In article >, Sanity Clause > wrote:
>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.

Note that when this system is cycling, you can hear it going ka-thump,
ka-thump in back. It should be one of the things that you listen for
when the car is starting up. If you can hear it going on continuously
all the time it should be pretty clear something is not right.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."