We can't find the fault with my erratic idle. From cold is fine, then after a short run the RPMs can drop 50 and back & forth.
Then settle down for a min. and start again, you can feel the acceleration is not quite right.
Sometimes it's fine for a while, then again at a light, very erratic.
We have: changed; leads, plugs, EGR, with vac solenoid.
2 cans fuel jet cleaner. No difference.
Someone has mentioned cracked manifold, but this has been discounted.
Any more suggestions would be welcome.
(one chap suggested changing plugs again, faulty one)

 

Watch fuel trims as it does it. What happens?

BTW wrong forum.

 

I've moved your question from General Tech Help to XJ40 forum. It is a model specific issue and XJ40 owners are more likely to read it here.

Graham

 

JagV8,

That's good advice, but unfortunately the XJ40 is OBDI, so if it's possible to monitor fuel trims, it would require a diagnostic system that few owners possess, such as the Jaguar PDU.


dstjohn,

First of all, have you checked the Vehicle Condition Monitor for any stored Diagnostic Trouble Codes (DTCs)? Turn the key to position II (Run) without starting the engine. Press the VCM button on the trip computer to the right of the steering wheel, and watch for a message to be displayed in the small window below the speedometer. Write down any message that is displayed. The message may be in the form of FF44 or Fuel Fault 69, but your number may be different.

If you have one code stored, there may be others, but only one is displayed at a time. To check for additional codes, with the key at position II, pull the Instrument Pack fuse in the center console fuse box. On our '93 I think it is fuse 9, the second fuse from the left. Watch for the speedometer and tachometer needles to drop below zero, then count to ten and replace the fuse. Turn the key to off, then back to position II and press the VCM button again to check for another stored code. Record, then repeat until no codes are displayed.

If you don't have the AJ6 4.0L Diagnostic Manual, send me a private message with your email address and I'll send it to you. The file is too large to post to the forum. The manual explains the functions and parameters of all the engine management sensors and gives diagnostic routines to track down the cause of all the DTCs.

Off the top of my head, a couple of likely suspects are the oxygen sensor (O2S) and Coolant Temperature Sensor (CTS).

The Engine Control Module (ECM) ignores the O2S while its heater is warming up to operating temperature, which takes 20 seconds or so. If your stumbling begins around 20-30 seconds after startup, I would suspect the O2S.

The ECM relies on the CTS to determine cold-start fuel enrichment and the air-fuel ratio (AFR) once warm. If the CTS is stuck, or there is a problem with its wiring harness or electrical connector, the ECM may receive incorrect engine temperature readings and apply an incorrect AFR. In your case, I would suspect the CTS might be stuck at a cold reading since the engine runs fine at startup. Another related possibility is the Intake Air Temperature sensor (IAT), but I can't recall many confirmed reports of that sensor failing.

It would also be worth checking your fuel pressure regulator (FPR) for leakage through the diaphragm. Pull the vacuum hose off of the FPR and check for wet fuel while cranking the engine, which indicates that raw unmetered fuel is being inhaled into the intake manifold and the ECM cannot correct for the overrich condition.

Another possible suspect is the Idle Air Control Valve (IACV), which sits on top of the intake manifold and clogs up with carbon. It would be worth checking yours and cleaning it well if necessary. The stepper motor that controls the valve is also known to fail, but can be replaced with an identical and inexpensive part from a Renualt Megane:

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Typically an erratic or rough idle can be caused by an intake air leak or vacuum leak, but since your engine runs fine when it is cold, an air leak doesn't seem like the most likely candidate.


Please keep us informed!

Cheers,

Don

 

With many Thanks,
Now the weather here turned nasty, & last night minus -10C or 10 below F
the car drove home with no erratic revs at all!
I stopped at home left it in gear, nothing, solid 7.5 rev.
I know it will return. But it is curious.

 

So it runs better in freezing temperatures?

 

Yes, what does that tell you?

 

Move farther north?

I'd go through all the air intake associated components with a fine tooth comb maybe clean or replace the oxy sensor make sure the whole intake pipe is clear of dirt. I know theres mesh screen just past the filter box where it clips to the rest of the pipe perhaps check that first.