Engine bay heat - surging idle/ breakdown

["Jan Wikström" <104707.3644@CompuServe.COM>]

This is exactly the same behaviour my XJ12L showed a year ago, before I
re-cored the (partly blocked) radiator.

The overheating due to the radiator made the engine compartment hotter than
usual, which I assume contributed. The air conditioning was on strike,
which eliminated the return fuel cooling. Result: hot fuel and vapour lock
- inevitable when the pump is very large and the fuel recirculates at a
high rate, picking up heat from the engine compartment. The tank drain plug
got uncomfortably hot to touch - say 50 deg C.

Once, in an effort to charge the pipe with cool petrol and get going again
(I was on the roadside a long way out in the bush, one stinking hot day) I
disconnected the delivery pipe after the filter (pump NOT running); out
came a hot aerosol of petrol that didn't wet anything as it evaporated
immediately. Pumping into an empty drink bottle, it took over two minutes
to get the pump to draw from the (rather low) cooler tank and establish a
flow that had no bubbles in it. Even then, it took some time to get up a
pressure that would open the relief valves and push the vapour back to the
tank.

Like you, I assumed that changing to the other tank would produce a
temporary cure, but this worked only if the tank was over 3/4 full. The
final clue came when I  (in desperation) opened the filler to help the pump
out by blowing into the tank: the tank turned out to have a mild vacuum in
it.

It turned out that the carbon canister under the left front wing, whose
vent hose had long perished, was blocked by crud. As a result, the engine
vacuum connection caused a vacuum in the vent lines from the tanks, and the
pump was just cavitating (still full of hot fuel, as was the suction pipe
from the changeover valve) and failing to overcome the vapour pressure in
the injection rails.

Cure: 1. Recore the radiator; this cured the overheating problem and
sharply reduced the vapour lock (now occurs only on parking after a run in
very hot weather). 2. wash out the canister with petrol and leave the
vacuum connection off. And yes, 3. fixing the air conditioning certainly
helps heep the return fuel cool.

- Jan

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