Tech Question - Butterfly valves.

[ratman]

Hi all. Need a bit of technical help. I am in the process of swapping over the throttle chamber on my S3. Having looked at the replacement it is the same part number as the original so all seemed well. When I looked at the butterfly valves I noticed that the replacement one had a small machined hole in one of the plates. The original one has the same hole but this appears to have been filled with a self tapping screw. Does anybody out there know
a) The reason for the hole ?
b) The reason for someone filling it with a self tapper ?
Regards Ian.

[andyf007]

ratman said:

Hi all. Need a bit of technical help. I am in the process of swapping over the throttle chamber on my S3. Having looked at the replacement it is the same part number as the original so all seemed well. When I looked at the butterfly valves I noticed that the replacement one had a small machined hole in one of the plates. The original one has the same hole but this appears to have been filled with a self tapping screw. Does anybody out there know a) The reason for the hole ? b) The reason for someone filling it with a self tapper ? Regards Ian.

Hi Ian

a) The hole is there to compensate for the idle control valve being on the other side, which, in effect is another hole or air bypass. If both butterflies were shut and there was no air bypass, then you wouldn't idle. By blanking off the hole in the butterfly it is forcing the near side cylinders to draw air through the rocker cover breather pipe when idling(not good).

b) Mis-guided?

Andy

[Roy_S2]

andyf007 said:

a) The hole is there to compensate for the idle control valve being on the other side, which, in effect is another hole or air bypass. If both butterflies were shut and there was no air bypass, then you wouldn't idle. By blanking off the hole in the butterfly it is forcing the near side cylinders to draw air through the rocker cover breather pipe when idling(not good). Andy

Actually, this can't be the reason as the plenum is a single chamber so therefore the idle valve would supply both banks. I would presume the hole to be there in case the idle valve packs in and closes up in order to provide a small amount of air flow to keep the engine running. Just my theory.

Roy.

[John Mac]

If there is the possibility of "back pressure" being created in the throttle body,a fuel/airmix explosion could occur and a relief for this is needed via the small hole drilled in the butterfly plate. Although not having looked at my own set up, as the butterfly plate cannot swivel and open on its own accord because it reaches a "stop", then I think this is probably the reason for the hole and the built in protection.
In your case the previous owner may have realised this but left only one open "just in case ".
Don't hold me to this but just having read up on various systems this is one of the possibilitys given.

[andyf007]

Roy_S2 said:

andyf007 said: a) The hole is there to compensate for the idle control valve being on the other side, which, in effect is another hole or air bypass. If both butterflies were shut and there was no air bypass, then you wouldn't idle. By blanking off the hole in the butterfly it is forcing the near side cylinders to draw air through the rocker cover breather pipe when idling(not good). Andy Actually, this can't be the reason as the plenum is a single chamber so therefore the idle valve would supply both banks. I would presume the hole to be there in case the idle valve packs in and closes up in order to provide a small amount of air flow to keep the engine running. Just my theory. Roy.

I stand corrected then, I'd always assumed that the plenum was in 2 halves . However, my point on the butterflies is still valid. The idea of matching the airflow through each is more to do with the balance at the airflow meters. If you block the air going through either the idle valve or the hole, it affects the airflow meter opening on the relevant side and thus the signals being sent to the ecu.

Andy

[Roy_S2]

I`m not saying your wrong Andy (don't worry ) as this may well be the reason. However, I can't see there being any problem with one airflow meter reading an amount of flow and the other one shut. Surely the ECU just adds the two values together. I suppose its one of those things that we will just never know the REAL answer to.

Roy.

[ratman]

Well thanks for all your help. As always my general car knowledge is improving by leaps and bounds as well as learning the vagaries of TVR's. I have swapped the throttle bodies over and whilst the lumpiness at lower revs has disappeared I now have an idle problem whereby the idle will intermittently rise to about 2500 rpm for no reason and then settle back to 1000. I will try the usual suspects and have already run ti without the throttle pot connected with no change. My one thought is that there could be air leaking around the paper gasket as I temporarily put the old one back. Anyway I will leave out the self tapper after the advice and thanks again - I just hope I don't have to act as referee between andy007 and Roy_S2 at some stage !!!

[andyf007]

Well, like Roy said, it's one of those things we may never know the answer to, but I've a feeling it was put there for a reason (whatever that may be), so best left unblocked.

It will most likely be the idle valve playing up, you can unplug it the same as you can with the throttle pot, just to check. I've had some erratic problems with mine and it's unplugged at the moment, but I think I've found the problem with it. It appears to be a faulty engine temp sensor causing the ecu to open the idle valve ('cos it thinks it's cold), and it revs to 1500 (cold start revs). I only came across that one because of another thread on here about engine running temperatures that matched some other symptoms it's displaying.

Andy

>> Edited by andyf007 on Wednesday 25th February 11:36

[ratman]

Well Andy I took off the throttle body again and had a good look at it. I discovered that the butterfly valves weren't quite shutting off the airflow so adjusted the stop and the bearing housing on the lever mechanism until they closed off fully. Idles perfectly now so that seems to have been the problem in my case.
Thanks again for the help and needless to say not fitting the self tapper.