Every so often I have been hearing at idle a horrible noise coming from the lower right side of the engine compartment. I have shut the engine off each time this has happened until tonight. I opened the hood and found that directly behind the AC compressor, directly below one of the AC hoses is where the noise is coming from. It blows straight back (or down) onto the motor mount. It is an intermitent sound, bssssst........bsssst,bsssst........bssssst....... bssssst sort of like that....?

Does the AC compressor have a pressure relief valve? If so, what would cause the excess pressure to be building?

David

 

Yes it does and it typically opens as a last resort short of something popping.

It vents excess pressure. Common things that might cause it to open would be running the AC and not having any air flow through the condenser coil in front of the radiator. ----> Check your engine cooling fan's clutch.

Another more common cause is the system being substantially over-charged with refrigerant. At idle, there's a lot less air flowing through the condenser to cool the hot refrigerant as it exits the compressor. Said pressure can run well over 400psig.

It will find somewhere to come out.

Hope this helps.

 

Today I was talking with the mechanic who, for years has worked on this truck for the previous owner. Anyways, during our conversation as he was describing exactly what you have, I decided to peek under the hood.........I made a discovery......

Seems that the previous owner didn't like waiting for the engine to warm up and attempted to speed the process up by placing a (radiator sized) piece of cardboard between the radiator and the condenser coil.

I took it out and problem solved! All is well once again.

 

There ya go then.

Many do run with a blanket of some sorts in the colder regions with proven success. Seems there's discussion on the proper design and placement so as to minimize odd fan blade damaging air currents. I'm ignorant of it though.

 

I pop the hood, leaving it on the hand latch and then tuck an old bath towel (color complements the paint) Shut the hood on the top of the towel and all is done.

It looks like it was made for it and does not bug the fan or the rad. Last year I went the the cardboard and it vibrated a leak in my rad which took a teaspoon of pepper to reseal.

Achoo