Hello experts, I had a weird thing happen a few days ago. I was having issues with my jag maintaining temperature so I replaced the thermostat to fix the issue. Now the jag keeps the temp at an even 170 degrees (the thermostat is rated for 180). Before changing the thermostat the Jag would start great (warm or cold) and idle around 700 RPM, slightly less when warm. Now, it starts and will only idle around 500 RPM and stalls. Once warm it idles around 700 RPM again.
Any thoughts/ideas would be greatly appreciated. I’m not sure how changing the thermostat would cause this issue unless maybe it’s related to moisture in the distributor. Since the thermostat is above the distributor and a minimal amount of antifreeze leaked on the distributor during replacement.
Within the last few months I performed the following tasks:
It should be about half open when cold, and fully closed with engine hot. It is important that the idle is set with engine fully hot, and the AAV fully closed…
The AAV provides extra air with engine cold to counter cold engine drag; more air is the only factor to increase idle rpms - and without enough air the engine cannot idle. In principle; cold ide should be 200 - 400 rpms higher than hot set - indicating that the AAV is working correctly…
That said; your hot idle is set too low; specs say 800. Nothing ‘wrong’ with a lower idle, but it may cause starting problems - particularly if the AAV malfunctions…
The change in idle is unrelated to changing the thermostat - lower air temps will slow cold idle…so…? However, it is unusual for an xk engine to run cooler than thermostat temps. Have you measured the coolant temp at the gauge sensor with an infrared thermometer - the dash gauge is not very precise…
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Actually, the specs say 750 +/- 50 rpms except for some markets, David - but the state of things sometimes require higher idle for satisfactory start and idle…
What is puzzling is his remark ‘start great (hot or cold) and idle around 700 rpms, slightly less when warm’. With a functioning AAV this is ‘incorrect’ - the whole point of the AAV is to increase cold idle. And how can it idle ‘slightly less when warm’ when the idle is set (to 700) with the engine warm…?
You are absolutely right that lean fuelling will lower idle, but will then usually cause misfiring. Otherwise, with correct fuelling (and ignition) the rpms are dictated by the amount of air entering the engine…
Is he referring to idle ‘in gear’ rather than ‘park’…?
This is why I thought that maybe the AAV actually worked, but the enrichment did not (fully).
Our problem is that it happened after the thermostat change, so it should be related to him working in the area.
Could the timing have changed? Now more retarded, so it would start great but run lower… and die as soon as the AAV isn’t adding enough extra throttle anymore.
But when hot it runs good. It should be a warmup issue then. Leaves us with AAV and extra fuel (I’d say the cold start injector works). And I can’t see how a AAV fault could relate to a thermostat change so it’s not first on my list. Suggesting to check the wires and for air leaks surely can’t be wrong?
Update - today the engine started working correctly again.
Absolutely no mechanical changes were made. I wonder if the AAV was stuck in the closed position and it released. Very odd. I’ll see how it works over the next few days and post another update.
I think they do that. My AAV is coming out as well; I used to have 700 and by now have 900 hot, and 1100 at cold start. But then again it starts better and idles higher than before so I‘m thinking that it doesn’t close all the way whilst it was stuck closed before. I am lucky to have a spare!
On the other side my car can idle at 400 in gear and not all that rough and it won’t even die in steering lock.
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‘Better’ starting is the intent of the AAV, David - increased idle is a necessity to overcome cold engine drag.
Ie, the air required for starting differ hot and cold; the AAV varies air ingress ‘precisely’ according to engine temps - to enable ‘feet off’ starting at any time. For some reason the xk engine is very sensitive to air ingress while starting - using the pedal usually cause the engine to baulk…
Unless the AAV closes completely when hot, the range of the ide screw (in conjunction with correct throttle gap) may be insufficient for setting idle as desired. And if the AAV then move; air ingress may be excessive - causing starting difficulties…
My view is that if the AAV does not close completely; remedial action is called for. How much the AAV opens when cooling down is less critical - it seems to vary a bit with individual AAVs …
