Auxiliary Air Filter of Injection Pump

[Sleek Lemur]

Hi All - I'm really getting good value from this forum !

Our service manual says "Before installing the new auxiliary air filter, check with the engine running at operating temperature, that the auxiliary air of the warm-up device has closed down."

Can anyone explain how I check this ?

Many thanks, as always.

[Pawel66]

Basic is: you listen to it.
When you start engine cold, you will hear a relatively noisy "Hishshshshshsh..." near the IP. Not "Hisssss...' but "Hishshshshsh...". It should die away after 2-3 minutes or so. This is when the auxillary air flow is shut off.

There are also other, more sophisticated methods, related to the lenght of the WRD push rod, but the basic is to listen to it.

Those other methods: please review posts search with "WRD" and the Technial Manual. You are Full Member, so you have access to Starting Aids - it is there.

[mrfatboy]

With FIP air filter off and hot engine running there should be no loud hiss of air being sucked into the WRD at the opening.  You can also place you palm over the opening and feel for any suction.  Should be none.

[Jordan]

Unscrew and remove the auxiliary air filter.  Start you engine cold and if you place your finger over the opening where you unscrewed the air filter you should feel suction.  If you keep your finger there the idle will drop a lot and may even stall.  As the car warms up you should feel less and less suction from that hole.  Once warmed up there should be no suction.  Now you know your WRD is working properly and you can reinstall the filter.

[wwheeler]

I think it is fair to say that there are probably many cars that still have a tiny amount of suction even once completely warmed up. That is just wear between the air valve and the bore on these 60 year old pumps and not harmful. In proportion to the amount of suction when cold, this leakage would be like 1%. With this tiny amount of suction, the engine rpm should not change when you cap it with your finger. 

[Jordan]

I think it is fair to say that there are probably many cars that still have a tiny amount of suction even once completely warmed up. That is just wear between the air valve and the bore on these 60 year old pumps and not harmful. In proportion to the amount of suction when cold, this leakage would be like 1%. With this tiny amount of suction, the engine rpm should not change when you cap it with your finger.

What Wallace said.  I have the tiniest bit of suction from mine when fully warmed up but it does not alter the idle when I cover the hole so I figure it is OK.

[Sleek Lemur]

Thanks to all, ever-helpful!

Question, is this process a necessary step in changing the Auxiliary Air Filter, or rather a separate test for the WRD.

Forgive the constant posts - am learning the car !

[Jordan]

It's more to check to make sure your WRD is operating correctly since you have the filter off.  There are no real steps in changing the filter other than unscrewing the old one and screwing back on the new one. 

I believe the 230SL filter is different, although they seem to look the same, than the 280SL so make sure you have the right one for your car.

[Sleek Lemur]

Many thanks to all !