After much much searching and questioning it turns out there is actually very little information out there on the troublesome little part that is the 'Water Slide Valve / Additional Air Slide Valve' which controls Idle air intake on L-Jetronic equipped E30's.So if you too are in the same boat and wondering 'what the f**k does this weird little H-Shaped valve in my engine bay do' or you're simply having idling issues then this may be of assistance:
The OEM part number for this is: 11531287303 - Part 11 in the following Diagram.
Although it actually looks slightly different, as the one in the diagram is a marginally different version of the same thing.
When this valve is operating correctly the 'Air' end should be open when cold - allowing extra air into the intake when idle, when the engine gets up to temperature the valve should close on the air end as the hot coolant running though the coolant side of it heats the element up.
It should look like this when 'open' and the engine is cold:
You should be able to see a spring and a slot for the air to pass through.
When the valve is closed you should no longer be able to see this return spring and air should no longer be able to pass through:
Yes, for those eagle eyed out there that is indeed an oven glove I'm wearing!As will be explained shortly...
It is a common issue that this valve may become either stuck open or closed which can cause Idle issues, the engine doesn't get enough air when idling cold as the valve is stuck closed causing low/erratic idle or the engine gets too much unmetered air when warm causing high idling because the valve is stuck open.
This is explained and probably better worded in the WIKI : AIr Intake WIKI
I had two of these valves.One which was already in the car and one which I found as a replacement after being adamantly informed by my specialist BMW mechanic that it was faulty and it needed to be replaced to sort out my high idle when cold.Now if you read my above sentence about how the valve functions you will see that the valve cannot really cause a high idle if malfunctioning when the engine is cold. This lead me to be somewhat sceptical about this diagnosis and I also had no way of knowing if my replacement 34 year old part would actually function any better than the one I had in my car already.I needed to find a way to test the functionality of the valves...
Time for the oven glove! I figured I'd stick both valves in the oven at 100 Degrees for about 15 mins to simulate the conditions of the engine heating up.As the oven warmed up I could actually see the internal mechanism moving and by about 10 mins later both valves had fully closed, functioning entirely as intended.After removing them from the oven what was really surprising was how quickly the valves then opened up as they cooled down.Turns out that I am lucky enough to have two fully functioning valves, but equally unlucky that despite the advice given this is not unfortunately going to solve my idling issue.
If after testing in this manner your valve does not seem to be functioning the bad news is that they seem very elusive and hard to get hold of, although they strangely appear to still have been manufactured upto 2016!.Another avenue is to either completely bypass it and live with a slightly odd cold idle or try possible alternative alterations (which I cannot vouch for) but seem like plausible ways to update this slightly archaic system somewhat.One of which is to install a later Motronic Idle Control Valve (ICV) as outlined in this article from the 'E30 Restoration Bible' by Andrew Everett:
I'm hoping that this information is of at least some use to anyone with an old 320i or similar which has this pesky valve.
Need technical Q/A then you're in the right place
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