I have a '99 XK8 with a non S/C 4.0 AJ27 V8 It is equipped with an air assisted fuel injection system that at cold starts and up to about 158 degrees F injects additional air into the fuel injectors to get a cleaner burn. In all of the threads dealing with the P0171 & P0174 codes indicating a lean condition on one side of the other I don't recall having every seen any mention of a bad seal at either of these air injection fittings, a leak in those lines, or a faulty air control valve that would continue to bleed excess air into the system when ever the engine loads and vacuum levels are high enough to allow the system to work even though the operating temperature no longer needed that air and the software would not expect it to be there.

According to the description of that system the assisted air injection is supposed to go to from full open to just 50% when the water temp reaches 140 degrees and close completely at 158 degrees.



" The system uses manifold vacuum, under cold start/part throttle conditions, to draw air through a modified injector nozzle, producing a jet which mixes with mixes with the fuel spray to increase atomization. At higher engine loads, the vacuum is insufficient to have this effect. The air injector assistance supply is controlled by the engine management system"

If the valve malfunctions and fails to close it seems to me that it could cause a lean condition at low speeds and engine loads where even a small amount of leakage would trigger a code. The same would be true if their were a small leak at the individual connector in the lower injector rail that carries the air supply to each bank of injectors or anywhere along the individual 'corrugated' plastic line that feeds each side. Since each side is independent of the other it would, it seems, be possible to show a lean condition on just one side.


Conversely if the valve never opened it would seem that it could cause a too rich condition until engine temperature rose to the appropriate temperature. In either case a faulty air control valve should trigger a code on both banks whereas an air leak could be confined to just one side or the other.

I've seen numerous mentions of VVT seal, manifold leaks, valve covers gaskets, etc., but never any mention of the air control valve and its plumbing as being equally culpable in a lean or rich code being thrown.

Am I on the wrong wavelength with this?

 

Another cause of DTCs P0171 and P0174 is an exhaust leak on the respective bank ahead of the oxygen sensors.

I personally have never seen a fault with the air system, but that should not suggest it has never happened.

 

I've not seen it either but I cannot see it being impossible.

The exhaust leak seems fairly uncommon, compared to the many other causes of lean codes.

I really really wish they'd fit more sensors to help identify the cause!

Interestingly (maybe), my STR does not have extra air injection (AAI/SAI). USA/NA only, for some years, I think.

 

I've moved your question from General Tech Help to XK8/XKR forum.

Graham