I have a 1996 XJS that suddenly began idling >3500 on start and does not reduce in rpm.
Disconnecting the ECM reduces idle to ~1200 and car does run OK.
I have been trying to find an ECM replacement but cannot find ECM # LHE1410DJ.

Any insights?

 

You will most likely need a TPS reset. You can only do that with a device loaded with Jaguar specific software.

A dealer should be able to help you, but call ahead.

Many shops may have that capability as well, but again, you'd have to call again.

Before doing this, I would strongly suggest removing the throttle body and cleaning it out from both ends. This can only be done thoroughly with the throttle body off the car. It is held in place with four 10mm bolts, so it's rather simple to remove. There is no gasket required, just watch out for the small coolant hoses under the throttle body. A pair of golf tees will be enough to plug the hose up once off the throttle body.

Your problem is very common. You can unplug the battery from the car for 100 years and nothing will change. You can cross the positive and negative terminals and nothing will change. The only suggestion I have for you is to be absolutely sure you do not have a vacuum leak. At that idle, the leak would be pretty big. Be absolutely sure that all hoses are properly fitted to each other from the MAF sensor all the way over to the throttle body. This is likely the only other realistic possibility.

If you feel like throwing parts at the car, you can replace the IACV which is cheap, the MAF, which is more expensive, the oxygen sensors, which seems to be rather difficult to find and lastly the coolant temperature sensor, which is very cheap.

Come to think of it...you should check on the coolant temperature the car thinks its seeing. Can you access the OBD2 and see what the ECU sees? (cheap ELM27 dongle and an app for your phone is the cheapest way to go) A failing coolant temp sensor will cause this problem. The part is $20 and is so simple to change, even a caveman can do it!

Replacing the ECU will absolutely not solve your problem.

 

I have more information on this thread.
The ECU was not disconnected but the idle control valve was disconnected from the ECU and the car idles at 1000 rpm. With the connection it idles at 3500 rpm.
ideas?

 

Is it a V12 or the six cylinder?
The V12 has an idle adjustment available on the driver side throttle body.
Not sure why the TPS would affect your idle.
I would be looking in a number of other places before messing with the TPS.
Try checking if the throttle plates are closing freely by disconnecting the linkage.
If it is the TPS, you don't need a Jaguar Dealer PDU to adjust it.
What you need are two digital volt meters that need to be jumped into the TPS connector.
All you need to do is to balance the voltages for each trace by rotating the TPS.
The information above is for the V12 ONLY.

 

It is a 6 cylinder

 

The target is 700rpm for a warm engine at idle.

1. Did you check for vacuum leaks?

2. You’re thinking it’s the IACV. There’s a way to test it. You’ll have to search the forums, but you basically turn the key (not engine) on while unplugging and replugging the IACV back in three times. After that, when you start the engine, if it idles normally, the IACV is proved to be working well.

Once you turn the car off, the car will return to high

The fact that there is a change when you unplug it means that it is likely not the problem.

 

"It is a 6 cylinder"

Then sorry I can't help you.

 

Hi,
I have more information on this problem
My mechanic says that the idle control valve is not communicating to the ECM. When connected rpm at > 5000
He disconnected the idle control valve from the ECM and the rpm ~1100 and runs well.
He is saying that I need a new ECM.
Is this the case?

Also - and more importantly, is the car safe to drive when the idle control valve assembly is disconnected from the ECU?

Jim

 

Hi Jim,
as Vee menitioned in his post earlier the probability is high that the Throttle Position Sensor Adaption Test has to be completed on your car.
The ECU of the AJ16 engine does not control the IACV if the detected Throttle Position is out of range.
Unfortunately, the acceptable range of the throttle position at idle (closed) is very narrow and through the years the TPS at the idle position is drifting out of this range.
For the procedure please also see the attached screenshots.
You need a dealer who has the appropriate diagnostic software (PDU, WDS or IDS) to complete this adpation setting. It may be challenging to find one ...

 

Rpm at 1100 is not a good sign at all.

You do not need a new ECM. If that’s what the mechanic is telling you, you are at the wrong mechanic.

if you want to replace the Idle Air Control Valve, it’s a cheap part, but the 5.5mm bolts holding it in place will probably snap when you try to remove them.

Let me know if that’s the direction you want to go in.

 

+1 for TPS setup. where are you located? Maybe somebody can help locating a Jaguar with diagnostic tool nearby.