The last two weeks my1986 XJ V12 just started going downhill� I
have done something and can�t find what.
First the feedback monitor relay shorted (the diode inside) and for
an instant the relay that controls the supplementary air valve was
connected the wrong way� that was my doing�.
Replaced the relay but then the fuel pump was not working�.
Replaced the fuel pump, but old one turned out to be stuck and not
At the same time fixed some vacuum leaks that I had found.
But since then my idle went for a walk� It�s very low, extremely
rough, cyclic and nonexistent when cold, barely present when warm,
and absolutely nerve racking�
There is also some backfiring on the left side manifold.
It has a tendency to overheat on uphill cruise speeds, possibly
Also there is some pre-ignition happening although the car was
tuned four months ago.
Ever since I got the car the idle would fall very low after a long
drive and until now I thought that this was because of a stuck Aux
air valve, and even though it has a hot start sensor mounted on the
fuel rail hot starting was always difficult.
There is also a difference between how much gas I put in and how
much the trip computer says, about 5 to 8lt at a 45lt tank.
Otherwise the car is running just fine and three months ago drove
6,000 Km across Canada, Victoria BC to Montreal… a fabulous drive
ECU replaced six months ago (6CU, DAC 6337)
All injectors are clicking and injector hoses are new,
Distributor was serviced a year ago; at least that�s what I�m told,
Auxiliary Air Valve is working, rebuild with a 74 degrees Celsius
thermostat bulb two months ago,
Throttle links adjusted according to the book.
Engine breather filter is clean.
Air filters are clean.
Opened the throttle potentiometer and has no excessive wear,
Vacuum advance system seems to work fine,
Do not know if the vacuum advance timer works but the supplementary
air valve works when 12v are applied.
When I disconnect the air temperature sensor idle comes back to
When I push the roller microswitch on the throttle pot idle is good,
When I disconnect the vacuum-operated switch nearby idle is good,
When I cut off the vacuum from the left side fuel regulator, idle
is good, but there are signs of fuel leak in to the vacuum.
When I disconnect the jumper at the ECU, force the system in to
closed-loop, nothing happens,
Starter inhibit switch on the shift lever is not shorted and
switches on and off but when I manually push it, while being in
Park or Neutral and engine running, nothing happens,
When I disconnect the coolant temperature sensor things get worse,
It’s been two weeks of strugle…
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