yesterday wife and I took our new to us 1985 c4 for a ride to Knoxville to show the grandkids the car. the car run great 75mph highway about 50 miles no problems just before pulling in there drive way we heard a loud howling noise coming from rear of car got out and fount noise coming from under gas cap must be fuel pump. shut off car let it set 5 min. turned on no noise. Anybody seen this? could fuel pump be going? if so what is the new pump choices? thanks for help

….. I suspect that the fuel tank vent is not working … Evap + charcoal canister … what you heard was probably air being sucked into the tank either through or around the filler cap …..

what is the next step should I replace canister or is there a way to check operation?

I have an 85 and before my fuel pump went out I heard the noise. When hot the motor is whining, and vibrating, but after replace it is quiet .

….. The Factory Service Manual has the complete troubleshooting flow chart for the EECS … Evaporative Emissions Control System … But the most common issue is the vent tube that runs from the fuel tank to the charcoal cannister … it is fairly small in diameter and made of steel and is prone to getting clogged by rust and other gooey byproducts of evaporating fuel … try disconnecting it at both ends of the car and blow compressed air from the rear to the front … if you can isolate the crud from entering the fuel tank , you can try blowing air in both directions … once the air is flowing , the tank is vented again and should stop your whistling/howling noises as you consume fuel from the tank …..

I will give that a try Thanks

….. If you heard the noise with the fuel filler cap off , replace the fuel pump ! …..

No noise stopped with cap off but there was a lot of pressure behind cap I am thinking plugged tank vent might be the right call we are getting some nice weather down hear in SE TN over the next couple of days and I am going to do the testing and cleaning evap system I also have a hole lot of vacuum lines to replace that might take care of a interrelate surging idle I am having as well.

C409 I think you were wright I could not blow through vent line at the fuel tank so I sprayer carb cleaner in hose and used compressed air and got a bunch of rusted crud out hope that takes care of it I am replacing all my vacuum hoses tomorrow they are shot. Thanks for your help