Re: Erratic Idle/Surging: XJ-12

[ToToogood@aol.com]

In a message dated 97-02-03 13:03:19 EST, you write:

<<  2. Symptoms: No start/very difficult start <50 F. When started surging 
      from 750/1000 rpm. Very rough idle.
       >>
I have no experience with the XJ12 engine management systems, but on the
earlier E-Type, I would be looking for a large vacume leak.  You can use a
bottle of propane (like you would acquire with the portable soldering kits,
not the bar-b-que kind) to help find the leak.  Attach a length of rubber
hose to the propane tank, via the open/close valve.  Then, with the car
outside, with the engine running, open the propane valve a small amount and
direct the propane gas to potential sources of your vacume leak.  When you
find the leak, the engine RPM will change.

My guess is that the vacume leak is causing the engine to run partially
unthrottled and very lean.  The leak is acting like a partially open
throttle, causing the engine RPM to jump above normal.  The leak is also
causing a weak mixture, which causes the starting difficulty.

Since the mixture is weak, you are getting incomplete combustion in the
cylinders.  The remaining HC is then burning in the CAT, causing it to over
heat.  Since the driver's side CAT is overheating, check the driver's side
intake system first.  I assume you have not noticed any difference in the
braking system, and can therefore rule out a leak in the brake booster.

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