Is it really Dieseling? That rocking, rolling shuddering running? Or is it
running normally? If the latter then it is probably as Paul Root has said.
A design error led to the ignition relay still being operated, hence
powering the coil, after the ignition switch had been turned off. This
wasn't discovered on North American spec cars because the anti-runon system
was so good that it shut the engine off instantly instead. With the
ignition off the switch extends a fused 12v to one side of the valve, which
because the engine is running has a ground on the other side from the oil
pressure switch, so the valve operates. The valve shuts down the engine by
sucking the fuel out of the carb jets, after a couple of seconds the oil
pressure drops, opening the oil switch, which releases the valve with an
audible click, even though the ignition switch still has 12v connected to
the valve. The next time the ignition is turned on the 12v is disconnected
from the valve, so that by the time the engine starts and builds oil
pressure closing the oil switch, the valve cannot operate again until the
ignition is again turned off. It was only when problems develop with the
emissions plumbing, or the anti-runon valve itself, that the error is
apparent. By comparison, when UK cars got the ignition relay the problem
was discovered instantly as they had no emissions kit or anti-runon valve,
and the wiring was altered to correct the problem. It seems this change was
never made to North American spec cars. As well as hoses from the main
canister to fuel tank and rocker cover it should have one to the inlet
manifold, and the anti-runon valve should have one to the carb overflow
port. If you don't want to recreate that then you can do the wiring mod the
factory did to the UK cars. Actually they did two mods but only one is
required - they moved the ignition warning light connection (which was what
was causing the problem) from the white at the ignition switch to the
white/brown at the relay, and they also moved the coil feed from the
white/brown at the relay to the white at the ignition switch. Either mod
should have cured the problem, and moving a significant load from the
relatively robust relay to the relatively weak ignition switch seems
perverse. But by moving the coil load to the ignition switch they could
have left the warning light wiring alone and still cured the problem.
PaulH.
----- Original Message -----
From: "Scott Bradney" <scott_bradney@msn.com>
To: <mgs@autox.team.net>
Sent: Saturday, January 10, 2004 5:03 AM
Subject: Engine won't stop
> Help,
>
> I have a 1980 MGB that recently had the engine completely rebuilt but now
it
> won't shut off it continues to diesel (motor on) after turning off the
> ignition.
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