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my 92 idles a little high when warm and a little low when cold. Cold starts will idle around 7000 rpm and is a little sensitive to throttle, as it stumbles somewhat. When warm it idles at 10,000 so the engine is pushing too hard against the brakes. It’s a minor concern but wondering what might be the cause.
Check all the Vacuum Pipes for leakage, especially the Vacuum Pipe to the FPR (Fuel Pressure Regulator on 'B' Bank) that's the one that's on the Oil Filler Cap side of the Car
But if the problem gets any worse then get a Shop to check the Vacuum on the FPR just in case the diaphragm may be leaking, though I'm sure there could be many other things that it could be, like a sticky piston in the AAV
my 92 idles a little high when warm and a little low when cold. Cold starts will idle around 7000 rpm and is a little sensitive to throttle, as it stumbles somewhat. When warm it idles at 10,000 so the engine is pushing too hard against the brakes. It’s a minor concern but wondering what might be the cause.
700 rpm is a symptom of the AAV not fully opening, but in principle, I think that is an OK cold figure, if it gradually climbs as the engine warms over the first minute or so.
1,000 at warm is too high, and you reduce it by screwing in the large nut on the AAV. 750 rpm warm, engine in neutral, is about right. If this idle adjustment produces a cold start rpm which is too low, then that confirms the AAV is not fully opening at cold. This fault is quite common. All this assumes you have the throttle butterflies set correctly at the closed position.
700 rpm is a symptom of the AAV not fully opening, but in principle, I think that is an OK cold figure, if it gradually climbs as the engine warms over the first minute or so.
1,000 at warm is too high, and you reduce it by screwing in the large nut on the AAV. 750 rpm warm, engine in neutral, is about right. If this idle adjustment produces a cold start rpm which is too low, then that confirms the AAV is not fully opening at cold. This fault is quite common. All this assumes you have the throttle butterflies set correctly at the closed position.
would you spell out a AAV for me please? I don’t think that term is in my manual. What is the proper name of that device?
The AAV is aka 'The Auxiliary Air Valve' and if the Car is left to stand for sometime, then the Piston inside can get 'Sticky' and cause the Engine to 'Hunt' until She's warmed up
With the Revs of the Engine going up and down for no apparent reason, believe it or not it's quite a common problem, assuming you want to think of it as such as it very often sorts itself out by the time that you've got on the road
On one of my Cars this used to happen on quite a regular basis, until one day it suddenly cured itself but if it bothers you that much then you could take the rubber pipe off and give it a squirt of WD or Penetrating Oil
Be careful with the rubber pipe as they are known to go 'Crispy' and can unexpectedly break, which maybe could be the cause of your problem, if air is getting in from somewhere else
Which could then very likely cause the revs to rise
Replacing the AAV could mean a very expensive day out as they are IRO of about £200 (uk) although 'Grant' has a Fix that he may have told you about
The AAV (Auxiliary Air Valve) A 'Sticky Piston' inside this can cause the Engine to 'Hunt' until She warms up
I turned the idle screw I. Two full turns until it seems to bottom out and the idle barely came down to 850.
When the idle speed adjustment has little or no effect it's because something else is having a greater effect that is beyond the capability of the adjustment to overcome. On the Jag V12 this will most commonly be air coming in from 'somewhere' other than the AAV or throttles or a mechanical problem with the throttles: incorrect adjustment, binding, etc.
Less common, but possible, are problems with the ignition timing/advance system....as ignition timing can change idle speed and there are devices to alter ignition timing at idle. On your Marelli car the micro-switch on the LH throttle linkage comes to mind. The switch adjustment is critical as closed throttle (signaled by the switch) brings xxx-timing map into play. The instant the throttle is cracked open, yyy-timing map comes into play.
92 XJS V12 idle concern
