vacuum leak; possible culpret - Alfa Romeo Bulletin Board & Forums
 
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post #1 of 12 (permalink) Old 09-25-2016, 09:43 AM Thread Starter
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vacuum leak; possible culpret

My S3 always felt (rough idle and lack of power) like it had a vacuum leak. I could never find it. When I removed the windshield washer reservoir (starter swap), I noticed a thin disconnected hose leading down through the firewall below the high pressure ac hose. This had become disconnected, at some point, from a nipple just below the brake booster hose connection on the intake plenum. What is this for?
Since I have not yet replaced my starter and can't run the engine, I was wondering if this could be my problem...lack of power and rough idle! ....Or did I accidentally knock it off (I hope not) when I removed the ww res?! Could it have run, although poorly, with this much vacuum leak?

87 spider grad, 06 audi cab, 12 prius, 79 bellanca viking
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post #2 of 12 (permalink) Old 09-25-2016, 10:17 AM
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Where does the other end of the hose go to? Pics are helpful, if you can. It sort of sounds like one of the hoses that go to the carbon canister in the RH wheel well 'secret compartment'. It would then attach to one of the 3 nipples there. If the hose is old, it could have just come apart also, so trace all of the vacuum lines. You might be able to find a vacuum hose diagram in the FAQ section...

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post #3 of 12 (permalink) Old 09-25-2016, 11:05 AM Thread Starter
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This hose is connected to a plastic line that passes thru the firewall into the passenger compartment. just below the high pressure ac hose. NOT into the area behind the wheelwell

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post #4 of 12 (permalink) Old 09-25-2016, 11:14 AM
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Oh, ok then- that's probably the altitude compensator line; it should start by the ECU and it's connected.....somewhere in the engine bay :P I can't remember at the moment...

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post #5 of 12 (permalink) Old 09-25-2016, 11:41 AM Thread Starter
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Definitely goes to the plenum, right below the tap for the brake booster

87 spider grad, 06 audi cab, 12 prius, 79 bellanca viking
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post #6 of 12 (permalink) Old 09-25-2016, 12:24 PM
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Quote:
Originally Posted by mgsteve View Post
Definitely goes to the plenum, right below the tap for the brake booster
sounds like the connection to vacuum sensor? (that thing in the back, that likes to 'whistle while it leaks'!)

edit: this thing
http://www.alfabb.com/bb/forums/90609-post1.html

Dom - Alfa Spider 1990 S4 - formerly: Alfa 101 Sprint, 2600 Sprint, Montreal - family classics: Jensen Interceptor II, '58 Hooper RR Silver Cloud I, Shadow II, '60 Corvette.

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post #7 of 12 (permalink) Old 09-25-2016, 01:56 PM Thread Starter
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Where exactly is the vacuum sensor?

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post #8 of 12 (permalink) Old 09-25-2016, 05:25 PM
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Behind the seat passenger side just above the second ecu on the fender wall just forward of the wheel well. The panel held by carpet screws I believe.

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post #9 of 12 (permalink) Old 09-25-2016, 05:54 PM
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Quote:
Originally Posted by mgsteve View Post
Where exactly is the vacuum sensor?
Item #8, under the side panel behind the right door.
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- - Eric
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~ 1984 Spider Veloce ~
- -~ 1981 GTV-6 ~
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post #10 of 12 (permalink) Old 09-25-2016, 07:02 PM Thread Starter
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Thanks Eric, In your opinion, would the car run, but poorly, with this vacuum line disconnected?

87 spider grad, 06 audi cab, 12 prius, 79 bellanca viking
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post #11 of 12 (permalink) Old 09-26-2016, 08:35 PM
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I've messed with mine while hunting down vacuum leaks. The car will idle slightly better when the hose is properly connected. You won't notice a difference under throttle. The plenum hose just slides over the hard plastic line running down the firewall...no hose clamp but won't hurt if you add one.

[FONT="Microsoft Sans Serif"]Don - 1984 Spider Veloce[/FONT]
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post #12 of 12 (permalink) Old 09-27-2016, 07:27 AM
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The vacuum sensor tells the computer when the engine is running at high vacuum (part throttle/easy cruise conditions) and the computer will then advance ignition timing for better fuel economy.

As long as you plug the plenum port (so there isn't a vacuum leak) you will likely not notice a difference in driving the car. Fuel mileage might be a bit lower.

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