So I have a 86 TPI motor with 90k.

When the car is run and then shut down, for say at least 10-15 minutes there is a strange hissing noise that seems to be coming from around the throttle body but all the vacuum lines are good, no fuel is leaking. Car doesn't seem to have any issue with vacuum.

For all I know its normal but I never remember hearing that when I had my 89 iroc. Only a buzz from the alt when the key was on but that's it.

 

It is either the vacuum line for the EGR under the TB, or evap canister on the side, or the coolant line that goes through the bottom of the TB. Those are the only things I can think of.

 

Possibly the air filter aswell. I know mine hisses like crazy while the car is running, and when I shut it off, it seems to suck air until the engine has completely stopped turning. I wouldn't think the coolant lines would do anything in the TB though, that's weird. Mine has a TB coolant bypass, what could possibly cause it to hiss though?

 

He said it lasts 10-15 minutes, which could be vacuum escaping from one of the lines slowly, or in the case of coolant, it could have a leaky gasket under the tb or a cracked coolant hose that is slowly bleeding off pressure from the hot cooling system.

 

Oh wow, didn't even see the 10-15 minutes. Mine lasts maybe 4-5 seconds lol. Big difference! I'd definitely check all vac lines though. Even if they're all hooked up, they might be dry-rotted if they're original. Even if they just have little lines when you bend them, they need replaced.

 

so it appears that there is a tiny pin hole leak around the neck of the thermostat housing. lucky i have a new one but since im in there im going to put in a new t-stat. any suggestions?

i have a 94 lt1 in my 79 cutlass and i put in a 160 degree t-stat to keep her cool in the cali heat but i also retuned the pcm to see that.

how cool can i go with the tpi with out tuning?

i see i can get 190-180-160, i need it to pass a smog check so its gotta get warm enough to go into closed loop but i want to keep it as cool as i can get it.

 

Have you been running the A/C before shut down? As the high/low pressure sides equalize, you'll hear a hissing sound.

 

No ac in the car. I know the sounds you are talking about though.

 

180 won't need a tune, although it would help to have the fans turn on a little sooner.