Low Hiss In Front Left Wheel Well

[81SCRosewood]

Hi Pelican! Two days ago I completed a 7 1/2 hour drive up California and my 1981 911SC ran flawlessly as always. Yesterday, I fired it up and it idled like an old truck. Felt and sounded like it was missing on one cylinder. The car even shook a little. I turned it off and left it until the end of the day this Friday.

I did a quick visual check on the cap & rotor, ignition wires, etc. Everything looked fine so I started it up again to see if I could see or hear anything while it was running. It was missing again as before. Nothing in the engine seemed out of sorts.

A buddy of mine heard a low hiss coming from the front left wheel well. I confirmed and then shut the engine off. The low hiss stopped as soon as I turned the engine off.

I've never heard that low hiss before. Could this be the cause of a missing engine? like a vacuum leak of some kind?

I'll pull the spark plugs to inspect this weekend and anything else I can think of to check.

Any ideas or suggestions would be appreciated from the Pelican Community

Thanks in advance!

BTW: the low hiss is NOT from the tire

[DRACO A5OG]

Maybe your AC line? It lives there too.

[81SCRosewood]

Good suggestion. I removed the AC a few years back, but did leave the ac lines. Could this still be a factor?

[DRACO A5OG]

Only if compressor is intact and connected with the ac belt on. But I am sure you removed it.

The only other thing would be the Fuel Pump.

[jason2guy]

you might be hearing the fuel pump or maybe the brake booster vac line, if it somehow
developed a leak. i would look for a vacuum leak creating a lean condition or possibly something ignition related- cdi, green wire. youre going to need to check it all so thats a starting point
the vacuum leak im talking about is airbox, sleeves, intake runners.....

[Ronnie's.930]

It could also be a hiss coming from the battery if it is being overcharged by a faulty voltage regulator - might want to check that out and see if battery acid is coming out of the battery vent (ASSuming you don't have a sealed battery, that is).

If too high voltage is the cause, it could be cause of the rough running as well.

[emac]

I agree with Ron with the battery--but I would check the booster for a vacuum leak. Look for any loose hose
Ernie

[81SCRosewood]

Thanks for all of the suggestions and ideas. I'll do them all today and report back.

Thanks!!!

[ossiblue]

It's not likely an overcharging battery as the hissing would not begin immediately upon start-up nor would it stop immediately on shut-down. The hissing is from the expulsion of gas as the battery acid solution boils off, a process that requires a few minutes to start and to stop. This doesn't mean to ignore that as a possibility, just that your symptoms don't fit that possibility very well.

A more likely cause is a vacuum leak in the brake booster system, as others mentioned. That will definitely cause rough running, and will appear and disappear immediately upon starting/stopping of the engine.

Not sure where the evaporation setup is on an 81, but the hiss might be from a leak in the hose from engine shroud to the charcoal canister--if it's located in the trunk on an 81. If so, it could account for the hiss but would not cause rough running of the engine.