Engine

Idle Stabilizer, Tutorial

Can anyone elaborate on the theory of operation of the following 2 items?
Or, point me toward some reference material?

Idle stabilizer (32V engines)
Flappy valve (32 V engines)

Tom Csibor.



The "idle stabilizer" is a variable valve that controls the air flow into the engine at idle. Most (maybe all, I don't know) are made by Bosch and are "torque motors" - what looks like an electric motor, but with the armature constrained by a clock spring so it can only rotate about 1/4 rev. It is attached to a rotary valve that controls the air flow. More current means more rotation, hence more air flow and the idle speeds up. Problem is the clearance in the valve is very small and small dirt particles can cause friction. The computer says "speed up" and applies more current, but the motor sticks. Engine slows down more, more current is applied and suddenly it breaks loose, opens too far, computer reduces current, valve sticks open, well you get the picture. Result: idle surge, sometimes enough to stall, always annoying. Others like Hitachi have tried to make this type of thing work, never with any success. Almost all new cars have stepper motors working a valve on a lead screw. Reliable, but more complicated.

The "flappy" (where did it get that name, anyway?) is a butterfly valve that changes the geometry of the intake manifold. At low rpm - below 4,000 - the valve is closed, separating the two halves of the manifold into essentially two 4 cylinder engines. The tuning is then optimized for low rpm torque. Above the set speed the valve opens, connecting the two plenums together pretty much eliminating the Helmholz effect and tuning the engine for high rpm torque. The valve is operated by a vacuum diaphragm hidden (of course) below the central manifold which, in turn, is controlled by a solenoid valve pulling vacuum from the resevoir and energized from the engine controller. A lot of new production engines have this feature. Note that each plenum is not connected to the cylinders of that side of the engine, but is connected to the end two cylinders of one bank and the middle two of the other bank, which is what it takes to get a 4 cylinder even-firing engine.

Hope that helps,
Gary Casey



Hi Gary,

You must either be a technical writer or a teacher/professor. That was the best and most understandable description I have seen. I now understand what is happening and why the surging happens. Others on this list also gave answers for which I am thankful, but assumed or took for granted that I knew more about this valve than I do. The explanation of the armature constraint and the increased current to overcome the friction/blockage now explains perfectly the surge phenomenon. It always seemed to me that 'something' was constantly overcorrecting, causing the idle fluctuation, but couldn't understand what.

As a side note, I took the suggestion from all the great listers that replied yesterday. I removed the vacuum line running to the stabilizer/actuator and doused it with WD40 and let it sit for a few minutes. I then disconnected the ground strap. After connecting the hose and the strap back, my problem went away within a couple of minutes of running and has not returned. Thank you all.

The only remaining engine problem I have now is a total lack of low end power. It is almost as though I have an extremely lean or rich situation and I have a terrible lag until I reach about 2000 - 2300 rpm. I checked the 'Flappy' and it is working fine. It Blips on startup, closes and remains so until high RPM (about 4000). This was a mystery when I first bought the car and even after replacing rotors, caps, wires, injector seals etc.. it still continues.

Again, many thanks to all the great knowledgeable folks who replied.

Tom Csibor



Not sure if different types of idle speed regulators were used in different years but the one on my 85 doesn't work against a spring. It basically has a centre tapped coil. Power on one side will open the valve and power on the other will close it. The system works by feeding a pulses to each side. When the pulses are equal (a square wave I guess? I have to get a scope on the thing) the valve will sit in the mid position. To move the valve in one direction or the other the width of the pulse will get longer on one side while getting shorter on the other. Gary's comments about dirt and sticking are spot on and equally applicable of course.

Colin