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High Idle XJR

 
  #1  
Old 01-18-2019, 11:22 AM
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Default High Idle XJR

My beloved XJR has always been blessed with a smooth idle, at precisely the textbook speed. Recently a pronounced intermittent misfire developed, culminating in a couple of refusals to start.
I read the codes, which produced something of a list, including Crankshaft position sensor, misfire, and coil related codes.
Having not hitherto replaced the Crankshaft position sensor, I cleared all the codes and fitted a new Crankshaft position sensor. Bingo!, all misfires gone, the car pulls like a train, perfectly smooth idle, and no codes.
The problem? It now idles at around 950 rpm in gear, 1350/1400 in neutral. I did not touch anything other than the Crankshaft position sensor, and was working from the side of the engine bay away from the throttle body, MAF, supercharger etc
Any ideas guys? It idled higher from the first turn of the key after fitting the sensor.
 
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Old 01-18-2019, 11:53 AM
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Check the fuel trims - hot engine, parked. Idle & about 2500rpm.
 
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Old 01-18-2019, 02:05 PM
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Most likely needs the TPS reset. It can be done manually, by loosening the screws to the TPS and rotating it slightly.

A one-hour labor bill by a Jag dealer will likely be your best solution.

I'm not sure why a new crankshaft sensor should affect how the TPS is read, bit I have had a similar issue when replacing the Idle Air Control Valve, and it always seems to require a TPS reset to get the idle back to normal.
 
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Old 01-18-2019, 03:33 PM
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Doesn't appear to make sense but may be worth trying - but do the trims first as it's almost free & DIY.
 
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Old 01-18-2019, 03:58 PM
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Could be you have knocked a vac pipe off somewhere causing an air leak - as per @JagV8 checking the STFT may show this.

Also, check that the throttle is actually fully closing at idle.

Also, as it's straight forward try putting the old CKPS back on and see if the idle returns to normal. I can't see any reason it would (but you never know!).

 
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Old 01-18-2019, 08:29 PM
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B1mcp has given you some solid recommendations.
 
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Old 01-19-2019, 08:48 AM
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Thanks guys. I will pursue these and report back.
 
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Old 01-19-2019, 11:32 AM
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Other then the common mechanical interference not allowing the butterfly from retarding to the idle stop

If you put your hand on the butterfly shaft tower and rotate it to the mechanical stop the TPS should read 0.60 + or - 0.02 volts DC on the Green / Yellow wire with the key to the on position but engine not running . This Green / Yellow wire is the middle wire on the TPS connector but is underneath the TB but can be tapped into as it comes over the fuel rail in the wire bundle . A scallop cut in the insulation will work

Another way to read the TPS is through the ECU connector at socket Red 12 and Red 7 and you'l l be looking at a resistance value

As you rotate the butterfly slowly through it's full range you should not see a bip on your meter indicating a open spot on the wiper arm , This bip would going to zero voltage or high resistance depending on how you are reading it

Once the ECU sees a bad spot on the TPS it rejects the signal input and defaults to a less then optimal set of ECU maps to regulate the engine

You had a TPS CEL fault to indicate this

See page X

http://www.jagrepair.com/images/Trai...20-%202000.pdf







Editing
 

Last edited by Lady Penelope; 01-19-2019 at 12:58 PM.
  #9  
Old 02-12-2019, 12:22 PM
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UPDATE
Having been distracted from the XJR for some time, I started her up this morning for the first time in about a month. Started first turn, and ran sweetly, but still too high a tickover.
I attached my code reader to look at the live data. Car quickly switched to closed loop on both vanks, and showed a STFT of 24% on both banks, with LTFT at -1.6% for both banks.
What caught my eye was the throttle position which showed 10.8% with my foot off the pedal. Pressing the pedal produced a corresponding movement on the graph, but I am now wondering what the throttle position should read at Idle. 0%? 5%? 10.8% seems high and might point to the need for a TPS reset?
I have checked under the bonnet, and cable is not snagged or too tight, and as I mentioned before, all I did was change the CKPS.
 
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Old 02-13-2019, 06:35 AM
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AJ16 engine is at around 13% when the car is idling with nothing on the gas pedal, so if anything, it looks a bit low.

The trims are obviously out. I wonder if the car is staying in open loop?

What do the oxygen sensors look like?
 
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  #11  
Old 02-13-2019, 06:31 PM
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Agree with Vee that Throttle Position looks OK.

I'm still thinking Air Leak. STFT at 24% (that's the max it can go to) on both banks shouts air leak to me.
 
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Old 02-13-2019, 09:41 PM
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From what I have observed from others the TPS is close in range for beng on the idle stop at the 13 % reading , never seen 0 %

Vacuum leak checks :

Cap the line on the top of the intake manifold # 3 cylinder going to the brake booster , this will limit your brakes in test drive . I think it's referred to as a banjo vitting with a squirrelky seal . Maybe a ball of interference tape

Cap the line on the bottom of the intake # 3 cylinder going to the EVAP system below the air filter , I found one of the EVAP hoses missing on mine

Cap vacuum line behind fuel pressure regulator

Editing

The EGR valve has a error followup position sensor if it is stuck open in a uncommanded position , It is in your ELM - 327 device as a PID








The Idle air control valve does not have a error followup position sensor so you would have to read with a meter to see if you have a open pole winding on the 4 wire stepper motor , To remove it on the supercharged model and clean the poppet valve is a chore from my understanding



 

Last edited by Lady Penelope; 02-13-2019 at 10:27 PM.
 
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