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My idle is steady at 750 when warm and in P. It sits at 500 in D. Except for the occasions it sits at 900 in D. One or the other idle speeds only with no wandering. This happens about one time in five stops.
All this is with the A/C off. This car has no relays for the supplemental air valve. Vacuum is 17 in. with no known leaks.
What can cause an occasional high hot idle like this?
AAV is already hot in this situation. I considered it, but it can't be opening and closing. In a hot engine it has no way to do anything but stay stuck where it is or close more.
Someone recently had the overrun valves playing up and causing this sort of problem, if you have them, try blocking their airfeed from the back of the airbox off.
Greg
Someone recently had the overrun valves playing up and causing this sort of problem, if you have them, try blocking their airfeed from the back of the airbox off.
Greg
Thanks for the info Greg, already disabled the overrun valves. No difference.
My idle is steady at 750 when warm and in P. It sits at 500 in D. Except for the occasions it sits at 900 in D. One or the other idle speeds only with no wandering. This happens about one time in five stops.
What can cause an occasional high hot idle like this?
Just a thought, you might have an intermittent malfunction in all the myriad little pipes and airswitch gizmos that litter the A bank network of vac pipes and the cruise activation.
If you have a Lucas car, then you might have feedback to the manifold from the vac pipe system too. You could try definitely blocking all of these and seeing if the problem goes away.
Greg
Just a thought, you might have an intermittent malfunction in all the myriad little pipes and airswitch gizmos that litter the A bank network of vac pipes and the cruise activation.
If you have a Lucas car, then you might have feedback to the manifold from the vac pipe system too. You could try definitely blocking all of these and seeing if the problem goes away.
Greg
Relocated the vacuum solenoid to beside the windshield washer bottle where I could access it. Replaced all the hoses in every direction ( two trips to the parts Store ). On my normal route it seemed to have decided on 900 rpm!
It also seems to be more responsive to moderate throttle where it was previously iffy. Thanks for the suggestion Greg!
Relocated the vacuum solenoid to beside the windshield washer bottle where I could access it. Replaced all the hoses in every direction ( two trips to the parts Store ). On my normal route it seemed to have decided on 900 rpm!
It also seems to be more responsive to moderate throttle where it was previously iffy. Thanks for the suggestion Greg!
Glad it seems to have helped, good for you. Assuming you have a Lucas car, I honestly believe the best thing to do is to get rid of the lot and just connect the vac capsule on the dizzy directly to the throttle tapping underneath the A bank throttle body. I did it years ago on advice from the Great David Marks, JEC technical guru, and it is miles simpler and better.
Greg
This is a Lucas car. I'm just re-reading S-58 to try to follow what's going on with the vacuum advance. It seems to me that this system is designed to prevent vacuum advance just when you want it most. Sudden throttle opening. "Wanna go fast? Patience, child, it's coming, just ease off and it'll come" PAH!
I also was poking around in the manuals that came with the car. One spec the shocked me was the fuel requirement. This high compression HE V12 calls for REGULAR UNLEADED! 87 octane! What!? Obviously a selling point at the time but it required hobbling the vac advance system this way.
I'm going to do some testing today. I'll try the change you suggest. Is the lower vac tap manifold vacuum or throttle vacuum? I have a pair of vac gauges mounted so I'll reconnect them and determine this.
I'm expecting a significant change! I'm left with a decision to be made about the 45 second timer function that dumps advance vacuum at startup. I'd like to retain this function.
We got the HE in 12.5:1 comp, while the USA got 11.5:1, and we also had a fuel usage of "Unleaded", which in those days was our 91RON.
I run that vac advance hose the same as Greg, and I have incorporated the vac dump valve into that line, so vac is dumped quick time when accelerating.
I replaced my capsule with a 15deg unit, and lengthened the pull rod accordingly. I simply could not get a V12 capsule at the time, so a Lucas unit with 15deg was OK.
A) Is the lower vac tap manifold vacuum or throttle vacuum?
B) One spec the shocked me was the fuel requirement. This high compression HE V12 calls for REGULAR UNLEADED! 87 octane! What!? Obviously a selling point at the time but it required hobbling the vac advance system this way.
C) I'm left with a decision to be made about the 45 second timer function that dumps advance vacuum at startup.
Check the vac capsule is actually working, if you can suck air through it, it is not.
A) The spigot to use gives manifold vacuum I am not sure what the difference is you mentioned. It is the only small one under the A bank throttle body.
B) is that a MON number or a RON number, as I think the MON number is lower than the RON number for identical fuel.
C) I say dump the timer! I dumped mine, absolutely no ill effects.
My car has no hesitation when I floor it, another reason to dump the cat's cradle of tiny tubes and gizmos!
Good luck
Greg
My vacuum advance is working properly, smoothly advances when supplied with a rising vacuum source and hold vacuum indefinately.
Running the dizzy vacuum straight from the throttle body lower tap to a tee feeding the vacuum advance and my gauge it's like night and day! So much tighter throttle response! Pulling 18in ( 45mm ) at this tap at idle and this tap mirrors manifold vacuum at all throttle positions.
Grant, I tried incorporating the dump valve, but it buzzes like a kazoo! I can't have that! Every time it dumps, "fzzzzzuuu!" Does yours not? I also note that, as expected, a rapidly opened throttle results in a precipitous drop in vacuum as metered at the vac advance. Is this not sufficient? Is the dump valve crucial here in this configuration?
I'm going to go for a test spin and ensure I'm not pinging and that it doesn't misbehave on deceleration with high vacuum and thus high advance. I do think I need the dump valve. Maybe I can get a replacement and hope that it's not musical.
For reference and translation purposes; here in the US I'm running premium fuel. 91 octane. Mid-grade is 89 octane and the specified unleaded is 87 octane.
Edit: my timer appears to be non-functional or otherwise disconnected. Vacuum solenoid passes vacuum at all times and there is no voltage on the leads feeding it either just after ignition nor afterwards. I assume it should receive 12v to activate and vent to atmospheric pressure.
Great news. congratulations. I have had no trouble over 15 years with a straight vac connector and no gizmo, so I am sure you will be fine. 91 Octane MON (the US system) is about equivalent to 96 RON (the Old World system) I believe. So no worries on the fuel quality. Wonderful when they go as they should, isn't it!
Greg
My dump makes no noise at all, and I simply wanted that line to drop vac in an instant, rather than wait the milliseconds for the capsule to bleed off, probably just me being me, and pre computer days, I only had common sense and written scribes to work with.
You will luv this car even more as you get better aquainted.
My dump makes no noise at all, and I simply wanted that line to drop vac in an instant, rather than wait the milliseconds for the capsule to bleed off, probably just me being me, and pre computer days, I only had common sense and written scribes to work with.
You will luv this car even more as you get better aquainted.
So today I sorted out the dump valve and its working as expected without any noise! It was late when I got this all done ( I'd been rebuilding my fuel sending unit) so I cleaned up and we went out for dinner. This change should have no effect on performance but it did.
This gives instant vac dump on rapid throttle opening And normal response under other conditions. It looks crazy to me but it works wonderfully. Smoother acceleration and seems to have eliminated a hesitation at partial throttle. I'm not sure why but I'll take it!
More important, the car LUVS it, and since the car is Female, why on earth would a mere male question anything a Female has to say. Maybe the owners that are still single, and have yet to appreciate the Female "common sense" approach have NO idea what I am referring to.
Having finally replaced my antique HT leads I fired Jessie up hoping for a perfectly steady idle. She definately appreciated the expense and trouble I'd gone through, but she let me know that if come close, but not quite pleased her perfectly. Very different sound particularly on cranking. Instant start ( 1/2 second crank ) and smoother than ever. Power bump has me spinning a wheel in starts that previously would have been boring affairs. I'll have to retrain my right foot. But there was still a lot of misses.
I revisited the changes I'd made to the vacuum system above. I wasn't pleased that my arrangement gave me full vacuum advance at idle and I was suspicious that this was part of the occasional skip I had. I modified the super simple dump-valve-only setup to incorporate the vacuum regulator as well.
This made Jessie very happy. Almost perfectly steady at idle with a missed beat once or twice in 10 seconds. Next stop, fuel injector seals. Those made the most difference of any repair I made in the XJ6. I'm hoping they sort out this last bit of missing.
08-31-2016, 06:46 AM
My hot idle speeds - two of 'em
