Dear Jag Enthusiasts,

After a cold start my 2003 XJR jumps out of the garage due to a high cold idle speed. There are no error messages and the car drives drives fine otherwise.
What should I be looking for to lower the cold idle speed?

Thanks,

Stefan

 

The car (PCM) is likely adding fuel based on where sensors say the accel pedal is and/or TPS reads and/or the MAF reads so use OBD live data to look for clues.

 

How long are you giving it after starting the car before moving it? Normally they’d settle down after a few moments of running to a sub 1000rpm idle, so maybe check the obvious things like there’s nothing caught on your pedal or lift the plastic cover over the throttle body and check the cable isn’t snagged.

 

It might be worth the time to do a hard reset and force a throttle relearn.

 

Plug a code reader in & see what temp it shows when you first start the car? I'm wondering if a temp sensor is out & the car thinks it's a lot colder than is so cranks the idle up further than needed.

 

Normally, the idle would scale back after a short while. I take it that is not the case in your situation.
There could be several things causing that, however my first thought would be to let it idle a while to
see if it would scale back by itself, observing how long it takes to do so.....if it does.

 

Stephan, all good suggestions above. Can you tell us what revs it goes to and how long it takes to fall back to the standard idle?

Normal process is + or - 1000rpm for 30 seconds then down to 500-600rpm - cold starts are high fueled and the EGR valve activated, this feeds warm exhaust gas back into the engine to speed up cold start.

There's a couple of things you can do. If you have a code reader that displays throttle position in percentage - you can see it climb and fall as you press the accelerator pedal, see what it is at wide open throttle or WOT (it does this without the engine running of course, just ignition on) you can reset it via an insert on the throttle cable bracket is located on the throttle body. A lot of posts about this procedure.
Cleaning the throttle body bore and plate, a soft cloth and some carb cleaner to wipe any residue from the bore or plate edge, careful gentle cleaning without forcing anything, the plate will let you access the edge if pushed gently
Also, a hard reset....
This reloads the base throttle body calibration map into the ECU and the engine relearns the fueling - don't be alarmed on first start after the hard reset because the ecu/throttle go through a high idle startup that decays back to almost stall point then back to it's happy place, let it idle for 10 mins to store the new map. Once this is done it should be back to normal operation.

If not.... There could be an issue with the EGR valve, faulty fuel pressure regulator, an unmetered air leak, a faulty MAF sensor etc so any codes at this point would be helpful.

 

Hi All,
Thanks for your excellent suggestions. This is a 2003 XJR with 139k miles.
The temperatures are now in the 40s. The initial RPM was 1000, which dropped to 800 RPM after 5 min and to 700 RPM after 7 min.
I plugged in a code reader and discovered P0172 and P0175 while all systems are green per code reader. Life data monitoring is not supported with my tool. The stored readings are as follows:
Fuel System 1: Calc Load: 56.9%, ECT 185oF, LTFT B1: -3.9%, STFT B1 -24.2%
Fuel System 1: Calc Load: 56.9%, LTFT B2: -2.3%, STFT B1 -25%
MAP: 15 inHg
Engine RPM: 1789
Vehicle Speed: 40 mph
IAT: 55oF
MAF: 4.69 lb/min
TPS: 13.3%

History related to P0172 and P0175:
- Repaired MAF sensor connector pins in 2021
- Replaced MAF in 2021
- Replaced EGR valve in 2014
- Replaced MAP sensor in 2013

Next steps: Clean the throttle body, hard reset, EGR valve, air leak,...

Thanks,
Stefan

 

Lots of tools can read live data, even an elm327 (about $5) so probably worth getting one.