Technomotive Presents

The Diagnostic Port

Question for the week of September 22nd, 1997

(We're going to assume you already know the point and purpose of a wastegate in a turbocharger system, and how it works.)

What is the purpose of the boost bleeder solenoid?

The boost solenoid acts as a bleeder on the control pressure sent to the wastegate system. When this solenoid is open, the wastegate system "sees" less pressure in the system and doesn't open the wastegate as much as it would if the solenoid was closed. This causes more air to flow past the turbine wheel, which in turn compresses air at a faster rate, and thusly raises the boost pressure in the system.

Mitsubishi designed the wastegate actuator system so that on a totally stock car, boost pressure is about 7 psi. They added the bleeder to the system so that the boost pressure would be raised to 11 psi.

The normal operation of the ECU is to allow the car to run at the full 11 psi. However, when the ECU encounters severe knock for a long time (a possible indication of low-grade fuel), it will close the boost solenoid, chopping boost down to 7psi. This will give the engine a little bit of protection against severe knocking, as high boost simply aggravates that problem.

How much knock has to occur before the boost solenoid will close? Also, how come it takes so long for the boost solenoid to open again?

It is really a question of how much advance the ECU will pull out before it closes the solenoid. The ECU has two advance tables, a maximum and a minimum. These tables are indexed by RPM and air-mass/cylinder. The ECU tries to use the maximum advance possible. When it hears knock, it will start reducing a factor that it uses to figure out how much of the max or min advance value to use (we like to call it "octane"). When this "octane" factor drops below 50% (the point at which the ECU uses the average of the max and min values), the boost solenoid is closed. Now the ECU won't open the solenoid again until the octane factor rises above 75%. The use of marginal gasoline will sometimes require you to reset the ECU or wait out the rest of the tank until good gas is added before the solenoid will open again.

What about this mod I've heard people do that makes an LED light whenever the solenoid is closed?

Knowing that the solenoid closes whenever the ECU encounters a large amount of knock, some people have added an LED that indicates when the solenoid is closed. They believe that this LED will give them an indication that the ECU is detecting a lot of knock. However, some people are reporting some funny results. For example, the LED blinks during startup, and can also always blink or be on solid whenever the car reaches a certain RPM range.

Why does the LED blink? Why is it not just solid on or solid off?

The boost solenoid will try to "hide" the fact that boost is being reduced. If the ECU were to quickly chop the boost level from 11 psi to 7 psi rapidly, you would feel the car lurch a bit. So instead of shocking the engine, the ECU uses a technique called PWM (pulse width modulation) to gently transition the valve from 100% open to 90% to 80% to... to 10% to 0%. PWM relies on the "averaging" of air to make it seem like a two-position valve that is open 50% of the time is the same as an infinite position valve (very expensive) open halfway. This rapid on-off is what shows up as blinking on the LED.

Why does the LED blink at startup?

The solenoid defaults to closed when the ECU is off. When you turn the car on, the solenoid transitions softly from closed to open.

Why is the LED on (the solenoid closed), even at idle (when, supposedly, there is no knocking going on)?

There is a "long-term" knock value that tracks the long-term average of knock. When this value rises above a certain point, the ECU closes the solenoid. There is a specific set of conditions under which this term will get updated. Under high airflow (i.e., WOT) or cold engine conditions, this term is not updated. Therefore, the LED will not change state during these times. Thus, if you trigger the LED by driving around on a tank of bad gas, then shut off the engine, wait, and start the engine again after it is cold, the LED will reflect the state of knock left over by the previous bad-tank trip, and will be on.

You just said the LED will not change state under WOT. However, my LED seems to blink all the time above 5000 RPM. How so?

Turns out that the ECU is trying to do some air flow (not boost level) control. If the air flow exceeds a certain amount, the ECU will treat the situation the same as it does excessive knock - reduce the boost pressure. This reduces the air flow, so the ECU then opens the solenoid once again when the air flow falls below the target, increasing the boost pressure. It does this fast enough so that there is no oscillation, but a steady state is reached. This is the blinking you will see sometimes at higher RPM at WOT. If the LED stays steadily on above some RPM, you most likely removed the solenoid completely from pressure control system. The ECU desperately closes the solenoid permanently in vain (in vain because this action will no longer reduce boost) - a steady state is never reached.

I have a 2nd generation DSM and the LED is acting strange between 2700 RPM and 3700 RPM.

The ECU appears to be attempting to soften the impact of turbo spoolup. You know that nice sudden rush you get when the boost hits that makes owning a turbo so enjoyable? We think Mitsubishi was trying to hide that. The whole design of the 2nd generation DSM seems aimed at making the car feel torquier and smoother at lower RPMs because "Americans just don't understand turbos, just big-block large-displacement engines."

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