Hello Jag enthusiasts!
I just finished the refurbishing of my V12 of theXJS 1990. Rectification of the cylinder heads, valves, change of the gaskets, change the distribution, injectors, fuel injection ramp, electric loom, hoses…. and many other small parts.
All is done, motor back in the car, started and….it is running, however once the cold start over and motor heated up, the idle run is very unstable, the RPM goes up and down quickly…..Can it be the problem of the auxiliary air valve, electronic….?
Any idea what is it from and how to solve it please?
Thanks in advance for your thoughts, guys!

 

Is it unstable in drive and reverse, or only in park or neutral?

 

With what has been done that list could be quite long.

BUT

The bvasics are:

AAV, NOT electric, see my paper in the archives on that, or ask and I will attach.
Throttle discs not set correctly, and the throttle rods not "balanced" correctly. Paper on that also in the archives.
TPS out of idle range. Another paper in the archives.
Spark plugs set at the WRONG gap, MUST BE 0.025", no iffs or buts.
Vac leak, Injector seals, Inlet gasket/s, any of the myriad of hoses, trans modulator and/or the hose to it.
The blanking cap on one of the spare spigot at the rear of the A Bank (AKA RH) Inlet Manifold.
The 90deg elbow on top of the AAV is cracked and split (more common than beer in a pub).

Now I need a coffee or 3, you will need to go through many. many things quiet;y and systematically.

ALWAYS remember this is a Double Six engine configuration, so as simple as it gets.

 

Another question. Does your car have catalysts and O2 sensors?

 

Further to Grant's post:
The onjection system has a feature called "over-run cutoff". This means that, for example, if you take your foot completely off the accelerator as you reduce speed to a traffic light, or to reduce speed for any reason, the ECU cuts the fuel until the RPM drop beow 1400 RPM.
If the car is idling, and there is an air leak, or if the AAV is not closing properly when the car is up to temperature, the revs will be above 1400, even though the throttle is closed. Therefore the over-run cutoff kicks in, and the revs drop because no fuel. BUT as soon as the revs drop below 1400 the fuelling starts again and the revs climb. This cycles until the extra air leak is fixed!
One question
Have you adjusted the AAV idle screw as far inwards as it will go?

 

I reckon since he is Maine, and a 1990, it will be Marelli and full of emission junk.

 

It says Marne, not Maine. There is a Marne river in France. Hence my question.
Jon

 

Yep, the joys of one eye, wrong place,

BUT, a 1990 would be cats and O2, no matter where it is.

Thanks for the Phone call Greg, and putting me right.

 

Hello,
It runs up and down on the idle. Doesn’t matter if with drive gear engaged or not - if no flap opened (no pressure on accelerator) the RPM jump up and down. When main flaps opens it runs steadily, but too high!
thanks for your answers.

 

Some details which could be important
1- The O2 sensors are installed
2- I installed “High performance” exhaust piping, the catalyst removed.
3- During full refurbishing I changed all seals and gaskets+ hoses (hardened to stone), however did not refurbished, set or replace the AAV (before was tuning just well). I feel I should begin by dismounting this and testing it. Any advice or guidage for it?

 

IS the idle adjusting screw on the AAV turned all the way in (clockwise)?

 

Here ya go.

Some light reading.

 

This guy sells AAV rebuild kits. His products are all excellent. He is regularly on the Jag-lovers forum.
https://www.ebay.com/itm/32460810686...mis&media=COPY

 

You could have a vacuum leak. When the idle goes up without using the accelerator, the injectors shut off. When the idle drops, the injectors go back on and the process continues.

 

Hi,
thank you for your tip - my problem is the cost of the transport USA/France and the importation tax. In the end there is few price difference compares to the new purchased from SNG Barrat for 260 EUR.

 

Thank you all for the big help to understand this problem!
I will take out the AAV and inspect it, check all this spaghetti of vacuum hoses, maybe change them, It is true that the most of them begin to be hard and might not be tightened enough to the nozzles causing possible leakage, even this might be really small and most probably do not cause THE problem, I must come from a bigger airflow.
Also I will check the correct connections of all these small animals around this big engine…if somebody of you have CLEAR schematic for XJSV12 1990 USA model I would appreciate …
By technical curiosity- I ask myself how it works when the AAV delivers the air in to one (left) collector only?

 

The seller is in Lithuania, so shipping won't be that bad. I don't know about tax, but they are in the EU.

Jon

 

The AAV is connected to both intakes via the balance pipe at rhe rear of the engine.

 

Hi Greg
thank you for this precision - I realized that when I saw it this morning too. This is the steel pipe bridge in the back of the engine.
It is amazing how you know, folks this car and specifically the engine in details!
thanks again for your help, I will take the AAV of and clean & test it, hopefully it will help!

 

Jindra
Do you understand how the large screw on the AAV adjusts the idle speed?