Dear fellow XJ enthusiasts and clever people on the forum,

I feel like I've asked a similar question before, but my memory lets me down. I never did get a proper fix anyway unfortunately..

Few months ago, before a big trip to England I did some maintenance to my 1991 4.0 XJ40. One of the final things I did was replace the fuel filter. After starting the XJ with the new fuel filter in place it developed an erratic idle. It's always been slightly rough while idling, but now the RPM seems to drop, the engine (and car) starts vibrating and then the RPM recovers, smoothing out the engine. This only happens in either park or neutral on a warm (or even lukewarm) engine temperature. The RPM sits nicely at a 700/750 and drops to 500, which starts the shaking. Listening to the exhaust it almost sounds as if it's misfiring, but I can't tell for sure. Driving the '40 is absolutely no problem, even idling in drive or reverse is absolutely fine. I haven't noticed any performance issues or an extravagant fuel consumption, relatively speaking ofcourse.. :-)

This started last August, in the months before I changed the following parts:
Spark: new spark plugs (Champion RC9YCC), spark plug wires, distributor cap and rotor arm.
Fuel: new fuel filter and o-rings, cleaned and painted the fuel rail, got the original fuel injectors ultrasonically cleaned and painted them (new o-rings and proper reinstallment, no leaks)
Air: new air filter, cleaned the air pipes, elbow and cleaned the throttle body without removing it. New idle air control valve, which Jaguar calls a stepper motor I believe, and thoroughly cleaned the body it sits in. Replaced the gaskets, but didn't smooth out the contact surfaces before doing so.
I then went on and cleaned the throttle position sensor, the red one below the throttle body, by removing the lower casing and spraying contact cleaner. It removed a lot of dirt. I didn't touch the sensor any further as to not change the adjustment. I cleaned the air intake temperature sensor and some other sensor that sits in the breather hose that goes from the oil filler pipe to the air intake elbow. All the cleaning of the sensors was done contactless and with either air intake cleaner or contact cleaner.
I further changed the engine oil and replaced it with a mighty fine 15w40 from Castrol. The engine appears to enjoy a lower viscosity.

I believe that should be about it. I hope anyone can come up with any more ideas as to what could be the issue, or more importantly, what could be done to stop the RPM from fluctuating. I can only think of either a vacuum/air leak (the engine does seem to whistle a little) or possibly misadjustment of the idle air intake or whatever it's called. I'm talking about the bolt that can be adjusted on the body where the idle air control valve sits.

Edit: no warning lights and the VCM doesn't read any fault codes.

​​​​​​Thank you and cheerios

Douwe
(Just in case my signature doesn't show up again; 1991 built XJ40 4.0 Sovereign)

 

Which things you have fixed between fine and erratic running?

 

I drove the car twice after having replaced the fuel filter and it was fine up until that point, then it started the rpm fluctuations. (Thinking out loud: could it be a faulty filter? Monday-morning production so to speak?)

 

filter is a such simple thing to manufacture that i would be surprised it went bad, and you drove car after all, i would rather search elsewhere.
maybe something went loose while you was working on it or cause of erratic idle got worst, does it smell raw petrol when idle goes bad?

 

I cleaned some bits of the engine bay last week, whilst preparing the '40 for a Jaguar meet at the track. I've checked a few times but can't imagine anything being loose and didn't see any loose items either. I assume you're aiming towards plugs, sensors or air pipes being loose? There's always been a hint of a fuel smell when the engine is running, but I figured that's normal. Tomorrow early in the morning I'm going for a drive and I'll check for any fuel smells while idling.
Douwe

 

some ideas
check air temperature and especially coolant temperature sensor readings, disconnect maf while fluctuation occurs, (you can do the same with temperature sensors), spray starter fluid around to check for vacuum leaks, check gearbox oil level and condition, it could also be fuel pump on its way out but you would have to hang gauge to check it.

 

Hey Douwe,
Ill second xjack on the vacuum leaks, since I have made these mistakes myself- The tube from the air injection pump to the filter box- its on the bottom and easy to forget and the tube itself can get fragile and crack since its made of fabric, Check that the intake boot is fit squarely and completely over the throttle body, the bottom is impossible to see if its not on right. There are also several small vacuum lines toward the front and around the bottom of the intake. I recall on my 92 that there is a tee in that area which will get brittle and crack but not break. There is also the vacuum line from the cruise control diaphragm onto the throttle body.
I suggest also that you may need to reset your IAC motor since you cleaned it. The procedure for resetting it is in the sticky section in this area of the forum. Ive done it, so it cant be too hard.
The only other thing I can think is just go back over everything and make sure all the electric plugs are completely connected and clean. Good luck.

 

I think cleaning the throttle position sensor did it. you just need to reset idle.