I know an EGR valve recirculates exhaust gases but I never gave it much thought (its just there). It's vacuum operated and it opens and closes but where the vacuum signal come from and when is it open and when is it closed?
Thanks , Gene

 

The vacuum line goes to your intake manifold. When the motor is running it produces a vacuum in there, the harder the motor works, the more vacuum. The vacuum lifts a diaphragm in the EGR. The more the diaphragm is open, the more air goes through the EGR. It will be open fully at whatever your max vacuum level is and will be closed or mostly closed when there is no load on the motor.

 

So if I disconect and plug the vacuum line the EGR stays closed. is that the wrong thing to do, will it hurt anything ?

 

There is an electronic valve that controls the vacuum to the EGR valve and thus how much it opens. It is not connected directly to manifold vacuum. It depends on the vehicle, but EGR is typically used in light throttle cruise conditions to reduce NOx emissions by lowering peak combustion temperature. EGR is also typically not active at idle or full throttle.

 

If you remove the electrical connector, you could throw a check engine code.

 

If you disable the EGR, the computer will throw an error code.

Some engines have tight enough tuning that if you disable EGR, the engine will knock from the increased combustion temperatures and pressures.

Also, excessive NOx could upset the operation of the cats.

Is there any reason you want to disable the EGR?

 

There is no Check engine code or computer on a 79 F-350

 

What a coincidence...the car I had the most trouble with when the EGR failed was a 1979 F150 with a 302 engine. It knocked like a popcorn popper until I replaced the broken vacuum line that activated the EGR valve.

 

My 460 acted kind of like the floats were stuck in the Holley carb and it was flooding, you had to drive it in traffic (stop and go) with 2 feet one on the throttle and one on the brakes when stopping to keep it running. I changed the EGR valve and the gaskets between the EGR plate and intake and carb, they were pretty much toast also.