'93 stalls on incline/decline/coasting; codes 22 and 24

[e2pii]

My '93 Legacy wagon (2.2 L, non-turbo, MT) has an intermittent stalling problem: It likes to stall on inclines, declines, or less often, flat ground (example, approaching a stop sign with the clutch in while braking- the RPMs will drop until the car stalls). It will almost always stall on a slope (uphill or downhill) if the clutch is in (either coasting, or to upshift or downshift). Whenever it stalls, it has always started up again.

I have only experienced the stalling problem since the speedometer broke; but when the speedo broke I had only driven the car ~600 miles (over the course of 6 months) -- so I do not know that the stalling NEVER happened before the speedo broke.

The check engine light occasionally comes on, but is usually off.

On start up, (warm or cold), the car usually idles very fast- 1500 to 1800 rpm. The idle RPM usually drops within a few minutes of driving to around ~800 rpm. If the car is shut off then started again warm, it's a toss up whether or not it will idle high or normal.

I read the trouble codes, I get a code 22 and code 24. I checked out the IACV per the directions in the 1992 service manual, and the IACV seems to check out ok. I also reset the ECU (by these instructions: http://surrealmirage.com/subaru/engine.html#resetecu).

My guesses: could be a bad IACV (but it tests OK), or bad mass air flow sensor -- the previous owner put on a "high performance" conical air filter, so a bad MAF, due to extra vibration etc from this mod, is not out of the question. More likely, I'm worried it's a bad ECU. (Being a poor graduate student, I don't want to replace the $600 part just as a diagnostic.)

Is there a way to test the ECU?
Does the ECU get info from the speedometer? That would link these problems with the bad speedo- if the ECU algorithms are messed up because of bad/no info from the speedo, would it explain these problems?

Thank you for any suggestions!

[rallyak]

Sound like IACV, just clean it and put it back on. It gets carbon build on it, cleaning it with PB blaster and brake clean works well.
Also code 22 is knock sensor and that sensor will adjust timing advance and also would help it run better.

You also mentioned that the speedo broke so you should have a code 33 which that doesn't change much with 5spd, but will let the rpm not to drop right away at a light or stop.

Ecu I believe is fine.

[Legacy777]

Welcome to the BBS.

If your speedometer does not work, the ECU will do some weird things and could affect idle stability. I would recommend trying to get the speedo working if it's not, reset the ECU, and then see if you still have the problem.

The ECU doesn't really care if the speedo reading is off, just as long as it gets a speedo reading.