THE BOOST SOLINOID, what does it do? do we really know?

lscislowski
Learner Driver
Posts: 6
Joined: Thu Dec 28, 2017 8:16 am

THE BOOST SOLINOID, what does it do? do we really know?

Postby lscislowski » Sun Dec 31, 2017 10:34 am

Hi guys
I want to ask a question that I seem to never be able to get a answer for, and please if you do not know, or had a friend of a friend that once said this, please keep quiet, let the people that know contribute and you might actually learn something.
We have all seen it, "help help, my WRX is only boosting to 7psi" posts, and the answers, "clean out the boost solenoid, check the vacuum lines, reset the ECU and so on". But what is the actual function of the boost solenoid? and better yet, is there a reason your car has be limited to 7psi, or 10psi in some models?
We own 2 WRX's, I've owned a MY98 GC8 for around 8 years, and my wife has a MY17 STI. When I bought my GC8 (stock standard), it was from a dealer, had low K'm and very good condition. I did notice when I first got the car it seemed to lack power. The history was, this car was involved in a small accident which damaged the radiator, and overheated the engine. When I bought the car, it was fitted with a re-con engine and a new radiator, and my99 front bumper, when the dealer said re-con they actually meant to say 2nd hand. I put a boost gauge in to and it was only boosting to 7psi, should be 13.5psi on a MY98. New to Subaru's it took me a bit to work out the system, Boost solenoid worked ok when bench tested, ECU would power up the boost solenoid when the car was started. turned out there were some nice new vacuum lines fitted around the turbo, and no restrictor pill. Got a pill, put it in and it worked fine.

Couple of years later, one day driving along and under acceleration the car suddenly lost boost, down to 7psi. the only problem I could find is the ECU would not earth out the boost solenoid. Please note that yes, I did have a +ve 12v and -ve 12v at the solenoid plug when using a multimeter and the plug was disconnected and the car was idling. Once you plug in the solenoid you sill get 12v +ve when you probe the wires but no earth. (The -ve I was getting when the plug is disconnected is just a very low current signal the ECU uses to check the solenoid has a circuit). This is a big trap for anyone using a multimeter and doesn't know that things need to be tested components electrical under load.
Now what is happening is the ECU has detected a condition in which it thinks it would be safer to reduce the boost on the car than running at full boost.
This is why there is a boost solenoid on your wrx, it is to protect the engine if the ECU thinks the conditions are not right for full boost.

My question is what are these conditions, what causes the ECU to limit boost. In my experience it doesn't always throw up a fault code to tell you why its limiting boost.

Please note, WRX's also have another protection in them, if you overboost by approximately 3psi higher than the standard boost, it will go into fuel cut when the engine reaches high RPM. this feels like you are accelerating and just turned the engine off when you hit 5000RPM. Found it out once when a vacuum lime broke off my wastegate.

As it turns out the reason that my car was limiting boost was bad fuel. What I didn't realise at the time is I only just filled the car up, when the problem started, and couldn't fix it until the fuel was all used up, filled it up again and on the first drive from the servo to home the car came good. Yes, I always put 98 in this car, which is stock. I had filled up from a servo I don't usually fill up from. It did say 98 on the nozzle, but I believe it was only 91. I did find out I was not the only one who had problems, several other people had problems who had filled up from that servo around the same time. Please note for you who are saying "how can a servo mix fuels up" . I am a mechanic and have seen diesel cars filled with unleaded before, and the owner can show me a recept showing me it was defiantly diesel they put in it.

Now since I have owned this car I have had 3 instances where I have driven out of a servo, with in several km of driving the car has limited boost. Problem is always fixed when the car is filled up again. Which must suggest bad fuel. So ok, what is happening is the car is detecting more pinging than normal, to protect the motor it detunes itself. On these forums we are always trying to blame faulty components of the boost system where in actual fact the boost solenoid is doing what it was designed for, to protect your engine from a different fault.

Had another problem, car boosting at 7psi again, coincidently I did just fill it up, and was blaming bad fuel. This time the car was also idling very rough as well, a week later thought I better take a look at it, turns out had a broken vacuum line which had nothing to do the wastegate side of things. Repair the leak and the car came good straight away. Now, there was no fault codes or engine light, but from what I can see the ECU must have picked up something wrong, unmetered air causing a mixture problems, which could suggest maybe a accuracy problem with the AFM or bad reading from the o2 sensor or just a leak, it did not throw up a fault code but was enough of a problem to limit boost just to be on the safe side.

So, does anyone have any information on the actual functionality of the boost solenoid, maybe some Subaru technical information, and under what conditions will it restrict boost? What should people be looking for when they have a problem? And yes I understand you may of had a problem, and it turned out to be a broken vacuum line of the boost solenoid or something like that, but most of the time the solenoid is doing what it was designed for, protecting your engine from a different problem.

mv75
WRX STi Driver
Posts: 839
Joined: Sat Dec 27, 2014 9:26 pm
Location: Brisbane Northside.

Re: THE BOOST SOLINOID, what does it do? do we really know?

Postby mv75 » Sun Dec 31, 2017 9:35 pm

This makes no sense. The boost control solenoid has a very specific and documented function. To bleed pressure away from the waste gate control arm and to regulate it at a specific psi as read from the map sensor based upon ecu programmed duty cycles for that solenoid. The boost control solenoid is just that, a solenoid. It doesn't know your boost, it's just a valve.

BTW, gc8's are notorious for bad earths. You need to clean them up and possibly run new ground cabling due to age and wear on the wires. The main grounds are super easy to find. You'll have two coming from the battery, one goes to a bracket just above the starter motor, just take it off, clean the area of grease and bolt it back on. The 2nd is to the fender wall right next to the battery, undo it, sand the paint a bit around the hole, clean all the grease and crud and bolt it back on. That should improve the system noticeably just doing that.

Also if you're in low performance mode, the ecu has a setting called IAM. Ignition Advance Multiplier will lower itself based upon knock sensor feedback on a learning table. At about 7 IAM you'll loose boost. The scale goes from 1-16, higher the better. This is why an ecu reset will enable boost again as you then reset it to its default of approximately 8 iam and clear the learnt values.

lscislowski
Learner Driver
Posts: 6
Joined: Thu Dec 28, 2017 8:16 am

Re: THE BOOST SOLINOID, what does it do? do we really know?

Postby lscislowski » Mon Jan 01, 2018 3:55 pm

Thanks mv75 for your input, it is very interesting and it is good to hear from people who know their stuff. Sorry maybe I've mad it a little bit to hard to follow. And please correct me if any of this is incorrect. The boost solenoid is just a solenoid, and its function is to bleed pressure away from the waste gate control arm as you said. Now unless I am missing something this solenoid can either be on or off. Now it gets its supply power 12v+ from the Main Relay (according to GC8 wiring diagrams), it completes the circuit through the ECU, (plug B137 wire number 3). So the ECU supplies the earth to this solenoid, thus can turn it off or on.
Now, as we know, mechanical wastegate actuation pressure on a WRX is 7-10 psi depending on model, if we were to bleed pressure way from the wastegate we can achieve a higher pressure 13.5-17psi, again depending on model. This is what the boost solenoid does when it is in the on position. The way we regulate the amount of bleed off is by the restrictor pill, different size for different models gives us different max boost pressure, all the solenoid does is it turns the bleed on or off. At least on a GC8 to my knowledge.
The ECM does have the ability to turn the solenoid off, in which case the car would not bleed pressure and only run on only waste gate pressure, 7-10psi. I want to know under what conditions would this happen?

Like I said, Ive had 3 separate instances in the last 8 years where I have driven out of a service station and with in the first few k'm, my boost has been restricted to 7psi, and all 3 times the boost returned to normal 13.5 psi once the fuel was used up and fresh fuel was put it. I cant see this being a coincidence.

bigBADbenny
Impreza RS Driver
Posts: 207
Joined: Tue Mar 20, 2012 11:43 am

Re: THE BOOST SOLINOID, what does it do? do we really know?

Postby bigBADbenny » Wed Jan 03, 2018 8:35 pm

Have a read here:
subaru's knock control strategy explained
http://www.romraider.com/forum/topic1840.html
And here:
How Subaru's Factory Boost Control System Works
https://cobbtuning.zendesk.com/hc/en-us ... stem-Works

bigBADbenny
Impreza RS Driver
Posts: 207
Joined: Tue Mar 20, 2012 11:43 am

Re: THE BOOST SOLINOID, what does it do? do we really know?

Postby bigBADbenny » Wed Jan 03, 2018 8:36 pm

Have a read here:
subaru's knock control strategy explained
http://www.romraider.com/forum/topic1840.html
And here:
How Subaru's Factory Boost Control System Works
https://cobbtuning.zendesk.com/hc/en-us ... stem-Works

Return to “General Technical Talk”

Who is online

Users browsing this forum: No registered users and 22 guests