XJ XJ6 / XJR6 ( X300 ) 1995-1997

high idle

Old 03-30-2019, 06:43 PM
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Default high idle

I've been driving Jags since 1982. And work a good bit on them as an amateur. But this is a new one on me. The ECM died about a year ago on my 1996 VDP. It was an intermittent thing though and took a long time to diagnose. I was able to find one with matching numbers (makes it plug and play) and that fixed the problem. I thought it would be prudent to have a spare on the shelf so found another one on Ebay with matching numbers so bought it. I swapped the ECMs a few days ago to test the spare. The engine ran just fine except the idle is not about 500 rpms higher. I did a hard reset but have not rechecked to see if that made a difference. Also put new Champion plugs and new QYL coils just because.

Interesting note.....I bought the spare ECM several months ago and have yet to find another one on Ebay with matching numbers????

Any thoughts out there?

Old 03-30-2019, 07:28 PM
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The ECU is " oriented " to match the TPS setting

The ECU part numbers has nothing to do with orientation , but software differences between different areas of the world using the same ECU hardware

Some areas do not require a EGR

The TPS is kind of fixed as it has very little adjustment as the mounting holes have very little wiggle room

On paper the tolerance of the sensor middle wire return signal to the ECU is 0.60 + or - 0.02 volts DC on the idle stop

Where your TPS was originally at is an unknown but your replacement ECU was probably " oriented " very close to the TPS value at idle stop

An example would be a TPS at 0.61 and your original ECU at 0.61 as the null setting , your 1st replacement ECU by chance still had the same 0.61 and worked as a plug and play

Now the current 3rd ECU may have a orientation of 0.59 and since your TPS doesn't get below 0.61 the ECU believes it is not at idle stop hence the higher RPM

To add a complicating factor the IAC step motor may not be allowed to step down to its final almost closed position but the position before that as it schedules down , or the IAC may be bad and staying open

The Fix :

Drill out the size of the mounting holes on the TPS so it can be adjusted to match your 3rd ECU

Clean out the throttle body of crankcase vent residue that prevents the butterfly from contacting the idle stop

TSB on replacing the return spring on the butterfly

See page 11 of the TSB as issue # 3


Last edited by Lady Penelope; 03-30-2019 at 09:35 PM.
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