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Old July 16th 18, 07:36 AM posted to rec.autos.tech
Sanity Clause[_3_]
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Posts: 11
Default Difficult starting

micky wrote:

> I saw on a TV court case where a shop told a Volvo owner that his loose
> gas cap had cause his something something pump to run all the time and
> that caused it to burn out and set the check engine light.


Leak Detection Pump.

It's Part of the EVAP (aka Evaporative Emission Control System), and
tests the fuel tank to make sure it can maintain pressure and vacuum at
certain levels, which means fuel vapor (fumes) won't escape.

It's not a typical spinning-electric-motor type of pump, but a set of
solenoids, diaphragms, valves, and orifices that are activated by the
ECU. Close the external air vent, open some vacuum ports, and measure
how fast the tank gets sucked down to a set level. Open/close other
valves, then pulse the diaphragms to slowly pressurize the tank, and
measure that. If the readings don't change fast enough, or stay steady
long enough, the system assumes there's a leak, and the Check Engine
light will turn on. Cars are usually only tested once per "warm-up
cycle", and only for a few minutes, so all leaving the gas cap loose
does is set the light, not "burn out" the not-really-a-pump.

That concludes today's lesson. Class dismissed.

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