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The next project

posted 1 Oct 2012, 07:04 by Robert Crane   [ updated 1 Oct 2012, 07:06 ]

The above shot shows the current state of the exhaust by-pass valve, which is located at the rear middle of the engine. At the bottom of the image you will notice a small metal pipe. That pipe should be connected to a rubber tube. 


The exhaust by pass valve is highlighted in the above diagram (item N).


On closer inspection you can also see the tube that is suppose to run from the valve down to solenoid valve (S in the previous diagram) at the rear right of the car.

When the car was inspected the rubber pipe was reconnected but that still failed to make the exhaust by pass valve operate. The prognosis from the mechanic was:

"Replacing the missing pipe will not make the exhaust by pass valve work alone, I tried a pipe on there and it still did not operate."

So what did that mean? Again, the response from the mechanic:

"With the exhaust by pass valve, it is currently closed at all times, whilst this will not damage the car it will limit the overall perform at high revs (albeit not to a noticeable degree) It also keeps the exhaust quiet all the time.Seeing as the valve still did not operate when I put a pipe on it, I can only assume it is the operating solenoid or wiring to the same, not a massive cost either way."

So it would seem that the issue does not lie with the actual exhaust by pass valve, although there is no guarantee of that but that would be the most logical place to start fault finding.

Ok, so what does this exhaust by pass valve do anyway? 


Here's an image found on eBay of the complete unit.


Basically what happens is that a vacuum is created through the small metal pipe to at the top of the unit (on the left in the above image). That then causes the rod to retract (move to the left in the above image). That then opens the valve in the exhaust system. Ok, now what difference does having the valve open and closed make? Well from - http://www.the355.com/mambo/content/view/18/27/

"The bypass valve opens up at higher RPM, allowing the exhaust gases to take a more direct route to the tail pipes, thus increasing power and noise.This means your 355 is quiet around town but when you open it up it screams somewhat."

Ah ha. So with it closed (as it always is if the valve fails to operate for any reason), then it doesn't sound as good and doesn't have quite as much power. That makes sense as the sound has always been 'more subdued' that expected. Now we know.

So how does the bypass valve actually operate? A vacuum is created at the inlet manifolds of the engine, this vacuum is used to feed a vacuum reservoir (item P in previous diagram) that stabilizes the pressure of the vacuum which then feeds a solenoid (item S in previous diagram) to control when a vacuum is present. The solenoid is opened and closed by the ECU and when closed there is no vacuum and when open there is vacuum to the valve. The solenoid has two pipes: one from the reservoir and the other goes to the diaphragm on the bypass valve - when the engine goes over 3500 RPM then the ECU supplies 12 volts to the solenoid which opens and that in-turn lets the vacuum get to the diaphragm moving the actuator and opening the butterfly valve in the exhaust.

Ok, so it seems there could one of three problems here:

1. The valve is physically jammed or mechanically broken and not able to open
2. The solenoid is not getting power from the ECU to open and close.
3. There is a leak in the vacuum feed.

Thus, the first most logical step to perform is to determine whether the exhaust by pass valve opens and closes. The actuator arm should therefore be able to move up and down freely. If that is good the would need to create a vacuum through the pipe to see if the valve will open. Now initially the mechanic said they connected the pipe and didn't get the valve to open but that could still seem to be a problem with the valve or the vacuum. So the only way to know is create a separate vacuum and see whether the actuator rod on the exhaust by pass valve operates. If that is all good, then the problem is further back (as mechanic initially suggested) and more investigation would be required (and another posting!).

So we have a plan.

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