Wastegate, how does it work?

[Wallace White]

Larry -

I'm not quite sure what's wrong in your case, but here's how an
MC-engine's wastegate works:

The bottom of the wastegate has a valve that sits in the exhaust
manifold (upstream of the turbo's turbine). That valve lifts up,
vertically, to open, allowing exhaust gases to bypass the turbine. 

Going up the shaft of that valve, we find the wastegate chambers. It's
like one big chamber split in half by the wastegate diaphragm. The lower
half is always connected to intake manifold pressure--the number you
read on the trip computer's bar gauge. The upper half has a spring
pressing down--the wastegate spring--and it's normally connected to
vacuum. 

The spring is sized so that when the intake manifold pressure is about
1.2 bar, the force on the lower side of the diaphragm is greater than
that on the top side. Thus the diaphragm moves up, and the valve down in
the exhaust moves up and vents exhaust pressure. I don't think the
pressure on the exhaust valve contributes much to the force balance...
(is that right?)

Now the electrical part: there is a wastegate frequency valve (WGFV), a
solenoid valve, that connects the upper chamber to either vacuum (the
normal position) or to the intake manifold (the energized position). You
can see that when you connect the top chamber to the intake manifold,
you now have equal gas pressure on top and bottom of the diaphragm but
you still have that spring on top, so the diaphragm is solidly pushed
down and the wastegate valve stays shut. 

If things were left in this state, you'd get overboost every time you
stayed on the throttle. Instead, when you are at WOT, the ECU cycles the
WGFV at about 6 Hz, varying the duty cycle towards the goal of reaching
a certain intake manifold pressure (~1.4 bar stock MC-1).

A few things go wrong:
- The diaphragm can tear. This results in overboost, every time.
(Happened to me.)
- One of the hoses can melt shut inside. If it's the top one, this will
also result in overboost, since the pressure in that top chamber is
usually very low. (Also happened to me.)
- The WGFV can stop working. (Hasn't happened to me... yet.)

I hear some people pinch the lower hose to ensure that they _can_ build
boost. With that hose pinched, the wastegate shouldn't crack open at
all. 

At overboost (1.6 bar, I think), the ECU does a last-ditch effort to
keep things under control and yanks the fuel pump relay. That's the jolt
you felt. 

- Wallace
  '87 5kcstq 170k

"Smeins, Larry" wrote:
> 
> My 87 5kcst has been suffering from low boost for a long time.  I've gone
> over the boost trouble shooting items on the web page addressing boost to no
> avail.  I even spent several hundred at the dealer and the best they could
> do was get the boost to 1.2 bar at 6000 RPM and that improvement soon went
> away.  Even with low boost, presently 1 bar at 5000 ft altitude, the car
> runs fine so I have been living with it.  I've always been suspicious of the
> wastegate or its control but haven't dug into that item, yet.  A couple
> weeks ago, on a very cold morning, I think the wastegate stuck shut.  I left
> my drive and as I pulled into traffic I gave it about 3/4 throttle and to my
> amazement I felt my self being pressed back into the seat like I'd never
> experienced before in my Audi.  I looked at the instrument panel boost
> display and saw 1.6 bar at which point the engine cut out a bit and recaught
> again at 1.5 bar.  I think it was hitting the overpressure cut out.  A few
> miles down the road I turned a corner and when I pressed the accelerator in
> my normal fashion the front tires broke loose.  Wow, I guess I'd forgotten
> what these 5k turbos could do.  After the car fully warmed up the boost
> dropped back down to 1 bar max and is staying there again.  I think its time
> to look into the wastegate, so, some questions.
> 
> I believe the wastegate is controlled by a combination of mechanical spring,
> manifold vacuum/pressure, and pulse width modulated electrical.  How do
> these inter-relate?
> 
> If I remove the vacuum line what will happen to the wastegate operation.
> Will it default open, close and become purely mechanical, or something else.
> 
> The same question for the electrical control.
> 
> Is there some way to block the wastegate closed to verify that my boost is
> being limited by wastegate opening?
> 
> Any other trouble shooting tips?
> 
> Thanks for any help you can offer.
> 
> Larry