Upon starting a cold engine, there's a discernible humming sound that continues for, say, 45 secs. to a minute, and then disappears. The sound is definitely emanating from the engine compartment.

Occasionally the sound will briefly return for 5 secs. or so when idling for a stop light change, and the sound is then accompanied by a slight (50-100) rpm increase.

Could this sound be the SAI system operating? (How does that thing work anyway?)

relinuca

 

Yep, as you describe it, it's likely the air pump.

 

What DareDevil said.

Mostly likely the humming sound (with my cars it sounds more like a Hoover vac running in the distance) is the secondary air injection pump.

This runs at cold engine start to inject air into the exhaust system to cause the unburned gas to burn and heat up the converters quicker.

The pump runs from around 30 seconds to a minute, roughly. (By my observation when the pump shuts off the DME switches from open loop operation to closed loop operation.)

Anyhow, I have never heard the pump come on at any other times though.

If it isn't the secondary air injection pump, another explanation is that what you are hearing is not the secondary air injection pump but the gas tank vent system and the evacuation of the charcol canister. This can sometimes sound like a pump is running. (In some cases it can be a pump (pressure or vacuum) running that is used to check the integrity of the vent lines and vapor recovery system.)

(I've never heard any noises from my Porsches's tank vent/vapor recovery systems, but my 06 GTO's vapor system would emit 5 faint knocks -- like someone rapping a knuckle on the car's body -- in rapid succession when the DME was venting/evacuating the canister.)

If is it the gas tank vent/vapor recovery system that accounts for the noise, the engine can react a bit to this as it is being fed gas vapors from the canister which means that temporarily at least the engine sees a bit of a bump in the fuel in its fuel air mixture.

Sincerely,

Macster.

 

Yes -- that's what I've been hearing of late, too, and, although it's not that loud it is at an odd frequency that starts to get annoying...right about the time it shuts off.

-Eric

 

Occasionally, It'll kick on as I pull to a stop.

 

once had a girlfriend that could hum and

 

Most likely it is cooler now where you are. The onset of cooler weather always brings out these noise posts that most often turn out to be the secondary air injection pump that the new owner is hearing for the first time, or that the owner may have forgotten about since last time.

I sort of like hearing the sound, because when it stops I use that as a signal the engine is 'warmed up' enough to drive off, although of course I know very well it is not completely warmed up.

And it probably helps that at least to me the noise is not very loud and neither pleasant or unpleasant the ears.

Sincerely,

Macster.

 

Fond memories eh? Makes for a nice road trip.

 

Its sole function is to bring the cats up to temp, and keep them there, quickly. It sounds more like a table saw (cutting wood) when it starts to go bad.

 

Thanks for the useful and informative replies (and the old girlfriend comment! ha.), especially Macster and Daredevil.

relinuca

 

It's normal for the SAI pump to run at times other than cold startup.

 

Someone just asked the same Q on Pelican and it was suggested that it was his IMSB failing.

 

 

Video clips (with sound) or it never happened!

 

I vaguely recall coming across some info that backs your statement up but I could not find anything in my sources on this so I hesitated stating that the SAI pump could run at other times.

It may happen when the DME does its continous monitor check of the secondary air injection system.

With my Boxster I do not recall ever hearing the SAI pump come on any other time other than at cold start. In fact, a few times I had to shut off the engine before the thing timed out and upon restart the SAI did not come on again.

There are 3 possible selections for the SAI pump's output: upstream of the 1st converter; downstream of the first converter's inlet; and atmosphere (off).

Upstream and atmosphere (off) I understand, but downstream? Maybe that's used for the continuous monitor test?

I have a data logger than I can set up to capture the commanded SAI status...

Sincerely,

Macster.