86 Jaguar XJS V-12 convertible project

Car running after much hair pulling, brakes working, brake lights (broken terminal on brake switch at pedal, resoldered) and signals all working. Now for the one problem left that I know of.
Car starts great, runs great and idles at about 750 rpm until the temperature reachs normal running position (N on the stock gauge). It will start to surge at N position with 500 rpm difference. up,down up,down consistantly. I can rev it and it will come back to a high idle then it starts the surging again. I sprayed carb and choke cleaner at vacumn connections to see if I could find a vacumn leak but no dice. I`m thinking it has something to do with thermostat opening with the surging not starting until normal operating temp but what is causing it is a mystery to me. When I let the motor cool it idles fine again until it reaches normal again. As usual any suggestions are appreciated

Search archives about AAV. Sounds like classic AAV not closing all the way once warmed up (which is a gross vacuum leak)

I would start with ignition advance; disconnect it and see if idle is then stable. I am thinking that idle vacuum fluctuations are being compounded by the vacuum advance. Possibly the vacuum limiter or dump valve are connected incorrectly or faulty.

Did you do a manual trans conversion? I recall something about the need to have the ECM fuel overrun cutoff modified; don’t know if that could be it.

Thanks to both for a direction to go in. Ill check both ut and let you know what I find out. No manual trans conversion. Thanks again Rod

I checked aux valve as gregma suggested. I closed the idle bolt all the way, it started a bit harder than usual but idled at 400 rpm. As it warmed the idle steadily increased until it reached normal operating temp. up to around 1100 rpm. I covered the hole under the air cleaner and the idle fell off about a 100 rpm. As I continued idling, the rpms increased to around 1200-1300 rpm and that is when it started the surging from 1300 dropping to 900 and back to 1300 and back to 900 and so on until I shut it off. My manual states if there is any change in idle speed when I plug the hole I should rebuild the aux valve or get another. I found that they are kind of pricey. I am in the process of contacting johnjohns and waiting to here back about a rebuild kit. If I get to the rebuilding stage I will post the results of that rebuild. RGK20m3 suggested checking the vacumn advance, vacumn limiter and dump valve and that will be my next step. Meanwhile any more suggestions are appreciated. Thanks Rod

I would send your AAV to John and let him rebuild it. He’s an expert. That’s what I did, and I can safely say mine works perfectly. Takes a bit longer, but worth the peace of mind.

Many slight differences depending on year, and he can instantly determine.

The idle should have fallen much more than that.
With the AAV totally blocked a cold engine would stall and warm idle should be around 500 RPM.
I think you have an air leak somewhere.

Would an air leak cause the surge of 500rpm differential at operating temperature? I`m going to run the test again just to triple check myself. Thanks for the input. Rod

I ran the test again. Idle bolt turned in. Starts a bit harder but does start. Idle at start is 400 rpm and climbs as it warms up. Plugged the hole to AAV at air cleaner. Virtually no change in idle speed and keeps running. I leave it run and aux fan kicks on at about 1100rpm. Operating temp according to in car gauge. I plug the hole and I can hear a change in tone in the engine but rpm hardly drops if that makes sense. I try to make rpm lower by revving slightly to make it kick down and the surging begins.Still a 4-5 hundred rpm flucuation with the surging. Im a little suspicious of the valve on the air pump. When I got the car it was not connected so Im checking that next. Ill see if there is a test for it and if I find leaks or no connection Ill post the results. Thanks for the insight. Rod

What’s the status of your distributor vent system?

On idle the ECU will cut fuelling above 1.500 or something RPM and will resume at around 1.000 RPM

Air leak indeed, and quite a big one.

Did you check the PCV valve? From left hand air cleaner to inlet manifolds. Mine was stuck open and caused the same up and down surging rpms.

Both lines are connected at distributor. Filter clean

PCV valve has been cleaned. Molded hose that pcv valve goes into took a bit of rework but is connected and solid. Thanks for the input. Rod

Normally you should be able to hear the leak.

I suggest you remove and cap all vacuum outlets from the engine, including PCV valve, brake booster line etc. etc, and connect the only the vacuum advance (albeit it could also be the source of the leak) without all the delay valves etc. See if it makes any difference

BTW, are your throttle plates gap and rods adjusted properly?.

[quote="Rodney_Lons, Still a 4-5 hundred rpm flucuation with the surging

Short time to reply but I’d call your high rpm a vacuum leak. The leak causes the idle to go high…and then the overrun valve prevents higher rpm, and the idle drops back…Then it’s back and forth…high rpm…low rpm. Fix the vacuum leak and you should be good. SD Faircloth www.jaguarfuelinjectorservice.com

Yes, but is the line connected at the AAV elbow?

It’s not the overrun valve that does that, it’s the overrun circuit in the ECU.

Speaking of overrun valves, though, have they been checked on this car? That’s usually the one thing that can cause a massive vacuum leak without creating an obvious hiss.

Kirbert, the other end of the distributor vent is connected at the AAV valve. Im not familiar with the overrun valve but now that Im at least up and running I will definetly check it out. I don`t know where it is. Is it the valve mounted behind the air pump. That valve was not connected when I bought the car and I put it back the way I thought it should be. Thanks for the insight and advice. Rod

The overrun valves are at the front end of each intake manifold, hidden inside a little aluminum canister bolted to the front end of the intake manifold. Inside is a spring-loaded valve that lets air past the butterflies into the intake manifold whenever it feels like it. Over time the spring loses its ooomph and starts letting air in all the time, boosting the idle. My recommendation is to take each valve out, adjust the spring tight enough it will never open, and reinstall.

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