Pulling max fuel and dying

[relative4]

2005 LGT 5MT, 165K miles.  I bought the car 9 months ago; when I got it home, it had 162K miles.  I replaced the timing belt, which was overdue, and replaced the turbo with a stock IHI unit; the old one was running fine but I wanted to maximize my time before blowing the short block.  Compression when I did the engine work was about 95 PSI in all cylinders.  It's been running great since, with significant but not crazy oil consumption.  I'm still running the stock tune - don't want to go stage 1 until I have enough time in my life to rebuild an engine.

A day or two ago, on a hot start, it stumbled badly and took 5-10 seconds to start running normally.  Then tonight, on a 15 minute drive, it was running jerky.  A mile from home, it died.  I started it, and it ran worse, dying several more times before I got home.  By the time I got home, the sulfur stench was almost unbreathable.  Coaxed it into the driveway and it continued idling without dying, but still stinking a bit, while I grabbed my OBD dongle and fired up BtSsm.  It's pulling massive fuel - learn and correction are both max negative.  The weird part is, no codes, pending or active.  Even if the computer can't tell me anything useful, it makes no sense that it's not popping a P0172.

I've troubleshot lean conditions on these engines dozens of times, but this is my first time running rich, let alone insanely rich.  What should I be looking at?  Thanks!

 

Edited March 2, 2023 by relative4

[relative4]

I guess I should start by diagnosing the fuel pressure regulator?  How do I go about that?

[m sprank]

You can use a gauge and check fuel pressure.  Could be pump controller/wiring/pump/regulator/etc.  But... it could also be a dead engine.  Misfiring causing the rich. 

Rather concerning to me is that you decided to do all that work to an engine with 95psi compression.  That is within 5psi of misfire at idle due to low compression.  The engine was/is on its last breathes.  You should have skipped it all and been saving for a new engine. 

[relative4]

12 minutes ago, m sprank said:

You can use a gauge and check fuel pressure.  Could be pump controller/wiring/pump/regulator/etc.  But... it could also be a dead engine.  Misfiring causing the rich. 

Rather concerning to me is that you decided to do all that work to an engine with 95psi compression.  That is within 5psi of misfire at idle due to low compression.  The engine was/is on its last breathes.  You should have skipped it all and been saving for a new engine. 

That possibility is why I provided the history; I am conscious of the risk.  However, the ECU is not reporting any cylinder misfires - I keep the misfire/roughness counts on my BtSsm monitor screen configuration.  I'll dig out the pressure gauge and tee in.  Fingers crossed.

[m sprank]

Have seen Subies misfire and not be detected by the ECM.  But it is rare.  Good luck!

[xt2005bonbon]

Two different scenarios caused this on my cars: first one was an obvious leak between throttle body hose and throttle body. Second one was poor ground at the MAF wiring harness. The latter happened on two cars.

Easy test is to have the car idle, run BtSsm on your phone and take your phone with you. Open the hood, stare at the AFR correction gauge while wiggling MAF wires close to the MAF connector. See if the fuel corrections suddenly change. If yes, go to iwire and buy a new MAF pigtail.

[relative4]

32 minutes ago, xt2005bonbon said:

Two different scenarios caused this on my cars: first one was an obvious leak between throttle body hose and throttle body. Second one was poor ground at the MAF wiring harness. The latter happened on two cars.

Easy test is to have the car idle, run BtSsm on your phone and take your phone with you. Open the hood, stare at the AFR correction gauge while wiggling MAF wires close to the MAF connector. See if the fuel corrections suddenly change. If yes, go to iwire and buy a new MAF pigtail.

Thanks!  Unfortunately the fuel pressure is bang on spec at 36 PSI with the vacuum hose connected and 44 PSI with the vacuum hose disconnected, so I will check the TB hose and the MAF next.  Thanks again.

[relative4]

The IC-TB hose is secure, and the MAF readings stayed steady while I wiggled the sensor, its pigtail, and a bunch of wiring along the harness.

The MAF reading was 4 g/s and 1.3V.  The FSM specifies MAF airflow of 2-3 g/s but 1.0-1.7 V, so I don't think there's anything wrong with the MAF.  Please advise if there's anything out of the ordinary there.

So far I don't see any problems external to the engine.  Can anyone recommend next steps to prove whether the engine is giving up the ghost?  Other than another compression test, I'll get to that if I have to.

I used Torque to pull codes, and it did show a pending P0172.  Went back to BtSsm, still no codes.  Does BtSsm normally not show pending codes?

I don't currently trust BtSsm to be showing cylinder misfires/roughness properly, especially since I've never seen it count a single misfire in the whole time I've had the car.  Can anyone recommend other apps/tools to check cylinder roughness?

[xt2005bonbon]

BtSsm is really good. At least, it has been for me since 2014. Both cars had issues with misfires (tight valves in a particular cylinder) and it did report it very consistently.

You said the MAF reading was steady during the 'wiggling' test. What about the AF correction gauge? What was it showing? IIRC, there are two ground wires on that MAF harness. You may want to play with one wire at a time and recheck the AF correction.

Edited March 2, 2023 by xt2005bonbon

[relative4]

Reset the ECU and drove it around till it was fully warmed up, and did some WOT pulls.  Drives normally, fuel trims normal, no timing pulls or misfires.  Guess I'll make it my local-only daily till I can get it to act up again.

The FSM says to replace the ECU at this point.  Not sure I disagree...

[m sprank]

Gotta love an 18 year old Subaru.  Hope it all works out for you.