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Hello fellow M5 owners,
I had my suspicions about my intake flap, it seemed that lately my e34 wasn't pulling as hard as it did when I first got it.

Anyway, after reading some previously posted threads, and viewing a video(which I can no longer find, HELP) on the way it is supposed to function, I have come to the conclusion that it is supposed to do a selfcheck upon start up. Well it didn't.
I had the hose that you can see on top of the plenum replaced not too long ago, just because it looked aged. What else could it be? Is there something that is ALWAYS the cause?

Any other info would be appreciated. How many parts is it? Any other way to test it?

91 S38B36. US Spec
 

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On a 3.6 , the resonance flap should self test immediately after keying the engine on .

If it doesn't and all pipework is sound , then the solenoid is a likely culprit .

D
 

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Discussion Starter #3
O , and I forgot, is it dangerous to be driving the car with it not functioning?
 

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The flap is just a throttle body that opens and closes to let the air freely flow across the plenum at specific times.

It stays open at lower rpm's to create maximum torque and avoid air intake restriction. Once the flow reaches a certain velocity the need shifts towards more power, the butterfly closes and air flows straighter into the engine minimizing turbulence.

You can zip tie your flap shut and test it. You will see that your low end drops dramatically when the butterfly is closed but the car should pull (normally) hard passed 4k rpm's.

You can also connect a vacuum pump to the flap and see whether the flap is holding vacuum (this will tell you whether the flap does not remain shut under vacuum). You may also have a leak in one of your hoses, apply the same technique plugging the pump into various hoses to find leaks.

Visually, the flap should actuate as you rev the engine even when stopped (check with your hood open). When warm, simply rev the engine high enough to see the flap actuate.

Is your car not pulling as hard down low, mid-range, or in the higher revs?
 

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I think my flap diaphram is bad. When I start it it tries to move but just cant get it up.;) It is getting vaccum from the selenoid so I belive it to be the diaphram must be bad.

I think I will vaccum test to see if i can get it to stay up.:nono:

How much vaccum does it take?? Is there any info on that??


cheers
cherrsagai
 

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Really doesn't take that much.
You can test both ways, just put the vacuum gauge at the end of the hose and read vacuum there when revving the engine. Then apply vacuum to the flap and see how much is needed at a minimum to get it to close.

You may also have a bad check-valve or reservoir. Basically they can both get clogged and stuck. A good cleaning usually helps.
 

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A slick way of checking if it works while driving is to put a dab of heavy grease on the stop for the flap and go for a drive.

If the hose on top of the plenum looked aged, then all the other ones are too - and there are a whole mess of them under the plenum. A crack in any one will cause problems. Luckily, hose is cheap.

There was a very good file (a powerpoint I think) showing how to diagnose and repair this whole system on www.bmwe34m5.com but that site seems to have disappeared... If I can find it, I'll post it here.
 

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Discussion Starter #9
Allright. So it defifinitely does not function. (tried the open hood trick and revved the engine, no go).
How much should overhauling the system cost? A few hundred? A thousand?
 

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In most cases, you'll probably find it's just a broken or cracked vaccuum line. Not sure of the price on the mechanical bits (the diaphragm an the valve) - but the rest is just standard 7mm vacuum hose and connectors that can be found at any autoparts store for about $10 in total.
 

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If the flap checks out ok, what woods said.
Taking the plenum off isn't that difficult except for a few clamps underneath that are not so easy to access.
It gives you a chance to clean and check the throttle bodies at well!
I just replaced my vacuum lines (89 E34 M5), and the flap actuates on startup but only does it once, everything I have read says it should actuate twice for the self test. Any thoughts?
 

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I just replaced my vacuum lines (89 E34 M5), and the flap actuates on startup but only does it once, everything I have read says it should actuate twice for the self test. Any thoughts?
Hola Gary ,

Get the oil temperature up , open the bonnet and then blip/ fully open the throttle up to 4250 rpm whilst watching the resonance flap mechanism .

If it actuates it works.

D
 

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Hola Gary ,

Get the oil temperature up , open the bonnet and then blip/ fully open the throttle up to 4250 rpm whilst watching the resonance flap mechanism .

If it actuates it works.

D
Thanks for the quick reply David. I will give that a try but I don't think its working, I put a twist tie around the top of the shaft, it moves when slid down during the start sequence (actuating once), I re positioned it took it for a good drive, and it had not moved once I re-opened the hood (bonnet). I will give your method a shot.

Does anyone know how to test the electrical portion of this system?
 

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Electrical test is basically the same as what davidoli mentioned. Difference being, you'd unplug the valve that opens the vacuum to resonance flap actuator, and replace it with the test lamp. The rest of the test is the same. Blip the throttle as per post above and observe the change.

Keep in mind, when the engine is off, there's no vacuum in the accumulator. Imagine your valve is permanently open. Once you turn the engine, and build vacuum, you could see the activity that you've observed. But at higher revs, you'd see no change if the valve is stuck. Test above will show if there's electrical activity present at required times. Presence of electrical activity, and lack of mechanical activity could indicate a bad valve.

Hope this is helpful
 

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Thanks guys, I got around to testing things today, the flap does not actuate when blip the throttle (full open). I will have to replace the valve. Does anyone per chance have a part number? are these things still available?

Thanks, Gary
 

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Gary , it may be the diaphragm or it may be the control unit .

The diaphragm is no longer available from BMW as far as I am aware .

I have a good used one that I can send to you if required .
 

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The Diaphragm is fine, I tested it by applying vacuum, even before I started messing with the vacuum lines. I t even actuates on startup but only does it once....
 

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For future readers, there is no need to buy used or reman'd units - we make a full actuator from 100% new parts all sourced in the USA, assembled in the USA, with some material upgrades over the OEM unit: https://www.angryasssolutions.com/parts/intake-actuator

We've sold near a dozen units, one to a BMW master technician at a US BMW dealer.

For the vacuum side, I'd hardly recommend a cheap auto parts store refurbishment - there is more to it than lines. We have you covered there too, with optional upgraded materials like modern braided vacuum line, and stainless steel fittings instead of BMW plastic: https://www.angryasssolutions.com/parts/s38-intake-system-vacuum-maintenance-kit

The Diaphragm is fine, I tested it by applying vacuum, even before I started messing with the vacuum lines. I t even actuates on startup but only does it once....
Gary, you are concerned that the valve inside the plenum is not actuating? This is based on the fact that the actuator only cycles once instead of twice?
 
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