What solenoid is that?

[Austin]

On the passenger side front strut tower is a sound mounted (rubber insulated) panel. On this panel is mounted the oem boost control solenoid, the MAP sensor, and another solenoid with a vent/diffuser looking end on it.

Connector has a yellow/red stripe wire and a green/yellow stripe wire.

Vacuum line runs from the manifold to this solenoid, then from this solenoid to the MAP sensor. What is this solenoid?

Thanks!

[vrg3]

This is the pressure exchange solenoid. The diffuser-looking thing is just a filter.

The pressure sensor is used to read both barometric pressure and manifold pressure. The pressure exchange solenoid switches the sensor's input between the vacuum line from the manifold and the atmosphere (through the filter).

[Austin]

The pressure exchange solenoid switches the sensor's input between the vacuum line from the manifold and the atmosphere (through the filter).

Thanks very much!

When/at what times does the pressure exchange solenoid switch MAP sensor inputs?

I would guess on initial startup, the ECU gets a reading for current atmospheric pressure, and then continuously monitors MAP... is that right?

[vrg3]

That would make sense, but I do not have any concrete knowledge. I do seem to remember having read at least one source that said the ECU "periodically" checks barometric pressure.

Most cars equipped with manifold pressure sensors simply read the pressure when first turned on but before the engine cranks and starts, and use that as the barometric pressure reading. The limitation is that if you drive, for example, from near sea level to the top of a mountain, it will have the wrong information.

Since Subaru could have easily done this as well, but instead chose to have the pressure exchange solenoid, I have to say it only makes sense that the ECU would switch the solenoid every once in a while when the engine is running.

Incidentally, I do believe this solenoid is identical to that used on later model naturally aspirated cars that have pressure sensors for the automatic transmission control unit.

What makes you curious about this part of our cars?

[Austin]

What makes you curious about this part of our cars?

I'm still having det/pinging issues.

I have pretty extensive knowlege of the way the 02 WRX ecu functions, but not the 91 ej22t ecu.

My det is difficult for me to solve. It doesn't happen all the time. Happens much more when hot outside, but still sometimes happens when cold. Car has to be hot. Seems to happen much more if I go WOT at 3K or 4K rpm, won't happen if I go WOT at 2K rpm and build boost slowly.

The car also goes lean and bucks sometimes.

That makes me think it's TPS/tip in fuel enrichment related. I've cleaned the injectors, replaced the fuel filter. The fuel pump gets replaced tomorrow. I've replaced the knock sensor, coolant temp sensor (silly idea, but that sensor is cheap...), TPS sensor.

I'm starting to reach for ideas on why the car is still pinging at times... Hopefully it's insufficient fuel available, and swapping in a new fuel pump will solve it.

[Brat4by4]

Don't forget your fuel pressure regulator, if that goes bad, that could lead to a lack of sufficient fuel pressure. Do you have an air-fuel ratio gauge hooked up to the car? Or a boost gauge? And I do believe there is more than one fuel filter (but I could be wrong) make sure they were all replaced.

Hope you get that fixed... pinging bad .

[Austin]

Oh, yeah, forgot to mention that. I've also swapped out the fuel pressure regulator with one from a MY02 WRX, about 10K miles old.

I've also replumbed the entire intake, replaced every rubber hose on the car, put on a new timing belt (the old belt had jumped a tooth at one cam...), Tstat, etc. etc.

I've cleaned the MAF sensor - that didn't seem to have any effect... wasn't dirty in the first place. If the fuel pump doesn't work I'll try cleaning the o2 sensor. If that doesn't work I'll replace the o2 sensor, then replace the MAF.

[evolutionmovement]

Brat 4x4, my N/A Legacy only has one fuel filter. My old GLs had two... You're not thinking of your Brat, are you?

[vrg3]

Isn't there one filter in the fuel tank and one filter attached to the driver side strut tower?

Austin - It sounds a lot like it could be due to the ECU learning badly due to a bad oxygen sensor... One way you could check that is to unplug the oxygen sensor and then reset the ECU by disconnecting the battery for a long time. When you start the engine up the ECU will use its built-in maps but retard timing a little bit to be safe.

[ciper]

Just to note, NA legacy have this sensor INSIDE the ECU.

[Austin]

Isn't there one filter in the fuel tank and one filter attached to the driver side strut tower?

Austin - It sounds a lot like it could be due to the ECU learning badly due to a bad oxygen sensor... One way you could check that is to unplug the oxygen sensor and then reset the ECU by disconnecting the battery for a long time. When you start the engine up the ECU will use its built-in maps but retard timing a little bit to be safe.

There is a suction strainer (which definitely qualifies as a filter) attached to the fuel pump in the fuel tank, and the regular one on the driver's front strut tower.

I have reset the ecu a few times during all this, but I've never left the o2 sensor disconnected. I'll try that.

[evolutionmovement]

OOps. Never thought of the strainer. I was thinking external filters. Is the fuel tank one a regular maintenance item? I've never changed mine (221K).

[vrg3]

To follow up on the pressure exchange solenoid... It looks like the ECU checks barometric pressure when you first turn it on (right before cranking), and also whenever manifold pressure starts to rise above atmospheric (i.e. every time you start to boost).

Some old Chrysler turbos have a similar setup with the pressure exchange solenoid, except that the ECU fuels based on manifold pressure (there's no MAF sensor or anything). It just periodically checks barometric pressure every so often, and if you pay attention you can tell because the ECU continues to fuel based on the pressure sensor, even when the sensor is measuring atmospheric pressure, so the engine hiccups a little. :)