XJ-6 idle problem seem to be pretty common, but I haven't seen any reported like the one I'm having.

My 1996 XJ-6 idles between 1100-1300rpm, but only when the engine is "cool". It idles fine after a cold start (900rpm), and after warming up (700rpm). But if I drive it someplace and park for 30-60 minutes, it will idle fast on restart until it warms up.

In detail, the problem manifests like this - on a cool restart the idle will start around 900rpm, then step up (every 2 seconds or so) to 1000, 1100, 1200, then step down to 1100 and back to 1200. After sufficient warm up (usually a few minutes on the road) the idle speed will drop down to normal (700) in one step.

So far the dealership has incorrectly diagnosed the problem as:
* idle air control motor (replaced)
* throttle position sensor (replaced)
and has performed several "set" procedures, checked for vacuum leaks, and checked/cleaned the throttle body for oil residue. We've also tried temporarily replacing the ECM with a shop spare, but the problem persists.

After the first two costly-but-ineffective repairs the dealership assured me they would get to the bottom of it "at no additional cost", but at this point they've pretty much given up, and are now recommending replacing the ECM with a remanufactured part, at standard shop rates, with no guarantee that it will fix the problem.

They're shooting in the dark and expecting me to pay for the bullets. I'm hoping somebody here has seen this before, or knows enough about this engine's idling mechanism to suggest some things that could cause this problem.

Any thoughts much appreciated!

 

Did the dealer do a TPS calibration after the TPS replacement? It is done with WDS or IDS software.
It would be interesting to know what the TPS parameter reading was when the system fails to idle down vs. other times.

 

I believe so, but I'll double-check. Do you think poor TPS calibration could make the idle speed depend on a cold/cool/warm engine like I'm seeing?

That the problem is related to engine temp makes me wonder if it's due to some sort of incorrect measurement the ECM is getting from the engine about its temperature.

I can ask the dealer about that, but just so I know a bit about what I'm asking for, is there one value called "TPS parameter", or is it a collection of values? What does it indicate?

thanks!

 

Hi crs.xj6,

The first suspect that comes to mind is the Engine Coolant Temperature sensor, or ECT. The ECM uses the signal from the ECT to make corrections in the fueling based on engine temperature, which is most important at startup when the engine is cold. If the ECT is slightly out of range (resistance below spec), the ECM may be applying too much cold-start fuel enrichment.

Other suspects could be the Mass Air Flow sensor (MAFS) or Intake Air Temperature sensor (IAT). The ECM uses those signals to adjust ignition timing, so if one of those signals is off, it may be advancing the timing too much. Looking at the IAT function, if there was corrosion or looseness on its electrical connector the ECM might think the engine is colder than it really is and advance the timing unnecessarily.

I tend to not suspect an O2 sensor or a problem in the A/C compressor circuit because the ECM ignores the O2 sensors until they've achieved full operating temperature, and I believe it also delays engagement of the A/C compressor for some period of time after startup.

As Ross mentions, the dealer may be able to see where the problem is by viewing Live Data of the TPS, ECT, IAT, MAFS, etc.

Please keep us informed.

Cheers,

Don

 

If the TPS is not indicating "closed throttle" within a very tight tolerance, the idle control system is deactivated. So, no, miscalibration of the tPS does not exactly match your symptom of temperature dependence, but it will explain no idle control and open loop idle could manifest itself as your symptoms.

Of course, Don is right about the possibility of a host of other causes. I don't see how they could recommend a rebuilt ECM if the replacement one does the same thing! I do not recall my XJ6 ever idling at 1100 rpm + when the idle control was working except for a very few seconds on startup.

It might be a good experiment to unplug the ISSC while the ignition is off (it resets zero after power down), then crank the car and observe the behavior.

 

Issc?

 

ISSC Idle Speed Stepper Controller??? It is the stepper motor "idle valve"

 

is this the same part as the idle air control valve (or motor)? I know where the IACV is, so I believe I could do this experiment myself, but I'm not familiar with very much else that's under the hood. To be sure I understand, you're suggesting starting the car with the IACV unplugged, is that right?

Thank you all for the help, these suggestions seem like promising leads to take back to the dealership.

 

Have you slackened and readjusted the throttle cable? Or, if you have traction control, throttle cables?

Don't buy any more parts until you do

Cheers
DD

 

No traction control, and fortunately/unfortunately I'm not doing this work myself. If I'd know in advance how helpful this forum would be, though, I might have considered it... :-)

From what I've heard, the dealership mechanic has been all over the throttle mechanism, so I assume he's set the throttle cable as he sees fit, but I'll certainly double-check with him about that, thanks!

 

I think the proper name is the IAC. I don't remember where I got the idea Jaguar called it ISSC, but I am pretty sure I was wrong.

 

Hi Ross,

You're not far off. These terms vary by manufacturer and are easily confused.

According to the AJ16 Engine Management System Dealer Training manual, the Jaguar terminology is the Idle Air Control Valve (IACV), which regulates the flow of air through a variable bypass passage that is regulated by a stepper motor connected to a cone-shaped valve. Other manufacturers use different terminology, such as Idle Air Control (IAC), Idle Air Control Actuator (IACA), Idle Control Valve (ICV), Idle Air Speed Control (IASC), Idle Air Speed Motor (IASM), Idle Speed Control Motor (ISCM), etc. I think Jaguar may have even used some of those other terms in some documentation for all its various models. It's hard to keep track!

In Jaguar X300 terminology, Idle Speed Control (ISC) is a function of the ECM and occurs at closed throttle when road speed is less than 3 mph. The ECM adjusts the IACV, via the stepper motor, to maintain the target idle speed depending on engine temperature, engine load, and A/C compressor clutch operation. Other manufacturers use alternate terms, such as Idle Speed Control System (ISCS), Automatic Idle Speed (AIS), etc.

Note that IACV and ISC appear in the Jaguar AJ16 EMS manual, but do not appear in the list of acronymns in post 13 of this thread, so the list needs more updating:

https://www.jaguarforums.com/forum/j...inology-37054/


You know a lot more about stepper motors than I do, but for the benefit of others, the stepper motor has two coils that are pulsed by the ECM. The pulses are phased 90 degrees apart. The order in which the coils are pulsed determines the direction of stepper motor travel to either increase or decrease the valve opening to allow more or less air to flow through the bypass. The precision with with a stepper motor can be positioned gives the ECM precise control over idle speed. You can test the stepper motor coils with an ohmmeter, but if you apply 12V to test the motor you will probably just fry a coil. Your suggested test of disconnecting the IACV stepper motor electrical connector is the best starting point for a diagnosis.

Cheers,

Don

 

Don:
Also, it is worth mentioning, if you power down the car with the IASC removed from its bore, the stepper is commanded a large number of closed pulses, used to calibrate its position by bottomming out the pintle in the bore hole, and you will eject the pintle into the nether regions of your workspace. Ask me how I know about that!

 

Would you kindly share the procedure for slackening and readjusting both the throttle cable and traction control cable? Thanks.