[xj-s] 1994 6.0 V12 - Idle Speed and Gear Loss Issues - Related?

Hi All,

I have a couple of annoying problems on my 1994 XJS 6.0 V12 which
sometimes seem to go hand in hand.
The first is engine speed. Description: Start the car, leave it in
park. Engine warms up, idle drops back to 750 rpm after higher
initial cold idle speeds. Leave it in park or neutral and it will
run for hours, idling nicely around 700 -750, even when engine gets
Problem is that when put it in drive, reverse, or any gear, whether
or not you are actually moving, very soon after the idle gradually
drops down to 600 then 500, engine gets very lumpy and rough,
almost stalls, sometimes has, then the check engine light comes on
and it immediately starts idling normally - 700 in drive and 750 in
neutral/park. There is a noticeable delay when you prod the
throttle from standstill most of the time. The car rocks and
sometimes backfires. Sometimes the engine hardly reacts at all.
Once already spinning at speed though, the engine turns nicely.
Once driving, you can often feel engine power/willingness drop in
and out, but not a total loss. It is quite strange, the exhaust
note changes to a dull drone and there is a major loss of power. It
still goes but like a very sick car. Push the throttle harder to go
and it sort of wakes up, splutters/backfires and then goes. The car
feels flat/dull and not eager to go.

Second problem is a loss of gear when in motion. Driving along, in
gear, randomly the car loses gear engagement, it is literally like
being in neutral, rolling and the engine revs freely when throttle
is pushed. The gear cuts back in and out randomly. This happens
often in fits and starts, but generally disappears after a while
into the trip.

Parts replaced: distribtor cap and rotor, throttle position sensor,
ignition leads and spark plugs, both ignition amplifiers and both
coils. The additional engine ground strap is fitted.
I have reset the ECU any number of times and disconnected and
checked contacts on the fuel injection ECU in the boot, which is a

Plausible ideas?: Vacuum or air leaks? The gearbox is GM-4L80E,
electronic 4 speed. Gearbox management issue? Crank positin sensor?

Any ideas would be greatly appreciated. As the 6.0 litre model is
rare in Australia I do not have access to spare ECU modules, e.g.
fuel injection ECU, for my car which means I am unable to test mine
against a new or same part from someone else.

I know this is a long message, thought I’d better give the full


Sydney, NSW, Australia
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In reply to a message from AdamT sent Mon 16 Apr 2007:

I should have mentioned the check engine code is FF44 then FF45. I
know this is fuel failure which can mean O2 sensors but have been
there done that also.
Sydney, NSW, Australia
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In reply to a message from AdamT sent Mon 16 Apr 2007:

Did you have a reputable mechanic install the TPS with the factory
PDU adjustment? If you didn’t were you able to dial in the TPS
voltages yourself? Was the TPS installed before you were having
issues or after? Did you purchase the o2 sensors online or from a
reputable source? There were some bogus Bosch sensors that were
being sold last year that looked genuine enough but were cheap
knockoffs and did not work. Can you remove the o2 sensors and run
the car through it’s paces to see if that improves anything? this
might indicate blocked cats & they might need to be replaced.
Are you using an aftermarket dizzy cap/rotor? So far we have not
found any acceptable replacements for the factory Marrelli caps and
the others fail at around 20,000 miles. Have you established that
the second fuel pump is engaging at 2500 RPM? It’s rare on these
cars but you might have a coil issue. Do a vacuum leak check all
around and make sure your dizzy is clean & dry inside. Let me know
if anything I mentioned solves the issue or not. We have some
complicated cars but yours sounds like it’s being a bit of a PITA.
I doubt it would be the ECU but if you need a tester I can send you
one with out too much trouble.–
Pacifica, CA, United States
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