Engine Idle speed In D


I drove a bit xj40 today.

The engine was warm and I had to stop at an intersection.

Drive position was in D and the brake applied.

Engine speed was at 500 rpm.

Vehicle nearly died.

Who or what controls the speed when the vehicle is stationary and when the gear is engaged?

Otherwise he runs objection free. Always starts well.

Thank you


The ECM is supposed to control the IACV.

Clean out the intake plumbing and decarbonize the throttle body and butterfly then set the butterfly correctly.
Then your car will probably settle down to idle properly.
Most people mess with the IACV or even replace it to try and solve idle issues like yours. Don’t do that or even consider fooling around with it until you clean up the intake.
The XJ40 is notorious for muck in the intake and requires a clean up from time to time. This is especially true in cars driven short distances in colder weather. Condensation mixes with oil vapour from the cam cover breather, forming a mucky goop in the intake plumbing.

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Hi. thanks for the information. but what is the butterfly and how do I adjust it? there is a guide. I’m still new to the xj40 … how can the engine control unit be read out VCM?

Stojan - this is a page you should add to your library


ignore section 10.6 at first, what you need is at 10.5

The butterfly is the brass plate inside the throttle body. When you remove the rubber plumbing, you will see it at the throttle body opening. Move the throttle cable and you will see it open. At first I would not necessarily remove the throttle body, just get in there with some carb cleaner and a rag and thoroughly clean the butterfly and throttle mouth. After cleaning you adjust the gap to .002"
If i remember correctly, you adjust it with the throttle cable stop.

DO NOT change any other settings, just do the cleanout and set the butterfly then see the results.

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Hi Stojan. I was having a similar problem a couple of years ago, I followed all the advice about cleaning the intakes and throttle body etc but the problem was still there. In my case it turned out to be a faulty/worn out alternator. At the normal idle speed around 750 - 800 rpm there was no output from the alternator, it was only kicking in around 900 - 1000 rpm. I would notice the dash lights dim a little and the needle on the charge gauge would drop to the negative side whenever I’d pull up at a junction or sit with the engine idling and then the engine would become lumpy and almost stall so I’d press the pedal to raise the revs and it would even itself out and the charge needle would swing back to the Pos side. I assumed this was BECAUSE the idle was too low but it was the faulty alternator that was making the idle speed to low. I rebuilt the alternator and the idle speed has been steady around 750 ever since.
I would check the alternator output at the usual idling speed of around 750rpm, while testing it turn on the lights and other gadgets to see if the charge rate drops away to the negative side.

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Many years ago I was having the sticky throttle issue when coming off idle on my 1990 4.0 liter XJ40. This was due to crankcase vapors and oil gumming up the throttle plate. Later cars had the issue even more commonly, and Jaguar came up with a mod to fit a plastic oil/vapor separator canister in the line to the intake elbow. This then routed the oil that separated out, down to a modified dip stick tube with a branch that connected to the drain hose.coming from the canister which was a grey device about 1.5 inches in diameter and 3 inches long with a nipple at each end. This exact mod did not fit fully to the AJ6 engine I had, but I was able to fit the dipstick tube with the branch and plumbed that into a T fitting I fitted into the breather hose I made up from hardware store bits. This must work as I have never had the sticky throttle syndrome since. Sorry for the poor photos, this is not an easy area to photo. Second photo shows the hose leading to the dipstick tube below the orange dipstick handle.

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Stojan. To understand the Jaguar you have to make a small investment in a " Haynes"the magazine will assist you greatly ,they are available on Ebay for under $30. There is also a CD online. The magazine has a write up on the most common problems you will run into with the car and step by step instructions. Good luck. John