legacycentral bbs

I have spent many hours researching and troubleshooting this problem and was hoping for some insight. I have a usdm legacy ss with a td04 and forester intercooler. i am running 11 psi on a proven accurate boost gauge. so for the issue; every 10 or so times i floor it, just as boost starts to build i will get that violent fuel cut like i am boosting above 13psi. this is on the onset of boost and absolutely not having to do with boosting over 13psi. I have a grimspeed boost controller and have tried switching back to the stock boost solenoid and the problem still persisted. even running waste gate pressure it still happens. i have no CEL and a brand new knock sensor/02. Whenever it happens I get ECU code 45 (MAP/pressure exchange solenoid) but only for about a minute and then it goes away. I guess what confuses me about the issue is the inconsistency... sometimes it pulls strong and hard and then other times just as boost begins to build it falls flat on its face. is this how a map sensor acts as it begins to go bad? I just wanted to get some input before i dropped the time and money trying to track down a MAP sensor or pressure exchange solenoid. Also please note that i am positive all vacuum lines are connected correctly and actually redid the entire system multiple times using a vacuum diagram on this site with all new vacuum lines.
Thanks

Make sure that you inspect the lines for small cracks or splits.

I have lately been having an intermittent issue with that as well. 45 pertains to MAP/PES, make sure you look at those and check the connectors as well.


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Solved the problem. Bad pressure exchange solenoid. I found one out of a forester xt at an auto wrecker and it was 100% compadible. Problem is completely gone and it builds boost smoother now.

Good to know

Welcome to the BBS, and thanks for the follow up and letting us know what the problem was.

what is the pressure exchange solenoid?

On the passenger side strut tower, there is a brown sensor with some vac lines coming off it. That's the PES. It has a line going in from the manifold, and one going out into the MAP sensor below.

hmmm. I have replaced mine once with an equally older and probably as crappy unit. Are there any newer subarus with a similar unit that will work?

Sure, subaru used them up to 1996 in the world ej20g imprezas

What the PES does is switch between atmosphere and the intake manifold. This provides the ECU a way to read both intake manifold pressure and atmospheric pressure.

I have found that the PES appears to be the same in 95-98 EJ22 and EJ25 cars as well. I swapped out one from a 95 EJ22 and one from a 98 EJ25D, and both worked the same. Some have a vac line going to the intake instead of the little black filter, and some have the end nipple pointing up instead of out, but they appear to all be the same solenoid otherwise.

Yup, they're pretty much all the same.

would this sensor cause a clicking sound on the BCS when the key is in the on position without engine running?

The BCS is what's clicking.