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Jaguar V12 AAV Bulb Theory

Jaguar V12

Auxiliary Air Valve

Thermal Bulb Theory & Implications

If you rebuild your AAV and install a bulb from a thermostat in place of the original bulb, will it work as well as it ever did?  No; the bulbs may look similar, but they operate differently.

The way a thermostat bulb works is described in detail at CalTherm's web site.  Basically, these bulbs work on the phase change of the wax inside.  When the wax melts, it expands greatly, pushing the plunger out of the bulb.  Before the wax begins to melt, the plunger moves very little; the normal expansion of the solid wax with increasing temperature is miniscule compared to the expansion when it melts.  Similarly, once all the wax has melted, the plunger will move very little with further increases in temperature.  Wax goes from completely solid to completely liquid in a relatively narrow temperature band, so the plunger does all of its motion within a relatively short temperature range.  And the difference between a 160 degree thermostat and a 180 or a 195 is the composition of the wax inside.  It's not merely an adjustment; you can't convince a 180 degree bulb to operate at 160 unless you replace the wax inside.

The AAV bulb works differently.  Thermostats need to move from fully closed to fully open in as narrow a temperature range as possible, in order to control coolant temperature within a narrow band -- which helps fuel economy, emissions, and engine life.  To do this requires the wax with the phase change characteristics.  The AAV, on the other hand, needs to operate over a very wide temperature band -- fully open at somewhere around freezing to fully closed just a short ways before engine operating temperature.  Hence, the AAV bulb doesn't contain wax; when one was cut open, it was found to be configured exactly the same as a thermostat bulb except that it contained some sort of oil rather than wax.  It's probable that this oil has expansion characteristics that move the plunger over the wide temperature range needed.  Craig Sawyers documented the motion of a brand new AAV as having linear piston travel from below 10ºC to 80ºC:

Chart Jaguar V12 Auxiliary Air Valve Piston Motion

You won't be able to simulate that action with a bulb removed from a thermostat, but you may achieve an acceptable operation.  When the OEM AAV was working as intended, it would hold the V12's idle remarkably steady from cold start all the way to fully warmed up.  If you install a bulb from a thermostat, you can expect it to operate correctly at room temperature and when fully warmed up, but it won't work quite right halfway in between and it may not boost the idle enough in freezing weather.  The engine will idle as intended when started in temperate weather.  Due to the rebuilt AAV not closing up at all between 70 and 150 degrees or so, the idle will get progressively faster as the engine warms up.  Then as the engine approaches the 160 degree point that this bulb was designed to operate at, the idle will quickly drop as the AAV closes up rapidly.  By the time the engine is warmed up, the AAV will be fully closed and the engine will idle as intended.

Plot complication: if the idle gets too high during that warmup period, the overrun cutoff could come into play as described on the AAV description page -- except that, with some EFI systems, the overrun cutoff is inactive until the car is fully warmed up.  Overrun cutoff problems notwithstanding, an idle higher than 1500 is too high regardless; your car will be trying to pull away by itself in D.

To avoid high idle problems during warmup, the following recommendation is offered: if you choose to rebuild an AAV using a thermal bulb from a thermostat, close off part of the port in the cylinder while you're in there.  Since most of the port that's open at room temperature will still be open when the coolant is at 150ºF or so, you don't want the port to be large enough to cause an idle over 1500 or so.  When rebuilt, the AAV will work almost as a step change in idle, providing one idle when cold and another when operating temperature is reached.  While not as ideal as the original gradually-decreasing idle as the engine warmed, it should render the car fully operational.

There are two methods available to reduce the size of that port opening.  The first and most obvious would be to cover part of it up while you have the cylinder out.  It is suggested that you cover only the portions of the opening more than 26mm from the bottom of the cylinder; in other words, if you have the 73352 AAV, you can leave that entire wide slot open.  One workable plan may be to cut a piece of steel ribbon, such as the stuff used to seal shipping boxes or even the unslotted portion of a large hose clamp, and curl it to wrap around the center section of the cylinder and cover part of the port with each end.  Clip it on, and perhaps apply some JB Weld to make sure it doesn't go anywhere.  Another plan might be to wrap wire around the cylinder, providing enough coils that most of the port is covered and perhaps relying upon air to flow between the coils.  Or, perhaps you can simply fill up part of the port with JB Weld.  Whatever you choose to do, make sure you don't interfere with smooth piston motion on the inside of the cylinder.

The other method of reducing the size of the port opening is merely to change the stack height when assembling so that the piston itself closes much of it at room temperature.  Assembled normally, the piston already closes the wide slot on 73352, which is why there's not much point to covering it.  But as long as you're installing a new thermal bulb and finagling a fitting on top of the plunger to push the piston, you might as well finagle that fitting a little taller so the piston is already partly closed to begin with.  Or, even simpler, you can just press the cylinder into the housing a little farther than originally intended, until the measurement from the top of the outlet fitting to the inside bottom of the piston is a little less than 70mm.  Note that you can opt to do this after the AAV is installed; just take the top hose off and whack it with a hammer!

There is one distinct advantage of using the stack height change method of controlling the port size: when really cold, you may actually get some more idle boost.  Although thermostat bulbs don't move much below their set point temps, they do move a little -- and that bit of motion will pull the piston down a smidge and give you a little more idle boost, since it will be uncovering part of the port that you had covered.

If the modified AAV works OK from room temperature up to operating temperature but doesn't provide enough air to maintain idle when really cold, there's little or nothing you could have done about it; if you had left the port larger, it might work better at freezing temperatures but it might idle too fast when partially warmed up.  The best solution at this point is to add an additional idle boost scheme by installing a thermal switch somewhere in the cooling system (or even just within the engine compartment -- an adjustable electric fan control would work) and using it to control a vacuum solenoid to allow a little more air into the intake manifold below the setpoint temp.  There is such an idle boost system already on many XJ-S's at the forward end of the right side air filter housing called the Supplemental Air Valve, but on some cars it works when cold and on other cars it doesn't -- perhaps explaining some of the differences in AAV port shapes!  If it's there, it should be easy enough to wire it up to work when cold; if it's not there, it should be easy enough to add.  It might also be a good idea for it to work when the engine is at operating temp and the A/C compressor is running; some cars have two relays on the radiator upper support rail that provide this operation when the car is in gear but not in N or P.

While all this may sound like work, it must be pointed out that much of it may not prove to be necessary.  Several V12 owners have simply totally plugged up the port in their AAV's when the piston operation failed, giving them warm idle only, and the car seems to operate just fine!  The Jaguar V12 has been known to idle at 300 rpm without stalling.  If you rebuild your AAV to give you any sort of small port opening at room temperature and a closed port at engine operating temperature, chances are excellent you will be satisfied with its operation when reinstalled.

Once you've read and understood all this, if you decide to proceed with installing a thermal bulb from a thermostat in your AAV, the next thing you'll need to know is how to get your AAV apart.


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