Today was AAV day. Both the XJS and the 928. 928 is working to spec.

Not so sure on the XJS. It does close with coolant temp increase. I pulled off the B bank air cleaner and could feel a high vacuum on the AAV intake hose. Car warmed to operating temp. But some vacuum was still present. When I blocked it off, the engine seemed to run smoother.

My question. Should the AAV close completely? If it does not, would that not lean out the mixture? What compensating mechanism would the FI ECU use to compensate?

Cheers

 

Yes it should as far as the main valve in it goes, it does however control the hot idle speed (since its purpose was to add air for cold start) so it does have be the idle air bleed - so some vacuum will be present, the challenge with these is how much is too much - your idle speed may seem correct but it may not be the AAV doing it - instead of fixing things people are known to fiddle with the easy option. Classic evidence for AAV issues are hunting at idle - sometimes severe. You need to make sure you have no air leaks any place and that the throttle is correctly adjusted - there are a lot of things that must balance on the V12.

 

I have done the throttle settings. Each butterfly is at 0.002”, rods are synched. I have replaced all vacuum hoses, and have not found any leaks.

Interesting to know it also funtions as a bleed. I have not read that anywhere.

 

The bolt on the side of the AAV is your adjustment for hot idle speed. I think that's what BenK is referring to in mentioning the air bleed. I would call it an air bypass...as air is bypassing the throttle bodies....but that's just semantics, I reckon.

Cheers
DD

 

Right !

One experiment is to momentarily block off the AAV inlet entirely. Engine speed should significantly drop .....to about 400 RPM or so. If it doesn't, there's air coming in from another source.

That would be me. I removed the AAV and tossed it over the hedge a few years ago

Cheers
DD

 

No, not when air is coming in this far upstream.

The fuel injection ECU has a pressure sensor which reads engine vacuum; this is the primary basis/method of mixture control. More air comes in, the engine responds...and the sensor in the ECU responds in turn. The ECU then adjusts mixture as needed. The ECU cannot discern if the air is coming from the throttles, the AAV, or a disconnected vacuum hose.

Cheers
DD

 

I would get a mirror on a stick.

Run the engine to Temp, remove the AAV Inlet hose, look up its snout, and ensure that the piston has 100% closed that port, as per the snaps in my PDF earlier.

It that is NOT 100% closed, the ECU will see that as a Vac/Air leak, as Doug explained.

 

Yep - not fully closed is exactly how mine was - I've had it fixed but I'm not averse to removing if it causes problems in future. I think the challenge with these is that they are like any other part of the car systems - they don't like sitting in the same position for prolonged periods, got to use it or lose it basically.

 

Exactly true, that is why I converted mine to a solenoid.