My '87 V12 coupe has quite a difference in idle speed between hot and cold. When first started it idles really low, 450/500 rpm. When up to temperature, it idles at 1100/1150 rpm. Now I know that the idle adjustment screw is on the side of the Auxiliary Air Valve and that screwing it in should bring the idle down. Its already in most of the way, there are two or three threads left, and someone put some teflon tape on the threads, which doesn't fill me with confidence. I haven't tried adjusting it yet, and that's the next thing to do, but I don't have a lot of confidence that that will fix the issue. I'm suspicious of the function of the valve, given the low cold idle, but I wanted to see if there were other things I should be looking for as well? A vacuum leak would let in air and raise the rpm at all temperatures right?

 

Yes, a vacuum leak would raise the idle. I suspect that the AAV has become sluggish and isn't moving like it should. It's aluminium piston in an aluminium housing and they can corrode together and stop moving, or the wax capsule that drives it looses effectiveness with age. I suspect a new AAV will fix things for you.

 

As somebody that just pulled the top of a V12 this for sure is something that needs to be pulled off and tested, I've had mine rebuilt by a chap called Jonas out in Lithuania, he also provides the metal half moons to seal the back of the heads, teflon throttle spindle bushings, improved banjo fittings. You can buy AAV's new, depending what you think is expensive you may however get a shock. I've PM'd you Jonas email addr.

You need also to be meticulous about tracking down air leaks, mine must have had several.

The inlet manifolds were loose, the throttle body spindle seals weren't so hot, there were several crusty vacuum lines, some of which should have had clips on them and didn't. The vacuum take off points at the back of the manifolds weren't particluarly tight either, the throttle butterflies on either side were improperly adjusted to boot.

If you have the engine running get it warm, let it tickover then be very systematic and spray carb cleaner at various joints etc, if you get a sudden increase in rpm you know there is a leak there somewhere - pick your targets though this stuff is flammable. Don't go messing with the AAV until you have done this because adjusting the AAV with an air leak masks other problems. I would say though that if you have significant air leaks that your cold start RPM should also be affected by them somewhat - check for air leaks anyhow you can nuke a V12 with one enthusiastic press of the loud pedal if it is running too lean.

 

Another way to check for a leak, and may be a little more precise is to use propane. Take a propane torch and remove the bits at the end. Attach a hose to the spigot and on the other end,of the hose insert a piece of brake line with the end peened down to a very small opening. Turn on the gas and move the brake line to areas where there may be a leak. If there is a leak, the rpm will increase.

 

My thoughts.

Not knowing the set up for your market, I will go with the gut.

AAV is more than llikely causing issues.

A vac leak big enough to give those revs when hot, would be audible.

Plugged into the rear of the A Bank aiir cleaner assembly, is a Vac Solenoid. Used as part of the 15 minute timer, A/C kick up, etc, depending on market specs from 1987.

IF, and this is the gut, IF, that solenoid is active, then it is open to air in (vac leak). With a knackered AAV, this solenoid WILL give the cold idle revs you state, AND, the hot "idle" revs you also state.

Its got 2 wires, so simply unplug it, as once the volts is removed, that solenoid defaults to closed, see what happens, you have nothing to lose.

I use those solenoids for my "cold idle" air, as I do not run an AAV, and if I forget to switch them off, I see that rev range, as well as other weird stuff.

I still reckon there is a myriad of things that are not set correctly, as already mentioned, so back to the beginning, ONE step at a time, and finish when you finish.

 

OP says he is in the USA, no 15 min timer for USA market cars.

 

OK.

It would have other stuff though, and that solenoid would be there, and that is my point. If its NOT there, OR connected, GOODO.

Costs nothing to look and unplug it.

 

But it does have a 45 second timer, is this activated by the same system?

 

What I call 'stack up'.

A slew of minor faults, perhaps not noticed or perhaps intentionally masked...but eventually causing a problem too big to ignore or mask.

I'd bet the vast majority of old XJS V12s fall into that category..... until someone sorts 'em out.

Exactamundo !

Cheers
DD

 

The '87 in question will have the 45 second timer, yes, activated by voltage from the starter relay. It opens the valve in the RH air cleaner to let more air in to increase the idle. It also operates a valve which dumps vacuum to the distributor advance capsule.

It might also have the revised idle control system which A) adds a relay to operate the valve in the air cleaner when the compressor is engaged and B) adds a relay to defeat this function when the gearshift is in "P"or "N".

Cheers
DD

 

Thank you all for your insights, lots of things to investigate. The car definitely hadn't been cared for before I acquired it, last on the road in 2015. I've spent the summer redoing the fuel, suspension, and braking systems and was finally able to drive it for the first time two weeks ago. Unfortunately its supposed to snow this week, so its time to put it away for the winter, which means that I won't be able to work on it until spring. The cooling system will need to be gone through, so I'll add the AAV to that list. I think that while I'm replacing the coolant hoses would be a good time to replace all the vacuum lines. Yes its a lot of work, but I'd rather know they're all good.

I'll have to go look for the 45 second timer and its solenoid. To further muddy the water, the are is a 1987 model year, but it has an August 1986 build date. The air injection system has been removed at some point.

 

You have done well so far.

The "Build Date" is ALWAYS the reference date for any car. "MY" etc is more a marketing thing.

Parts and Service manuals are ALWAYS related to that "Build Date".

Air pump gone, may mean other things are also AWOL.

When you do the vac lines, ONE AT A TIME, then when they are all new, any iremaining ssues will NOT be from crossed circuits. The Jaguar Vac Schematics are vague at best.

 

after months of fiddling with my idle, 2000 rpm cold and 1200 rpm warm, i blocked off the air intake for the AAV inside my air cleaner. works like a charm, 1200 and 600 warm . my question will this damage anything else not having air to the AAV ??

 

I did that . Once. Ended up with a distributor cap full of engine oil, which was sucked upwards past the distributor seal.....because the vac source for distributor vent system is on the AAV hose.

So, find another source for your distributor vent. Is really doesn't (and shouldn't) be a strong vacuum signal. It just needs sightly lower air pressure to pull the air thru. I simply added a nipple to the backside of the air filter housing.

Cheers
DD

 

Thank You !

 

In this case, where the AAV is getting NO airflow, what is making your engine idle cold at 1200 and warm at 600? What is changing the airflow?

 

I agree, if the AAV Inlet is plugged, therefore zero air in, how is the engine doing those revs, and dropping when it warms up??

Am I missing something?

 

my guess is the air flow is through the manifold, big inlet in air breather, not the auxillary air vent. Just a quess. why is it called the auxillary air valve?

 

A little obvious, what you are missing is how the air gets into the manifold - via the throttle butterfly or some 'other' means - the AAV is one such mechanism.

Because that's exactly what it is, it allows more air into the intakes when the car is cold - bypasses the throttles hence the increased RPM when cold.

When the engine passes 80°C or so it should be pretty much closed, the idle RPM is then determined by the hex adjuster located in the AAV.

I'd suspect the RPM is getting managed somehow by the other myriad of solenoids and timers - that the RPM is behaving as stated with the AAV blocked though doesn't seem right.

The vacuum system on the V12 is real complex, and was changed depending where the car was sold so you kind of need to read this page to even begine to understand what everything is 'supposed' to do >> V12 Vacuum

Just blocking things up or disconnecting without understanding what or how things work will bite you eventually, probably in ways you won't anticipate.

 

I found out my AAV is not working at all. Do you recommend I spend the money and replace the AAV?