Hi guys,

My friend's 75 280 has always made this strange hissing from the intake: https://youtu.be/mjrzNhHvXL4.  Any ideas what this could be?

We took off the cone filter and when we did we could hear it from the afm, so we took off the afm, and could hear it through the throttlebody, when we took off the throttlebody the sound stopped (kept the engine running by pulling the temp sensor cable and left the tps switch in to run rich enough for it to not die).

We've checked multiple times for vacuum leaks using both a smoker and a can of starter fluid on the manifold and there are no leaks.  We also tried 2 other throttlebodies and both make that noise.  I've never heard if this issue - has anyone encountered it?

 

-chase

Put the factory air cleaner back on and see if it's still there.  Automotive engineers spend a lot of time figuring out how to quiet intake noise.  

People pay good money for "cold air intakes" to get that noise.

Take a legth of hose and use it like a stethoscope, move it around and see if you can pin point the noise. You may here it good at the intake opening but see if you can here it somewhere else.  Like around the manifold or injectors

22 minutes ago, Zed Head said:

Put the factory air cleaner back on and see if it's still there.  Automotive engineers spend a lot of time figuring out how to quiet intake noise.  

People pay good money for "cold air intakes" to get that noise.

The problem is the noise is there with the cold air intake removed.  It's also there with the afm removed haha.

5 minutes ago, Patcon said:

Take a legth of hose and use it like a stethoscope, move it around and see if you can pin point the noise. You may here it good at the intake opening but see if you can here it somewhere else.  Like around the manifold or injectors

The sound literally comes through the intake piping all the way to the throttlebody

You got my point backward.  The "CAI" lets the noise out.  The factory air filter keeps the noise in.  It's just normal air flow noise.

12 minutes ago, Zed Head said:

You got my point backward.  The "CAI" lets the noise out.  The factory air filter keeps the noise in.  It's just normal air flow noise.

The strange thing is my 78 has the exact same air filter and doesn't make that sound the only difference is I have the weber big-throat throttlebody

I believe he has the stock air cleaner so I'll pass the message along to give that a shot though

Edited August 14, 20222 yr by chaseincats

Double check that the PCV valve isn't stuck open and letting in excessive air.

Update for you guys.  The PCV valve even though its a few weeks old very much is letting air in both directions not to mention the pcv hose is apparently split towards the top.  He's going to get a replacement valve and snip off the torn portion of the hose.

When I hear how it went I'll update you all.  Thanks so far

I spoke with him and he bought a brand new valve but said he could blow through both sides there too.  He read later on that blowing into these doesn't tell you anything, you have to suck on it to get an actual answer of if its bad or not - is that correct?

13 hours ago, chaseincats said:

not to mention the pcv hose is apparently split towards the top.

This is the most likely cause of the noise.  The system is not "intact".  During normal operation of an engine with normal blowby there won't be very much flow through the valve.  Just enough to clear out the gases.

Edited August 15, 20222 yr by Zed Head
+ of

Update for you - the pcv valve was fine when he tested it per the fsm but the hose was still split which has since been fixed.

The noise is still there but with a new coolant temp sensor and pcv hose, the idle dropped from 2500 to 1300 with the thottlebody screw placed all the way in.  Changing topics from the noise to the idle speed, we've found all the vacuum leaks with the car dying at this point when you take the oil cap off.  With the idle screw all the way in on the throttlebody, could this be an ecu issue causing it to run so fast?

If we pull the temp sensor connector, the car's idle will drop drastically (maybe down to 400 or so) but that's simply because the car is running in max rich mode.  Any ideas besides vacuum leaks that could cause such a high idle?  There really can't be any leak areas left especially after smoking the engine and using a good chunk of starter fluid blasting the manifold...