I'm chasing rich codes again. Sigh.

I start the car when cold, and often it shows a very rich mixture, and the STFT is like -25% on both banks. (what really chaps me is that it doesn't happen all the time. This thing is possessed.) Anyway, for research I pulled the hose from the connection at the rear of the TB. I got plenty of manifold vacuum at the hose, and the mixtures immediately went to a positive 25%, indicating a vacuum leak. So far, so good. I plugged the hose with my finger, and the mixture immediately went to -25% again. I repeated this a couple of times. Then I reconnected the hose and it dropped back to around -25%. This does not seem right.

How does that valve work? I don't get a code for it, but I think I should test the operation if possible. Any hints? Also, I searched a lot online and can't find any pix of the valve itself. Denso makes a pile of similar ones for Hondas, but it appears that the Jag part isn't available. Is this a serviceable part, i.e. something that can be removed and cleaned? Or, is it time to bite the bullet and send the TB in for a rebuild?

TIA, and a Merry Christmas to everyone!

 

I've attached a snip from the training guide 881

Merry Christmas to all.

 

Have you ruled the MAF out? (And how?)

Also, does this car have a fuel rail pressure sensor & if so is it OK?

 

Thanks! That's very helpful. So, if I understand, there is always manifold vacuum at that hose. The valve opens when cold, allowing more metered air from the TB into the system, where it's introduced into the fuel stream by way of the little holes in the side of the injector end shroud.

Two questions. If the valve was closed, for whatever reason, during cold warmup, would the system read rich?

And, I noticed, by looking at another TB I have, that the electrical part of the valve can be adjusted a bit, and that turning it a few degrees one way or another causes the internal valve to open a bit more or close a bit more. Would adjusting this make any difference in the rich/lean condition?

 

I've tried three MAFs, the original Jag that came with the car, a new Genuine Denso, and a Brand X off of Amazon. All three with work, to one degree or another, and then go into "rich mode" at some point. The symptoms are similar. Running somewhat normally for a few hours to a couple of days. Then, especially at slow speeds with low throttle opening, it starts to run rich, and it gets worse until the Restricted Performance light comes on. Then, often, it will be just fine for the remainder of the day.

I have recently checked the fuel pressure and it was normal, IIRC something like 40 or 45 psi, fluctuating very little when revving the engine.

 

For what it's worth, the snippet posted by Michaelh above, doesn't say anything about increased fuel supply during cold warmup, just extra air to increase atomisation. The air is metered, so if for any reason that extra air was cut off, the car would know the air flow was lower than expected and I would expect open the throttle further to compensate. The trims are then an adjustment based on the oxygen sensors (which do nothing when cold of course).

All this assumes the car is in closed loop, which it won't be immediately after a cold start, so make sure any readings you are getting over the live data is in closed loop mode.

I would also expect any obvious fault on the AACV to generate a code.

I think all you can do at this point is experiment - for example make a note of trims and MAF reading at start up, then disconnect the AACV and see what changes at another cold start up.

I'm not sure at this point I would try adjusting the valve as you describe unless you have a guaranteed way to get it back to the original setting - it's never a good idea to introduce a new unknown variable when trying to fault find.