1992 Legacy Turbo makes very loud knocking noise that fluctuates with engine RPM'S but ONLY when defroster set to defrost or heat/defrost.

Started about 50 miles after dealer did head gasket.

NOT going back- they did plug wires and coil in response to my ?'s about low idle induced vibration that was NEVER an issue before gaskets et al and is still a problem.

Where to start??

Could it be vaccuum related??

What could they have missed (or misconnected) in the re. and re.?

Heater, fan and all controls work fine. No problem on vent or heat setting.

Car seems fine except for this dreadfully frightening knock

I searched but could find no reference to anything similar anywhere.

ANY advise greatly appreciated.

Sounds maybe like an Air Cond compressor problem... it's usually on for AC and defrost on most cars. AC compressor will knock with too much freon in it too. Others opinions?

 

1992 Legacy Turbo makes very loud knocking noise that fluctuates with engine RPM'S but ONLY when defroster set to defrost or heat/defrost.

Started about 50 miles after dealer did head gasket.

NOT going back- they did plug wires and coil in response to my ?'s about low idle induced vibration that was NEVER an issue before gaskets et al and is still a problem.

Where to start??

Could it be vaccuum related??

What could they have missed (or misconnected) in the re. and re.?

Heater, fan and all controls work fine. No problem on vent or heat setting.

Car seems fine except for this dreadfully frightening knock

I searched but could find no reference to anything similar anywhere.

ANY advise greatly appreciated.

Sounds maybe like an Air Cond compressor problem... it's usually on for AC and defrost on most cars. AC compressor will knock with too much freon in it too. Others opinions?

I second that

Sounds maybe like an Air Cond compressor problem... it's usually on for AC and defrost on most cars. AC compressor will knock with too much freon in it too. Others opinions?

Thanks - you nailed it - disconnected compressor and all better

-probably still hurtin' from the service?? and forgot to think

-easier to blame dealer

thanks again

there is a handy pressure guage for AC systems to see if the system is overcharged or not...me thinks its over by alot or way under...

Dave I moved your first thread here, and replied to it.

 

The knocking noise is the ac compressor. The ac system turns on when the defrost mode is selected to remove excess moisture from the air.

 

You can try shooting some more refrigerant/oil in there to see if it quiets down. If not, you'll need a new compressor. You can probably just keep running it.....I ran mine that way for a while.