The car is a manual '95 XJR, no EGR, no cruise, no air injection on this one. When I got the car, whenever I backed off the gas pedal the revs would hang anywhere between 700 and 2000 rpm. On a couple of drives the idle got lost completely and the car would stall when stopping at lights.

Today, idle is steady and the throttle never hangs. Here's what happened.

Diagnosis of the throttle position

First, get a suitable OBD scanner. Jog the gas pedal and observe the absolute throttle position reading. Does the percentage grow linearly as you depress the pedal? Does it go back to the same percentage every time no matter how you release the pedal? Is the percentage low enough (mine is 10.2% at idle)? Is the maximum percentage high enough (mine is 90.6% at wide open throttle)? Remove the traction control cover and adjust the cables properly. Test if it made a difference.

In my case the cables were routed awfully and adjusted far too loose. I rerouted the cables with as few bends as possible and took off the slack. Throttle reading was linear, but the percentage varied between 10.2 and 12 when the foot was off the pedal.

When the throttle gets stuck higher than the lowest value you have seen, pull the straight spring under the intercooler by hand and observe the percentage and the revs on the scanner. If it drops down to the lowest value every time, the sticking can be fixed by adjusting the spring force.

Adjusting the linear spring

Normally the right hand end of the straight spring is connected to the bottom hole of the aluminum bracket under the intercooler. The bracket has two more holes above that, which are probably for cruise control. I looked up two springs, wider, thicker and shorter than the original. I hooked the first spring on the top hole on the bracket, hooked the second spring on the mid part of the first spring and hooked the free end of the second spring where it belongs on the throttle body. All this can be done without removing any parts for access, although it can be fiddly.

Using the top hole allows for a longer spring and allows us to set up a higher pretension without getting too stiff for wide open throttle. But it all depends on the springs you are using.

Adjust the stiffness by changing the position where the left hand spring hooks on the right hand spring. If the traction control spring starts to give before wide open throttle, the spring is set up too stiff.

After this adjustment the throttle position always returned to 10.2%. I thought that was it as the throttle behavior became more consistent. But no, the idle revs were still hanging or missing when I test drove the car.

Diagnosis of the idle air control valve

I did this with the clutch pedal, but I guess the same effect can be achieved with the brakes on automatics.

I started the car, the idle was 700 and steady. I put it on first gear or reverse and slowly eased the clutch without touching the gas pedal. The idle adjusted to increased load as it should and the car started moving. Now, the clutch was fully engaged and the car had reached a steady speed but the idle didn't back down. I depressed the clutch and the revs shot up to around 1500 rpm before dropping down to 700 rpm after a few seconds.

The hanging revs were now clearly caused by the idle air valve since there was nothing else adjusting the amount of air going into the engine. Either the valve is getting the wrong commands from the ECU, or there's mechanical trouble with the valve. Since the sensor data for engine speed and throttle position were fine, I thought it must be mechanical.

Inspecting the throttle body and the IACV

Time to take the throttle body off the car.

Lift the front of the car on stands. Remove the rubber elbow between the MAF and the air duct. Remove the oil filter. Then detach the throttle cable. Remove the straight air duct connected to the throttle body. Drain the coolant (or pinch the coolant hoses connected to the throttle body). Unplug coolant hoses, IACV and throttle position sensor. Unbolt the aluminum bracket and the throttle body. And the throttle body is out.

The bore was a bit grimy, but the butterfly was moving freely and had a slight gap at fully closed position, looks good. Gave it a wipe.

The TPS rotates smoothy and the spring action is there. Gave it a good jog and put it back to it's place.

Off comes the IACV. The piston and the bore were grimy and sticky but not too bad. Wiped the parts clean.

Oddly the IACV piston had a polished spot on one side. This means the piston is not hitting the center of the bore. IACV collar appears to be millimeters smaller than the hole it's supposed to fill. I don't know if it's the wrong part or something, but with standard flat gasket it's not going to align correctly without care.

Aligning the IACV

When the IACV plug is connected turning the ignition on makes the piston retract a couple of millimeters. Turning the ignition off makes the piston, after a few seconds, extend out all the way until it hits a wall. Then, it backs up a couple of millimeters to it's resting position.

If the IACV is out in the open when you perform this test the piston will come off the cog and fly to the direction it's pointing along with its spring and sleeve. Hold it with your fingers unless it's bolted on.

To align the IACV I first centered it the best I could and tightened the bolts very loosely allowing some sideways movement. Then I connected the IACV plug and cycled the ignition while looking at the IACV piston through the throttle body hole. The alignment looked good and I carefully tightened the bolts.

I put the throttle body back on the car and went for a test drive. No more issues.

Conclusion

In my case the odd throttle behavior was caused by badly routed and poorly adjusted throttle cables, throttle linkage not returning fully due to insufficient spring tension for worn parts, and misaligned and dirty IACV.

For the first time during my ownership the car rides as it should. Looking forward to clocking some kilometers this summer. Yay!

 

Thread deserves a bump

 

Superb write-up. Many thanks! I've bookmarked it for future reference.

 

I have been using the car as my daily driver since the idle was sorted. And I'm happy to tell you the issues have not returned and the throttle behavior has been consistent. I'm quite enjoying the ride.

I then turned to address the intermittent miss at idle and low load. The miss has always been there and I have read statements that it is supposed to be there (especially on a '95, cam design or something) and Andy bracket might not help the issue either, but gave it a go anyway.

Britishparts.co.uk had reasonable priced coils so I went ahead and ordered a full set along with a new crank sensor.

First I replaced the crank sensor. Curiously, I noticed improvement in low load throttle response (take off, gear changes). It had no effect on the miss.

The coils were brand new Lucas branded items (as was the sensor), all black, no stickers, logo embossed on the side. I replaced the coils one by one testing the idle after each swap. They made zero difference.

So, I'm starting to believe the miss is not ignition related. I'll probably move on to O2 sensors, injectors, fuel pressure and maybe let a Jaguar specialist have a go. But that will all be in another thread and probably after the winter.

Cheers!

 

I went through a three months of misery trying to track down an idle misfire. In my case it occured only after the engine warmed up, usually became worse as the car was driven for longer periods, and was sometimes quite severe.

I don't have time right now to mention all the checks and replacements I made.

On nothing more than a whim I replaced the spark plugs. The problem was instantly solved. The existing spark plugs were only 7-8 months old and looked perfect.

I have no explanation for this.

I went from Champions to NGK but I don't know if that means anything. In the past I have run both types in this car with no problem.

Just thought I'd mention my experience.

Cheers
DD