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Old 06-26-2002, 04:16 PM   #1
Scooby Newbie
Member#: 1919
Join Date: Jul 2000
Location: Markham, Ontario, Canada
1994 Legacy Turbo

Default boost and open loop

Hey all!

It's been a long time since I've posted here. I have a '94 Legacy Turbo.

My Legacy is experiencing an odd problem. Sometimes at part throttle acceleration in 2nd or 3rd gear, the ecu doesn't seem to want to switch in to open loop mode if I start making boost. My A/F ratio meter keeps bouncing back and forth, and I can feel the engine hesitating. Eventually it will switch to open loop (A/F lights go green), and the car takes off. The strange thing is, I've never noticed this hapening in 4th or 5th.

There are three things controlling the switch over to open loop that I know of...
1. Throttle position (full throttle)
2. Engine RPM ( > than 4000rpm)
3. Pressure sensor ( > than atmospheric)

I suspect the problem lies with the pressure sensor or pressure exchange solenoid, since the engine will always switch over to open loop at full throttle and rpm's > than 4000. But I can't explain why it doesn't happen in 4th or 5th. The only reason I can think of is that revs build very slowly in these gears.

Any thoughts? Has anyone noticed something similar?

'94 Legacy Turbo
'73 Datsun 240Z Turbo
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Old 06-26-2002, 04:35 PM   #2
Scooby Guru
Member#: 15543
Join Date: Feb 2002
Location: san francisco, ca
90BJ Legacy LS ABS
AWD 946 Rio Red Jpn built


A quick lazy response would be, grab a precise set of feelers and make sure the tps is adjusted correctly. Then use an ohm meter and make sure the transition from closed to open is completly smooth on resistance withough any hickups.
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Old 06-26-2002, 05:00 PM   #3
out back
Member#: 1964
Join Date: Jul 2000
Chapter/Region: RMIC
Location: 84XXX


Is it still using the stock pressure sensor and boost solenoid? The stock solenoids are flaky and seem to get pretty unreliable after 8-10 years.

The TPS could be causing it, but you'd probably see it in other gears.

If your A/F is varying, it's possible that you could have a vacuum leak that's introducing a lean condition under load in the first gears, but not so much in the higher gears. I had a leak that would cause nasty hesitation in 2nd gear.

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Old 06-27-2002, 10:46 AM   #4
Scooby Newbie
Member#: 1919
Join Date: Jul 2000
Location: Markham, Ontario, Canada
1994 Legacy Turbo


Thanks for the replies so far...

I don't think the problem is with the TPS. If I floor it, the engine switches over to open loop no problem. The issue is with part throttle acceleration.

When the engine is in closed loop mode, the air/fuel ratio bounces back and forth between lean and just shy of rich. This is due to the nature of 02 sensors and the ECU trying to hit the ideal air/fuel ratio. I've attached an animated gif of what this looks like on the meter.

The problem is that sometimes it stays in this mode even when I might be making say, 5lbs of boost. I can feel it, and I can see it on the meter. What should happen is that above atmospheric, the ECU should switch over to open loop and I should see 1 or 2 of the green LED's on the meter light up and hold there, indicating a rich mixture. If I accelerate gently in 5th gear (easiest gear to do it in), I can observe the switchover as soon as the boost gauge hits about 1lb of boost.

So, I'm thinking it's either the pressure sensor, or the exchage solenoid (provides the sensor with a manifold or atmospheric sample). But I don't waste money buying both if it's only one.

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