1984 Ford F150

I have a 984 Ford F150 with only about 85000 miles on it. Its in good condition but I have just recently had a problem. Right when I turn the car on I hear a hissing sound coming from under the car. It sounds like air is being let out of the bootom of the truck. This continues until i turn it off. Can anybody give me advice?

My guess is that you have a leak in the exhaust system. If it were in the vacuum side you’d likely also be experiencing operating problems.

get this looked at ASAP. A leak in the exhaust could be allowing carbon monoxide into the cabin. You can’t smell CO, but it can kill you. It can make you fall asleep and crash or it can kill you directly. It displaces oxygen in your bloodstream.

If not an exhaust leak (which is a real possibility), it could be a noisy fuel pump. I have heard a number of Fords (and some Chryslers) with a really noisy, “hissing” fuel pump. Seems to happen a lot in mid-late '80s rigs.

The hissing might be coming from the automatic parking brake release diaphram motor.

Ford had parking brakes in that era that would automatically release if applied when the engine was started. This was done with engine vacuum applied to vacuum motor just above the parking brake pedal.

Start the engine, and stick your head under the dash and look above the parking brake, see if you can tell if this where this hissing sound is coming from.


This truck also had a smog pump with a divertr valve. these were known to whistle at times. LEE