump-bh5-be5-simple-job/

This explains in detail what pump you need, and how to install. So if your car is experiencing what feels like boost cut, or fuel cut (like hitting a wall when you put your foot down) then try a logging software like ecuexplorer (the only one that worked for my JDM 2000 Legacy B4) and get a VAG-com KKL cable to connect the car to your laptop. See if your getting a large drop in boost when it cuts out and if so, then its a boost problem. If not, consider changing your fuel pump or looking into other fuel related issues (Regulator, injectors).

Thanks for all the suggestions! -Kaelan