850NA purge valve operation

Post by **div4scpro** » 14 Jul 2011, 16:22

what is the duty cycle of the evap purge valve? mine is pulsing at idle at all times. when is it supposed to run and when not? could this contribute to lower mileage, idle and just off idle roughness? if the answer is when the ECU tells it to, when is that?

Free2drive! · Post by **Free2drive!** » 15 Jul 2011, 10:36

This might help

http://volvospeed.com/vs_forum/topic/25 ... ntry286015

Matt
94 855 t5

FCPEURO · Post by **FCPEURO** » 15 Jul 2011, 16:38

As far as I know, it is only suppose to purge when the pressure builds up in the evap system. It should not be doing it all the time.

Post by **div4scpro** » 19 Jul 2011, 06:38

so if the purge valve is pulsing all the time is that likely the valve is faulty or whatever is sending the signal is malfunctioning?

Post by **erikv11** » 19 Jul 2011, 23:23

You can speculate and analyze that question all you want, but if you want to know whether or not the purge valve is faulty, do the simplest of the available diagnostic tests: just swap in one that is known to be working. They are all the same, 94-98, turbo or NA. Borrow one from another 850 owner for 10 minutes, or get one at a junkyard shouldn't cost you more than $15, etc.

Post by **jblackburn** » 20 Jul 2011, 00:06

Wellllllll...that depends if you've got a round one or a square one. Volvo liked to randomly switch them up. But yes, I would agree for that diagnostic. I've never heard mine clicking on and off at idle (it's a square one).

Post by **jimmy57** » 20 Jul 2011, 00:19

Purge valves do not operate themselves. The EFI ECU or the ECM runs it. The base operation is to run it when throttle is open to assure the canister is cleared of accumulated vapors. By operating the purge valve at a low flow rate while momentarily stopping fuel mixture correction, the presence of vapors can be determined by observation of front O2 sensor(s). If the indication from sensor is rich then there are vapors and purge valve operation will continue with an other check after 90 seconds or whatever software time is programmed. If the O2 sensor indicates lean purge valve operation is stopped and another test is done later to see if it needs to operated then.
After a time idling with no purge valve function the control unit will do the test at idle as vapors will accumulate with idling too. The same routine applies but at idle the purge rate flow is always flow as the engine can't consume as much when idling. If purge valve operates at lot or continuously when idling after engine has initially idled for a time then the control unit is finding reasons from its monitoring cycles that purge valve should continue to operate as vapors are still detected.
If the purge valve or hoses are partially blocked the flow for testing may be unaffected but for regular operation it is not clearing canister. Low fuel level and really hot temps and especially these days with alcohol dosed fuels, the vapor pressure is higher and the rate of vapor accumulation in canister is greater and takes more purging.
As to duty cycle, the purge valve flow rate is controlled by duty cycle with fixed frequency. 10-12% idling with the rate going up with open throttle and vapor found present. Testing is at 7% if I remember though that is a software set % and can be different for different years. The lower pulse rate and the design of the valve can make them obvious when working or not at all audible when functioning.

You can test purge valve by taking it off car and blowing through it to see that it is sealed. Apply 12 v across terminals (no worries about polarity) and it should have little effort to blow through.

All I've listed here applies to electric purge valve systems. The vacuum operated with purge valve integrated onto top of canister do not purge at idle and do not test for vapors, etc. They just have a fixed opening when throttle is above idle and pull air through canister collecting vapors only if any are present or creating a manageable controlled vacuum leak if no vapors are present.