Ed Taylor

Seems that my diverter valve has died since it will not hold a vacuum and therefore no air gets to the cats. What would be the downside of plumbing the air pump to the cats full time, eliminating the valve and plugging the vacuum line?
TIA for yoour help.

Sean79 5spd

I don't realy see a downside. Not sure why the diverter valve is required at all. The only thing I can think of is that there may be conditions under which the cat doesn't operate well when there is too much O2. Doesn't seem likely though.
You have a later MY so it is possible the O2 sensor could be affected by the additional O2.

Ed Taylor

Wouldn't bother the O2 sensor as it is upstream of the air injection pipes on 87 cats.

Old & New

It may pop a bit from the exhaust when you are decelerating.

Z

The full throttle fuel mapping goes pretty rich on at least the S4 and later cars at the upper RPMs. With the diverter valve switched to always blow air into the cats, under those rich conditions the cats would get very hot. They could either melt or start a fire if that were to occur.

Ed Taylor

Now I'm confused. I thought the purpose of the added air which the valve supplies when a vacuum is applied, was to help burn off excess HC and to reduce NOx by reducing exhaust temp. Am I missing something?

BarryW

Under full throttle there is very little vacuum so the valve would be shut except at idle or deceleration.

Ed Taylor

Barry, Isn't it the other way around? Greater air velocity through the intake creates higher vacuum?

BarryW

No, the wide open throttle plate lowers vacuum. Vacuum is strongest when the throttle is closed with the engine running. The throttle plate creates the vacuum.