Auto Tranny Vacuum Sound when put in gear?

MPDano · MPDano's Garage

I have an 81 with a 3 speed auto. When I put it into gear, it sounds like vacuum leaking from the rear. Also, when I accelerate, you can really hear the vacuum or air pushing from the rear of the car. Any ideas what this is or is this normal?

rhjames · rhjames's Garage

is it a vacuum line?

or an exhaust leak?

--Russ

Danglerb · 02-09-2009, 08:58 PM

Isn't there a vacuum line that runs back to the tranny?

rjm65 · 02-09-2009, 09:36 PM

Quote:

Originally Posted by MPDano

Also, when I accelerate, you can really hear the vacuum or air pushing from the rear of the car. Any ideas what this is or is this normal?

Dude, your car is so quiet I suspect the air noise you hear is your exhaust! RMB and all those funny noises will go away!!!

MPDano · MPDano's Garage

Here is the video of when I got it running on all 8 cylinders. You can hear is as I accelerate away.

http://www.porsche928forums.com/videos/928_12.26.2008.wmv

Danglerb · 02-10-2009, 11:36 AM

That sounds like intake noise, like running without the air filter.

MPDano · MPDano's Garage

Your right, I do believe I was driving without the air box that day. I guess I should try it with the box on. It just sounds like it's coming from the rear of the car. Could be the exhaust fumes getting to me.

MPDano · MPDano's Garage

I still get that noise (vacuum) coming from the rear at takeoff and when putting in gear. Anyone else with an Auto can verify that theirs does NOT make that noise?

Mrmerlin · 07-06-2009, 11:30 PM

instead of guessing just get the car jacked up and then look at the driver side of the trans you should see a vacuum line connection to the vacuum modulator, start the engine and listen, then pull the line off and see if its sucking, no sucking means a break in the line , it runs along the top side of the TT up to the a connection at the rear of the engine

vdubr928 · 07-07-2009, 12:56 AM

Pass side on your 81 3 speed. Not sure about other years. The vacuume line that feeds the modulator is metal most of the way back. There is a 3 inch section of rubber hose at the modulator on the pass side of the trans. There is no reason you would hear an increase in leakage (if it is leaking) when you hit the gas. Actually you would hear more at idle as there is more vacuume available at idle than for the first few seconds of balls out throttle. You must be hearing something else but if it makes you feel better pull that vacuume line and check for leaks. A cheap section of line from napa will verify the line is not the prob. Chack for trans fluid in the old line to verify the modulator is not leaking. If neither of these problems are present then you can rest assured that you are hearing something else. Leaky Cats can cause an airy sound.

MPDano · MPDano's Garage

Quote:

Originally Posted by Mrmerlin

instead of guessing just get the car jacked up and then look at the driver side of the trans you should see a vacuum line connection to the vacuum modulator, start the engine and listen, then pull the line off and see if its sucking, no sucking means a break in the line , it runs along the top side of the TT up to the a connection at the rear of the engine

Isn't this the same line that runs to the front of the Spider? If so, Vac is present and working as this is what makes my shifts smooth and not like I have a Shift Kit installed (hard shift).

MPDano · MPDano's Garage

Quote:

Originally Posted by vdubr928

Pass side on your 81 3 speed. Not sure about other years. The vacuume line that feeds the modulator is metal most of the way back. There is a 3 inch section of rubber hose at the modulator on the pass side of the trans. There is no reason you would hear an increase in leakage (if it is leaking) when you hit the gas. Actually you would hear more at idle as there is more vacuume available at idle than for the first few seconds of balls out throttle. You must be hearing something else but if it makes you feel better pull that vacuume line and check for leaks. A cheap section of line from napa will verify the line is not the prob. Chack for trans fluid in the old line to verify the modulator is not leaking. If neither of these problems are present then you can rest assured that you are hearing something else. Leaky Cats can cause an airy sound.

All these lines are brand new (silicone). Cat is also brand new since I needed one for Cali Smog.

vdubr928 · 07-07-2009, 08:47 AM

Do you have a vacuume pump for testing? Pull the line at the engine and test for leaks. Very simple system that Mercedes used since the late 60's not much can go wrong here. The Porsche part isn't even adjustable on my 82 so there is even less going on here than what I'm used to. Line is easy to find since it's the metal one that goes to the intake.

My vacuume tester looks like this and doesn't work worth a crap to bleed brakes but is a great auto vacuume system tester. (since it will show loss) You should not have any loss through here and if you pull the line at the engine and see tranny fluid then you have a bad modulator. Does the car shift smooth? If so you probably don't have a modulator problem since it controls shift pressure.

http://cgi.ebay.com/ebaymotors/NEW-BRAKE-BLEEDER-VACUUM-PUMP-KIT-AUTOMOTIVE-TOOLS_W0QQcmdZViewItemQQ_trkparmsZ65Q3a12Q7c66Q3a2 Q7c39Q3a1Q7c72Q3a1205Q7c293Q3a1Q7c294Q3a50QQ_trksi dZp3286Q2ec0Q2em14QQhashZitem5881dc11d8QQitemZ3801 35805400QQptZMotorsQ5fAutomotiveQ5fTools

MPDano · MPDano's Garage

Quote:

Originally Posted by vdubr928

Do you have a vacuume pump for testing? Pull the line at the engine and test for leaks. Very simple system that Mercedes used since the late 60's not much can go wrong here. The Porsche part isn't even adjustable on my 82 so there is even less going on here than what I'm used to. Line is easy to find since it's the metal one that goes to the intake.

My vacuume tester looks like this and doesn't work worth a crap to bleed brakes but is a great auto vacuume system tester. (since it will show loss) You should not have any loss through here and if you pull the line at the engine and see tranny fluid then you have a bad modulator. Does the car shift smooth? If so you probably don't have a modulator problem since it controls shift pressure.

http://cgi.ebay.com/ebaymotors/NEW-BRAKE-BLEEDER-VACUUM-PUMP-KIT-AUTOMOTIVE-TOOLS_W0QQcmdZViewItemQQ_trkparmsZ65Q3a12Q7c66Q3a2 Q7c39Q3a1Q7c72Q3a1205Q7c293Q3a1Q7c294Q3a50QQ_trksi dZp3286Q2ec0Q2em14QQhashZitem5881dc11d8QQitemZ3801 35805400QQptZMotorsQ5fAutomotiveQ5fTools

Yeah, I got BB500's tester at the moment. It shifts smooth, but I will put the tester to it since I got it anyways. Also, what kind of pressure should I test at?

Mrmerlin · 07-07-2009, 06:13 PM

vacuume is spelled vacuum

MPDano · MPDano's Garage

Nope, Gore spells it with an "e" at the end, so it must be correct.

vdubr928 · 08:48 PM

40 or so

Normal should be around 30. If you want to be sure check what you have coming off the metal side at the rear of the engine. This will not only tell you what to test at but also you can check at the trans with everything hooked up and make sure your not leaking somewhere in between. ie. you should have the same at the modulator as you have at the junction near the bell housing.

If you have good readings, car shifts smooth, and no tranny fluid in the lines then you are hearing somthing else or a leak from somewhere else.

MPDano · MPDano's Garage

Thanks and I will give this test a go when i get a free chance. Can you watch the video I posted? You can hear the vacuum(e) sound or some noise.

ice-hot · ice-hot's Garage

MPDano,
I watched your vid. and thought I did hear something like a whoosh sound not a sucking vac. sound. (that sounded stupid) In any case I did hear something that I dont hear w/ mine 79US auto. I see you have replaced the vac. line to trans. already, that is the only vac. line going theree, but it could be the modulator itself.
The test described in other post should determine this. You are really just trying to find out if there is a break in the line from the plenum to modulator.
Good luck, John

vdubr928 · 07-08-2009, 09:53 AM

Ok, I did watch the vid. Are you running without an air filter in this vid? Or do you have a cone filter upgrade? It "sounds" like you are pulling allot of air from somewhere. (diagnosis by video is never that accurate) That being said here is my opinion.

Pull the modulator line off the trans. Drive the car....... Does it shift rough? Do you notice a pretty big differece in the way the trans shifts? Is it unpleasent? Probably you are going to answer yes to all of these. If so... Forget testing the modulator, It's working. (you checked for trans fluid in the line when you pulled it... A very rare failure on these)

Ok... Now we know what it is not. No other engine vacuum* goes to the rear and I will be honest with you. If the air leak is as big as it sounds it is pre AFM. You could check the brake booster but if it is leaking (that bad) you would need a floor jack pushing on the brake pedal to stop. Pre AFM means check the air box. If you have a leak post AFM that was sucking as much air as it "sounds" like you are then the car would run very poorly. (Read probably not at all and no way it would idle) Best way to find an airleak is a can of carb cleaner. start spraying near where you hear the sucking sound (that sounded stupid LOL) and when you hear engine RPM's drop then you founds your leak. Start at the airbox.