Erratic idle when engine warms up – help please!

The problem started with my 87 XJS Cabriolet [V12] last year after I pulled it out from winter storage.
The car starts without problem and it runs perfectly at 700rpm while is still cold, I can even rev it up to 2500rpms without issues. But the problem starts once the engine reaches warm up temperature, so as I step on the gas and the tachometer reaches +/-1300rpm the rpms start fluctuating between +/- 1700rpm and +/-600rpm nonstop until I release the gas, and at point the rpms settle at 700rpms and the engine idles smoothly again. This problem is constant and has rendered the car undriveable. I know it sounds like an issue caused by a defective Auxiliary Air Valve, but the AVV isn’t the problem.
This is what I have tried so far:

  1. Auxiliary Air Valve [Note that I tested it before ordering the replacement, which was working correctly, but since I had replaced it back on 2011, and to be on the safe side I decided to install a new one].
  2. Fuel filter
  3. Fuel pressure regulator [outlet side]
  4. Distributor cap and rotor
  5. Spark plugs and wires
  6. Battery [The old one wasn’t holding charge anymore]
  7. Injectors were serviced 5 years ago.


  1. Vacuum advance leaks [All the hoses and connections looked fine and secured].
  2. Throttle disks and linkage.
    This forum and Kirby’s book are always my first sources of advice and guidance; however, this time I haven’t been able to find the solution to this problem.
    I really hope you guys have an idea about the potential cause and solution to this issue, which is driving me crazy [no pun intended].

deja vu from last week.

No where in this list do I see you checking the Coolant Temp Sensor. That could be non-working or have a bad connection. They are pretty cheap, IMO it’s always good to replace after a while.

A bad one will make the ECU think the engine is cold when it’s actually warmed up, and make it run badly or stall.

You are correct Greg, I did not check the coolant temp sensor, and never even crossed my mind, which by the way I have never replaced.
I will check it out tonight -
Thanks for the advise!!

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Throttle position sensor misadjusted or worn. See Kirby’s book for adjustment procedure.

Also, the throttle cutoff can cause the problem, if the AAV is open too much. Here is an exerpt from Kirby’s book:
“This feature can easily confuse the novice mechanic that may not be familiar with it. What will sometimes happen is that
the car is running poorly, and the idle screw will have been opened up to maintain a reasonable idle speed. Then the
problems with the engine are corrected, and the engine then tries to idle faster – but hits the cutoff RPM, shuts off,
drops back to the cut-in RPM, kicks back on, and sits there cycling up and down indefinitely. The mechanic thinks he
has really screwed something up this time and begins all sorts of ill-conceived corrective measures when all he really
needs to do is close down the idle screw a bit. It’s even more confusing, of course, when the overrun cutoff feature is
temperature dependent so it doesn’t cutoff when the engine is cold but starts cycling upon warmup.”

That’s a good point! I will certainly check the AAV’s idle screw while the car is running to see if it makes a difference, and I will read tonight the adjustment procedures.
All these advises are providing me a fresh perspective to keep on looking for the solution.

Seems like 700 RPM is a little light when it’s still cold. Should be 1000 RPM and backs off to 750 RPM once the engine has warmed up.

The Cat is alive!!! As jal5678 suggested I adjusted the AVV to lower the RPMs, and per gregma’s advise I tested the coolant temp sensor and connector, which happened to be faulty, so I replaced them both and…that took care of the problem!!! I kept my fingers crossed the entire time as the car was warming up, and the hesitation is gone, and with all the other work I had done to it trying to fix this problem, the car is now running like champ!!!
This issue had me against the ropes, so I am very thankful to all of you for pitching in, and to gregma for suggesting what happened to be the solution to my problem.
Thank you all!!


Unless I’m missing something here not sure why you would lower the RPMs if it’s only at 700RPM when cold. It should be at 1000RPM and settle down to 750 RPM when warmed up.

You are right it should be 1000rpm and settle down to 750rpm.
BUT NOW I AM BACK TO THE DRAWING BOARD, I declared victory too soon!! After replacing the coolant temp sensor, I ran the car on the driveway for over ½ hr. and the hunting was gone, I stepped on the gas several times, and the engine didn’t hesitate at all.
But yesterday, I took the car for a drive and the problem was back, so again as soon as it reached 1300+/- rpms the engine started hunting with the rpms going between +/-1700 and +/-1300 nonstop unless I released the gas.
So, I am going to continue looking for the source / solution to the problem, and of course all your suggestions are welcomed and appreciated it.
I am starting today with checking the throttle position sensor adjustment procedure on Kirby’s book.

I spent many months getting my V12 running well…ended up being “all of the above”.

These engines are temperamental to say the least. But not that they’re bad, they are just from that unfortunate period of fuel injection when the ecu and electric sensors were not that sophisticated. Huge improvement in the 90s with all injection cars.

Sounds like a fuel cutoff issue. The EFI is only supposed to cut the fuel off at idle, which means when coasting above 1500 rpm or so. If the EFI thinks you’re at idle when you’re not, that’s a throttle pot issue. Adjusting may work, but I kinda doubt it; it’s more likely it’s gone bad. Going through the adjustment procedure should clarify whether it’s working properly or not.

If so, i advise switching to the new and improved red throttle pot, from later V12s. I did. The old black throttle pot is troublesome.

The adapter kit is NLA, but i found a used one.

Kirby - When you say “gone bad” you mean the “throttle potentiometer”?
As I was checking all the connections, I noticed that a previous owner had some work done to the potentiometer wiring, which were spliced and held together with electrical male/female connectors, which by the way weren’t making good contact, so I replaced them.


Post a photo so we can see.

On the picture you can see the 4 prom connectors I used to replace the old one worn out bullet type [shown on the picture]. “Important” only 3 proms of the connectors are utilized.
Note that the hunting issue was already present by the time I replaced the connectors

I see a lot of corrosion on engine metals, this could mean some electrical parts/wires are also quite corroded. You may have a big project on your hands, but if that’s all it is, it’s cheap and the car will survive. Just gonna be time consuming.

I knew what I was getting into when I bought it 10 years ago, is a nice cabriolet [targa] and overall it has been a great car….of course after taking care of years of previous neglect, and issues like the one I’m currently dealing with, but it’s worth it.
Luckily, I found a brand new TPS [black] which is coming next week. I looked for the red throttle pot as you suggested [which is available] but some of the parts required to adapt it aren’t, so I just purchased the black pot.
I will keep on working on it and update you guys.

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Update……While I was waiting for the throttle pot to arrive, I looked for damaged / compromised wires, and I happened to find at least one related to the Potentiometer. Upon repairing the wire I proceeded with the installation and adjustment of the new “Pot”, which would’ve been difficult without my wife’s help, because while she has making sure the voltmeter was making contact with the red and yellow wires/terminals, I was adjusting the Pot at 0.32v. After I put all the parts back together, I kept my fingers crossed for at least 5 minutes with the intent of gathering enough courage to start the car……only to find out that the “hunting” problem was GONE!!!
I just got back from a test drive and the car run very nice, it was responsive and smooth As Kirby suggested the Throttle Potentiometer was the problem. I want to thank everyone for your advice! This forum is priceless