1987 series 3 XJ6 coolant temperature sensor or auxiliary air valve the culprit here?

Seven years ago experienced poor idle after startup when engine was cold. After a few minutes of warming, engine ran ok. . After replacing CTS, along with its ‘pigtail’ wiring connector, problem was solved. Recently, I noticed whenever I start up the car, the engine would ‘stutter’ for a few seconds, at which time I would lightly ‘rev’ the engine a bit, just for a second, at which point all would be ok. Yesterday, when I started up the car, it idled badly. I had to sit in the car with my foot lightly pressed on the gas pedal for a minute or two until idle became smooth with no threat of stalling. I then drove the car 25 miles , making a stop somewhere for a few minutes. No problem. So, having read a few old posts, I’d surmise after 7 years, the replacement CTS has failed. I ordered one from a ‘usual supplier’, should arrive in a few days. But, I also read in an old post courtesy of Frank Anderson, the problem could also be a sticking auxiliary air valve. I’m anxious to get the issue resolved soon, but, at the same time, before removing the rubber hose on the AAV to access the possibly sticking portion of the AAV but also possibly damage the 34 year rubber hose, might I be better off waiting for the new CTS to arrive and install it, possibly curing the problem and avoid dealing with the AAV? Your opinions are welcome. Thanks
Mel R

Not a bad theory, the idle is a little wacky on mine once in a while also. Once in a while (about twice in the thee years I’ve had it) the idle drops so low it’ll stall from the power steering load doing parking lot maneuvers.

Several hours ago my curiosity got the better of me and I attempted to remove the rubber hose on the AAV. I loosened the clamp on the rubber hose, but found to my dismay the rubber hose seems fused to the AAV. As I’m not going to cut it off, I decided to put the clamp back on and wait for the new CTS to arrive and hope it will cure the problem.

Why not just test the CTS, Mel - using an ohmmeter, and the dash temp gauge is adequate for a crude test…

At room temp, the resistance should be some 2,5 Kohm, at 50C (122F) some 850 ohms, and at 80C (176F) around 325 ohms. High precision is not required at this stage - reasonable complianceindicates that the CTS is innocent…

It’s far more likely that the cause is the AAV, and some gentle exercise with a small screw driver wil at lest temporarily restore its function…

The symptoms described may be lean mixture due to CTS failure, but the engine may accept the leaner mixture as it heats up. Or lack of adequate extra air (AAV) will interfere with a cold engine - but as it warms up it doesn’t need the AAV. More or less identical symptoms, but two entirely differnt causes…

xj6 85 Sov Europe (UK/NZ)

I’m happy to report, Frank, you were right about CTS being the cause of the rough cold idle and not, fortunately, the AAV… New cts arrived in today’s mail. After removing the defective old one and installing the new, I turned the ignition key and smiled as the engine fired right up and idled at 1100. After a short time idled settled at 800 as the engine warmed. As always, thanks for your advice and encouragement.