85 3.2 Hissing Sound

[wnsgc]

Greetings,

Last week while attending my monthly PCA meeting, we had an informal car show. There were a number of 3.2 liter engines there, so I was able to look around and compare things. One thing I noticed was a "hissing" noise my engine makes, both at idle and acceleration and this noise was absent from the other cars/engines.

I've been reading up about vacuum leaks, as I suspect this may be the cause of the hissing, but I don't really know. If I do have a vacuum leak, is there an easy way to determine where it is? I've read about a "smoke-test" to find leaks, but I don't have access to such a machine.

I've also read that back-fires could be a sign of a vacuum leak. On downshifts, I do get what I'll call "soft back-fires", not really loud, but a bit more than a rumble.

Any and all thoughts are appreciated.
Thanks.

[Rot 911]

Stick your head in there and listen. Just avoid the fan area.

[gtc]

I used to get the same backfires with the stock chip.
No idea what the hissing might be. Maybe something in the AC system, e.g. the compressor clutch isn't releasing?

[GH85Carrera]

It is probably the anti-freeze dripping on the exhaust manifold and boiling off

[wnsgc]

Wow, I didn't realize we had such comedians in the forum...

Okay, this being said, is there any real advice or should I just turn the music a little louder?

[RRico]

watch out, one of those guys me be jerry seinfeld.

[York]

I'll try with some serious thoughts.

Does the hissing sound change as the engine rpm increases?

Is your oil cap on tight? When you take if off, does the idle change? Check all the hoses and clamps around the oil fill line. Make sure the hose at the back of the air filter box is connected. Check the hoses and clamps at the throttle body, for instance the idle control valve and all others that you can see. Also check the hoses on the back of the throttle body, there are several and some are pencil thin.

Another common vacuum leak is at the intake runners where they bolt to the engine, the gaskets get bad, tear from over-torquing the bolts and such. Mine were in bad shape when I removed the intake to check and replace the gaskets. If yours are original, they probably need to be replaced. If you do it, use a torque wrench, and re-torque after a week or so, but don't over torque the bolts.

You can try spraying short bursts of starter fluid around all areas of suspected vacuum leaks with the engine running to see if the idle speed increases. If so, you have a leak, but if the spray gets to the air filter, it can cause a false positive.

[Rot 911]

Originally Posted by wnsgc

Wow, I didn't realize we had such comedians in the forum...

Okay, this being said, is there any real advice or should I just turn the music a little louder?

I gave you some real advice. You need to narrow down where the hissing is coming from. Start the engine and stick you head in there and start listening.

[Por_sha911]

Vacuum leaks will usually manifest with other problems like rough idle and poor mpg. I the car runs smooth I would keep an eye on it but not fix what's not broke. Light popping of exhaust can be a sign of running rich but your exhaust may also cause that. I have a B&B system with no cat and as a result it will have some light popping on decel.

[mackskibum]

Take a hard look at the base idle adjuster- if this is out of adjustment or the o-ring has deteriorated and the ICV compensates, you can have a hiss at idle.

[Trog]

ditto!

Originally Posted by mackskibum

Take a hard look at the base idle adjuster- if this is out of adjustment or the o-ring has deteriorated and the ICV compensates, you can have a hiss at idle.

[rusnak]

you have a k&n air filter or modified airbox I bet.

[dentist90]

If you could actually hear a vacuum leak I'd be surprised if the car were driveable. That would be a lot of air leaking in!

That said, there are some small hose connections behind the throttle housing that could be off. Not sure how audible they would be if knocked loose.

[rusnak]

I agree, you need a good ear and some refrigerator water hose to hear a vac leak.

That's why I'm leaning toward it being suction from the intake. You can hear the intake pretty good through a K&N filter. Way more than through paper filters.

[wnsgc]

Thanks for the info, I've been away on vacation for the past week. I'll pick up the hissing sound this week. Regarding the questions, I have a stock airbox, but I do have a K&N air filter. I'll look and listen towards the back of the engine compartment to see if the hoses are in good shape.

More to come. Thanks.

[rusnak]

the hissing sound is the engine vacuum. Put your ear next to the airbox intake.

[rnln]

Do you have any modification in your car, even slightly? Is the hissing sound is like the jet engine sound but much less? If so, I have the same sound. It started maybe 2 or 3 seconds after the engine start. When driving, can't hear it since I have after margket lound muffler. Sometimes on hard acceleration, I can hear it lounder, but only some times. Car acts normal, and I don't know what is it either.

[rcaradimos]

as Mackskibum wrote..

Take a hard look at the base idle adjuster- if this is out of adjustment or the o-ring has deteriorated and the ICV compensates, you can have a hiss at idle.
__________________
Mackskibum aka Dave Austin
'66 911 Work In Progress (#303734)
'85 911 Carrera
06 325XI
'05 Envoy XL SLT

I went to a auto supply store and fitted some new o-rings on the screw, also cleaned out the passage in the throttle body, the screw allows air in from this passage (it runs north and south). It was clogged w/ old carbon and oil. The hiss is from air being sucked in Thur the screw area and not from the vent passage area. If you back the screw out all the way you should see an o ring on the screw, replace w/ new. If you take the boot off the top of the throttle body you will see the air passage that the screw allows air to by-pass the TB butter fly. It is about 1/4" in diameter and goes down to where the screw goes in the throttle body. While your in there clean the bottem of the butter fly plate ...you will see what I'm talking about..clean reinstall and fly like the wind.
Hope this helps,

[wnsgc]

Where is the "base idle adjuster" many folks are speaking of?

I'm looking in my Bentley Manual right now and do not find mention of it.
There is mention of an "idle speed control value". Is this the same as the base idle adjuster? I don't see screws with o-rings that I can replace.

Thanks.

[mackskibum]

The base idle adjustment is in the manifold just below the butterfly. I believe it is a 7 or 8mm socket that fits. It faces you when you are looking at the egine from the rear of the car. Sorry, I dont have access to a photo at the moment.