I took my brother out to drive my car a couple of weeks ago (and let him drive). We beat on the car prety good. Towards the end of the ride, the car started making a loud humming sound under the hood.


Initially, it sounded like a very small vacuum leak that was causing something metal to vibrate. After checking everything out and listening more, it sounds more like a vibration in the fuel system centered around the driver's side TB. The noise sounds almost exectly like the noise you get when you just crack open a faucet in the house - it is a high pitched and loud whistling/vibrating sound.

The only reason why I think it could be a vacuum leak is that the pitch of the noise tends to change based on engine load. But this is unlike any whistling vacuum leak I have heard before.

I had been driving the car off and on for several months without this noise. Recent changes fuel system changes include: stiffer FPR spring, DCS FP gauge assembly, set FP to 15 psi (the minimum allowed by the spring)

Any ideas on what could be causing this?

 

Sounds like your regulator is making noise. I'm not 100% sure, but I think it could be caused by not having enough pressure on the spring. You said you have it backed off all the way, well I think it may be too far.

Nice '69 by the way.

 

If I put more tension on the spring, FP will go up. It's at 15 PSI right now and the car is running rich on current stock computer.

I have a 7747 ECM modified with a HAM and the EBL Flash. I have been slowly replacing the old parts on the car and wanted to have all issues sorted out before switching to the new ECM. This noise makes the car undriveable because it is so loud at idle.

I may need to go ahead and replace the ECM so I can boost the FP. I will ultimately need 19 PSI to support the HP level of the engine.

 

You can jump the fuel pump relay with the car off to pressurize the system. This should make the regulator make the noise and then you can adjust the pressure from there to see if that's your problem. I understand the car won't run (or runs rich) with higher fuel pressure, you're just doing this to track the problem down. You'll like the EBL system. I have the "classic EBL" and I love it. Someday I plan on purchasing the flash version.

What's HAM?

 

Harness Adapter Module... GM changed the configuration of the ECM-to- wiring harness connector sometime in the mid 80s. The later model 7747 ECM won't physically plug into the '82 wiring harness. Later ECMs used pin-type connections, mine is a card-edge connector. The HAM modifies ECM board to accept a pin-type connector.

 

Try using either a piece of hose to your ear or a wooden stick to your ear to identify exactly where the sound is coming form. With the hose put the end close to the suspected sound, with the stick put it right on the suspected noise source.

Paul T.

 

It's a vaccume leak around the base gasket. As you open and close the thorttle it changes the way air flows through the TB which changes the pitch of the vaccume leak. Some times you can get a vaccume leak that is silent, but the IAC motor will be almost closed also causing a hissing sound. You can also do the ole starter fluid trick and see if you can find the leak that way.