Hiss between shifts
Thread Starter
Instructor
Joined: Apr 2018
Posts: 110
Likes: 24
From: Siler City, NC
Hiss between shifts
I have a 99 coupe 6 speed with around 130k and recently I’ve noticed a hissing sound between shifts that is very audible when the car is cold. It seems to go away after some time but always is there during the first 30 minutes of driving. I am also experiencing some high idle when coasting to a stop, then it will idle back down to normal. I’ve done some research and I’ve seen some posts that talk about air coming out of all the vents no matter what position the selector is in, however I’m not experiencing that problem, just hearing the hissing. I did spray brake cleaner around the intake and throttle body while it was idling but never heard any difference in idle. Maybe it’s normal but I don’t think so. Anyone experiencing the same problem and know where I should start looking?
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,613
Likes: 4,061
From: Cape Coral, Florida
If you can actually hear it must be a pretty big leak...the high idle is indicative of a vacuum leak and you can check the brake booster and PCV area...if you want to find it quickly it may be worth it just to take the car to a shop that has a smoke machine...you can find it quickly with that method and with a scan tool if you see your long term fuel trims high at idle that would nail down a leak too !!
Thread Starter
Instructor
Joined: Apr 2018
Posts: 110
Likes: 24
From: Siler City, NC
That's a great idea! I will call around and see who has a smoke machine. The reason I was wanting to see if anyone else had figured this out is because most vacuum leaks I have experienced were audible under acceleration, not when you are letting off the gas like you would between a shift. That is when this hissing sound is most audible on my car.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,613
Likes: 4,061
From: Cape Coral, Florida
A leak will mostly be heard at idle because that is when there is the most engine vacuum....as the throttle plate is opened engine vacuum will decrease...if you smoke a big fat stogie that can work too as a smoke machine...just blow the smoke into a vacuum line....LOL !!!
Burning Brakes
Joined: Jan 2006
Posts: 788
Likes: 105
From: Olathe Kansas
I would also check the lines to the Air Injection System.. They are at the pump, air filter housing, and exhaust.
The fresh air pump usually just runs on the cold engine.. so if it goes away once hot...?????
The fresh air pump usually just runs on the cold engine.. so if it goes away once hot...?????
Thread Starter
Instructor
Joined: Apr 2018
Posts: 110
Likes: 24
From: Siler City, NC
Foggy, I removed the AIR pump several months ago due to it not working at all. It was actually full of water and was throwing the 1410 code so I pulled it all out, plugged the holes on the exhaust manifold with block off plates, then had a tune done to remove the code. My mechanic put it on his diagnostic tool yesterday to see if there were any lean conditions and didn’t see any. I still want to throw some smoke at it soon.
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Thread Starter
Instructor
Joined: Apr 2018
Posts: 110
Likes: 24
From: Siler City, NC
Thats a good question, I want to do a torque tube rebuild at some point next year just so I have the peace of mind that everything is in good shape. I purchased the car with around 120K on it so I know its probably due a rebuild and a new clutch. I knew the original owner and he did take very good care of the car, thats why I didn't have any hesitation on buying the car, he had the complete history of everything that had been done to it. I built a homemade smoke machine last night and am waiting on the sealant to dry so I can run some smoke this evening and see if I can find a leak. I'll definitely keep you posted.
Running Guns & Moonshine
Joined: Aug 2001
Posts: 12,656
Likes: 7,296
From: CT
I have similar in the cold on my 99. I have determined it is not the engine at all. If I depress the clutch, no noise. It is only as I move the shifter and reengage the clutch. Sort of a low hydraulic noise as I thought of it this morning. I'll check my master cylinder reservoir later.
