Hey everyone,
I have been busy this winter. I had a bad leak on the drivers side valve cover so I stripped the engine down to get at it. I have just put everything back together, Injectors cleaned, new plugs etc etc. The car fired back up which was a relief but after about a minute the idle revs up then down up then down over and over. Any suggestions on where I should start my troubleshooting? Could I have mixed up a vacuum hose?
1992 xjs V12

 

my v12 has been doing the same thing has been doing this for maybe 3 years when the engine is warmed up it run fine. it would be nice to know why the car is do this

 

It sounds like it could be the auxiliary air valve, they stick with age.
There are loads of threads about it if you do a search.

 

Interesting. I have an 87 with the 5.3 V12. On occasion, usually when it's warming up, the RPMs will jump to 1800, then down to 400, in a cycle that takes about 4 or 5 seconds to complete. This will continue for about a minute, then it will go back to normal idle.

Somehow I can't imagine the AAV flaking out in this way, but I certainly have no better ideas.

John

 

I had 2 problems at the same time; the hunting and also randomly stopping and then not starting again.
Replaced the AAV and the CTS and it ran fine after that.

 

The 'flaking out' is really just the ECU's response to excessive engine RPM at closed throttle. Any source of air will increase engine speed but usually it's a fault with the AAV.

The engine idle speed increases when extra air is admitted via the open AAV. If too much air is admitted the RPM goes too high. If the ECU sees a closed throttle, via the TPS signal, together with a too-high engine speed, it (the ECU) will cut off fueling at xxx-rpm. This fuel cut off naturally drops the engine speed until the ECU reinstates fueling at yyy-rpm

Reinstating the fueling, in combination with too much air, causes an increase in engine speed until the RPM (again) bumps up against the fuel cut off, at which point the fueling is cut-off again....and the engine speed drops, again. Repeat, repeat, repeat.

Cheers
DD

 

Remove the LH air cleaner outer casing and the filter element itself. Do not remove the air cleaner base....the one that is bolted to the throttle body.

The air cleaner base has a 1" hole with a hose on the back side. the hole an hose is the air supply to the AAV. With the engine running block the hole with your thumb. Does the idle speed stabilize? If it does, the AAV is probably stuck open. If not, you have extra air coming in from elsewhere and/or a different problem.

It should be added that, with a properly functioning AAV, you should be able to adjust the idle speed right down to about 400 rpm. If you can't bring it down that low then you probably have extra air coming in from a different source....or a mechanical problem with the throttles.

It should also be added that, alth the AAV is the prime suspect, engine rpm oscillation can be caused by other faults. Go to post #11 in this thread and watch the video clip. In that case a faulty TPS was the culprit.

https://www.jaguarforums.com/forum/x...88-v12-193753/

Cheers
DD

 

Doug,

Thanks for that.

When I originally bought it, I pulled the AAV, cleaned it, tested it, then put it back in. Worked find for about 4 years, now the idle variation, plus I can't get the idle RPM below about 900.

Instead of messing with the AAV, I'm going to try replacing the TPS. Palm notes that a Hall Effect device is a perfect fit and requires no adjustment. We shall see.

Thanks,
John
1987 XJ-S V12

 

That might be a record


You almost certainly have extra air coming in from....somewhere



As you wish, but don't discount the AAV as a possible...and even likely...culprit


I think you might've misunderstood something. The TPS does require adjustment...though it isn't anything high tech to do so.

More later, gotta run

Cheers
DD