Hello,
Haven't had too much of a play with different scenarios yet but I haven't used the car for a couple of weeks and started it last night to see if all was ok. All ran fine for a minute or so then the revs drop to nothing then back up - over and over for a couple of minutes then settles down and runs fine. Had to use the car this morning to get to work and the same happened. I made a video of it to explain better! Not much to see but you can hear it.
Thanks!

 

are you disconnecting the battery when you arnt using it? as if you are its the sensors re-establishing there values,

 

No, I haven't disconnected the battery at all...

 

the best thing I can find from a mates car years ago with similar problems on start up was the aav was knackered but they are quite expensive, and 2nd hand are crap, I'm sure someone will know on this forum (when they get up) bottom right in pic. with the blue silicone elbo

 

My car does this albeit to a much lesser extent. I would suspect the AAV. This works the same as a thermostat and they do not last forever. As the wax melts the AAV closes reducing idle speed.

Does the idle respond to the idle control screw, which is on the side of the AAV. I would also check for leaks. Remove the AAV hose from the rear of the LH air filter and plug it, the car should almost stall. If it does not then you have a leak somewhere.

 

I fixed mine by using aluminum tape and covered the hole within the air cleaner, then poked a hole in the tape with a pin, it was just enough. AAV needs replacing, but this has worked for me for 1 1/2 years.

 

OK, so I was about to post a thread but now not sure if I need to... I'm having almost EXACTLY the same problem with a bit of a twist. For me the idle is great when the engine is COLD but at a certain point something clicks on or off and I get that exact surging idle behavior.

I don't want to hijack this thread so I'll just keep an eye on it, but please please please post the results when you find the problem.

 

On page 264 of the Palm book is a description of an issue very similar to yours which I was experiencing too.

"When the idle gets above 1800 rpm or so, the overrun cutoff feature kicks in and shuts the fuelling off until the rpm
drops below 1100 or so, then it turns it back on. The resulting cycling of the engine will get your attention! People get
very concerned when this happens, thinking it must be something far more serious than simply the idle speed is set too
high, but that’s all it is."



Check to see what your cold and warm idle speed are. If the surging is happening at about 1800 and lasting about 45 seconds your problem is isolated.

 

Regardless of whether the car does this cold, warm or hot the problem is still the AAV. Yes the the ECU cuts fuel with closed throttle at 1800rpm, but it's the AAV (or a vacuum leak) that causes the high idle.

Mine does not do this cold, it does it through the warm up process before the car is fully up to temperature. I'll fix it when I replace the ECU and ditch the AAV for a modern idle control valve.

 

keep us posted if you when you get started pls

 

It sounds like the problem on my car is different than the OP's.

My idle is not fast - goes from about 700ish to nearly nothing and back, will usually die eventually. I'll run the car for a while and get it completely warm and see if I still have the issue. If so, I would think that my issue is unrelated and should probably start another thread.

 

I would still suspect the AAV first. Disconnect it from the rear of the LH air filter (it just slips in). Check the vacuum at the inlet, if you plug the hole the car will almost stall if the throttle butterflies are adjusted correctly.

 

You are absolutely right about what's going on with my car.

I had an issue when installing a new TPS that I got worked out and the car was much happier. Drove it quite a bit yesterday and overall it worked like a champ... but had a bit of an issue idling right after starting it when sitting for a while.

Today it started fine but didn't want to keep running. It was much colder, FWIIW. I took one of my other cars and when I got back did some 'sperimenting.

Simple test - used a screwdriver to open up the hose connected from the AAV to the intake manifold, gave it about the same opening as the AAV would when fully open. Voila, perfect idle.

That thing has got to go. I see a number of ideas having to do with rebuilding it, but assuming that I can it won't last forever. I can only imagine the cost of a new one in five or ten years.

I suppose for now I'll keep mucking with the valves that I have. One works, sometimes... it did on the stove the other day and probably did in the car for a while but isn't doing so well today. I just hate messing with something with such a high failure rate.

Being a purist on original Jag parts - only when the original version isn't a piece of crap.

 

I have seen the AAV replaced with a cable operated heater valve run the cable into the cabin, it's a cheap fix.