I took the car in for service which included fixing the too fast idle. Now back it intermittently idles at about 1400 rpm which climbs to almost 2000 rpm if I put it in neutral. In other words around the side street too fast forthe speed limit without continually have to brake.
They replaced the module [ I recall] that controls the idle { What ever happened to the simple screw adjustment on SU carbs? } with a second hand one, Which obviously they will replace when they come back after Xmas. But is it worth putting another s/ Hand item back? Are new ones available? and if so do they cost a fortune?
I’d just like to be prepared so as to make a sensible decision when I take it back.
What is the vin of this car Ed? Lots of changes between early and late models.
Depending on the year (vin) the IACV (the “module”) can be cheaply replaced by using the same unit as fitted to a Renault Megane …although I wouldn’t bet on that being the problem.
You probably have a vacuum leak or the throttle butterfly isn’t closing fully or IACV hasn’t been set up properly or a combination of all three.
To set up the IACV, see the XJ40 book on the old site, 10.6 - Adjusting Idle Speed
Larry thanks , i’ll look at the vin in the morning, It’s extremely early about 3rd quarter of 1986.
I would prefer to use a new module if that is the issue.
I’m guessing you already checked but it looks like the 3.6 uses the JLM1741 (which is apparently NLA from JHP ) - however David Manners has an aftermarket one for 36 quid.
The rest of the models, across all years uses component JLM12185 which is a pricey little guy ($200+?) but as I said earlier, can be substituted with the Megane unit which runs about $20 or so.
The housing does get carboned up on these suckers and might be well worth a scrub, the stepper motor has a cone shaped nose that regulates the air through an aperture and if it the carbon in there is preventing it closing enough then …You can also test your unit, off the car, by running through the setup procedure and observing the nose/piston moving in and out.
Hi Ed, it just seems odd to me that they replaced the module and the fault is still present. I know it was a 2nd hand one and it’s possible that one was faulty too, but before spending more money I’d want to completely rule out all the much cheaper, and definite possibilities first. Every rubber vacuum hose from the manifold back to the other end is a potential source for an air leak, especially if they are the originals .Small splits along the length can ‘self heal’ when the vacuum increases and makes detecting them difficult. Spray a little WD40 around gaskets and any other fittings on the manifold and see if it gets sucked in.
I’m not sure how long you have been driving it with the fast idle but have you noticed whether the mixture is weak or rich. If the spark plugs or the rims around the tail pipes look white, pointing to a weak mixture, it would indicate an air leak present at all engine speeds and not just an idle fault, and you are only noticing it at idle speeds.
Yes , I can see what you mean and agree. The housing was cleaned. i try some of those ideas.
I wasn’t sure the old tailpipe colour method still worked with current petrol