Idle surge after long battery disconnect/1994 XJ40 4.0l

Hi,havent been on sight for awhile.My problem is idle surge 900 to 1500 after car had sat for awhile with battery disconnected.Ihave researched this alot so I have tried alot of things.No check engine light or codes.I disconnected the battery cables and connected them together for about 2hrs to do a hard reset?Then drove it for about 12 miles.I did remove the throttle body assembly to clean and also the IAC assembly.Did a reset on the IAC motor per post I found here.The TPS sensor is newer and checked the voltage at fully closed (0.61v).Even during the drive after getting up to speed when releasing throttle it would buck.Did drive it on expressway.this happened before awhile ago but was corrected just driving around the neighborhood.Just FYI I am a retired recently retire tech of 47 years,ASE Certified MAster .Just not this problem.Any help will be greatly appreciated.

It’s been a while but the TPS voltage sounds high to me?

actual manual says to keep it close to 0.60v to avoid ECM having to relearn idle everytime battery is disconnected.???TPS was replaced awhile ago,7 years maybe

By ‘reset’ do you mean this procedure?

"Provided by AJ6 Engineering, 60, Henshall Road, Bollington, Macclesfield, Cheshire SK10 5DN, England
Email, Tel/Fax:- 0044 (0)1625 573556

4.0 Litre

  1. Run engine until fully warm.
  2. Switch off ignition.
  3. After 5 seconds switch on ignition - wait 5 seconds - remove connector from stepper motor.
  4. Switch off ignition.
  5. Wait 15 seconds then replace connector on stepper motor.
  6. Repeat operation sequence 3 - 5 twice - on the last occasion do not reconnect stepper motor.
  7. Start engine and set base idle to 550 - 600 r.p.m. via air bypass adjuster.
  8. Switch ignition off and reconnect idle stepper motor.
  9. Restart engine - idle speed should now stabilise around speed shown below.
  10. Check idle CO as for 3.6 (if non-cat).
    Specified Idle Speed.
    Cold in neutral - 800 (Auto)
    Hot in neutral - 700 (Auto)
    Cold in drive - 650 (Auto)
    Hot in drive - 580 (Auto) (3.2L = 650)
    Hot in Neutral - 800 (Manual)

Note: Cold idle speeds will be slightly higher at very low temperatures.

If the above procedures do not result in stable idling check that the throttle potentiometer is correctly set. Whilst the ECU will automatically compensate for minor errors within the range of the adaptive memory, a badly adjusted throttle potentiometer can cause spurious drivability and idling problems."

An idle surge, or hunt (up and down), is what I experienced when I tried to use a Renault Megane IAC stepper, which some J-l’ers have done successfully. When I gave up on that and replaced with a Jag-spec unit the problem went away. Because you have not recently replaced yours, my guess is it has gone south on you.

that is the would think bad IAC would set engine code BUT.Will try new one.By the way I have a Rat Fink coffee cup I use every morning.Thanks for the advise.

That is awesome!! Gotta love the Fink. :+1::+1: Keep us posted as to the outcome.

Knowledge is power, and collective knowledge is - well, a LOT of power!

Check the hose connection from the IACV at the intake elbow. That plastic tit is easily crushed by over tightening the clamp and causing a small vacuum leak.

In fact check the security of all your hose connections between the MAF and the throttle body - as well as the hoses themselves.

did already check.replaced boot from MAF to throttle body when I did the chains and guides.

I have the same symptom on my 94 aj6 xjs (same engine and ECM). pulled the elbow to the throttle body. My MAF was slightly high, at .72 volts. Rotated it down to .45 volts. Also, screwed out the valve inside the IACV, and found the tip had considerable carbon on the tip, which I cleaned. Upon reassembly, Idle is stable on battery reconnect. Jazzboy, did you find the issue/ a solution for yours?

Corrected “MAF to TPS”

Hey there,been kind of busy with other projects.So to update I took the IAC out and inspected it(which I thought I already did)and found the plastic shaft on the pindle had cracked and split.Replaced it with a new one and vehicle runs fine now.Didnt even have to do the idle learn procedure.I would like to thank everyone for their help.

Thanks for the update. The power of the Fink (you still using your Rat Fink coffee cup?).

Yes sir,just about everyday unless in dishwasher.then I use my MG mug. :wink: