EGR vacuum solinoid

**John Robinson** · 08-20-12, 03:54 PM

I have looked at my FSM until I am blue in the face. I cannot for the life of me figure out which side of the EGR vacuum solinoid gets the vacuum from the manifold and which one then goes to the EGR valve. When I look at the nipples for the vacuum hoses the one that would be closest to the intake manifold has a small brass piece with a very small hole in it. I am guessing that is the one to the EGR since with that little hole it would be very restrictive as to how much vacuum ther EGR would get. If someone could verify for me the correct location of those vacuum hoses It would be very helpful.

**boomdriver** · 08-20-12, 06:22 PM

Its does not matter...
I'm sure they come plugged in a certain way from the factory but its just an on/off vac switch....so vac can pass either way. The orifice is the same...it does not matter which way the flow comes from..it still does the same thing. The switch is either open to allow air flow..or not.

**John Robinson** · 08-20-12, 07:13 PM

Ok but when I suck on the side without the orifice I can pull air from the other side but if I reverse that I can not pull any air. Wouldn't that mean that something mechanical has to happen to open the one with the orifice. And what does that do. Would it not go to the EGR to pull the pintle open?

**toptechx6** · 08-22-12, 04:49 AM

Have to disagree with Boom on this one. The port that allows vacuum to pass goes to the EGR valve, if you check again you should see that it is vented to atmosphere when the solenoid is closed (normal position, no power applied) rather than connected to the other port as John stated.
The other port is connected to manifold vacuum and will not allow vacuum to pass with the solenoid closed. When the ECM wants EGR gas to flow it grounds the solenoid allowing vacuum to be applied to the EGR valve. When the ECM decides EGR is no longer desired the solenoid closes cutting off vacuum from the manifold source, if the port going to the EGR valve was not vented to atmosphere the EGR valve would remain open because vacuum would not be released from the diaphragm, this would cause stalling at idle or other drivability issues.

**John Robinson** · 08-22-12, 05:59 AM

Thanks for the replys. What I am really needing to know is which vacuum line goes were.

egr vacuum routing.jpg

**boomdriver** · 08-22-12, 08:47 AM

Originally Posted by toptechx6

Have to disagree with Boom on this one. The port that allows vacuum to pass goes to the EGR valve, if you check again you should see that it is vented to atmosphere when the solenoid is closed (normal position, no power applied) rather than connected to the other port as John stated.
The other port is connected to manifold vacuum and will not allow vacuum to pass with the solenoid closed. When the ECM wants EGR gas to flow it grounds the solenoid allowing vacuum to be applied to the EGR valve. When the ECM decides EGR is no longer desired the solenoid closes cutting off vacuum from the manifold source, if the port going to the EGR valve was not vented to atmosphere the EGR valve would remain open because vacuum would not be released from the diaphragm, this would cause stalling at idle or other drivability issues.

I understand what you're saying and for a very long time thought the same thing...
what changed my mind was that fact that the C4 EGR valve is a negetive back pressure type that uses the neg pressure in the diaphram along with the spring to operate and return the plunger. the neg pressure EGRs will all have a "N" stamped on the top in the part number. This is supposed to assist in its return and not make it subject to vac conditions holding longer than it should. If the source vac is not maintained, the Neg EGR bleeds down to close on its own.

I've had it both ways. The vent is there, obvious, everything works and hoses swapped around never made any difference at all. I get no codes, no other issues. My egr valve is currently sticking and leaking causing a bad misfire that comes and goes, but trading the hoses had no effect on that either.

**toptechx6** · 08-22-12, 12:07 PM

Originally Posted by John Robinson

Thanks for the replys. What I am really needing to know is which vacuum line goes were.

egr vacuum routing.jpg

Answered your question here John, should be port "B" in your illustration, see the vacuum diagram on the fan shroud for confirmation.

Have to disagree with Boom on this one. The port that allows vacuum to pass goes to the EGR valve, if you check again you should see that it is vented to atmosphere when the solenoid is closed (normal position, no power applied) rather than connected to the other port as John stated.
The other port is connected to manifold vacuum and will not allow vacuum to pass with the solenoid closed. When the ECM wants EGR gas to flow it grounds the solenoid allowing vacuum to be applied to the EGR valve. When the ECM decides EGR is no longer desired the solenoid closes cutting off vacuum from the manifold source, if the port going to the EGR valve was not vented to atmosphere the EGR valve would remain open because vacuum would not be released from the diaphragm, this would cause stalling at idle or other drivability issues.

**toptechx6** · 08-22-12, 12:16 PM

Originally Posted by boomdriver

I understand what you're saying and for a very long time thought the same thing...
what changed my mind was that fact that the C4 EGR valve is a negetive back pressure type that uses the neg pressure in the diaphram along with the spring to operate and return the plunger. the neg pressure EGRs will all have a "N" stamped on the top in the part number. This is supposed to assist in its return and not make it subject to vac conditions holding longer than it should. If the source vac is not maintained, the Neg EGR bleeds down to close on its own.

I've had it both ways. The vent is there, obvious, everything works and hoses swapped around never made any difference at all. I get no codes, no other issues. My egr valve is currently sticking and leaking causing a bad misfire that comes and goes, but trading the hoses had no effect on that either.

Just confirmed that you are correct Boomdriver, the EGR valve will bleed off vacuum without opening the apply line, thanks for the clarification. I maintain there is a correct hose routing to the solenoid however, if manifold vacuum is routed to the EGR valve port you will have a vacuum leak since it is open to atmosphere. Certainly a small leak, but only one port on the solenoid will hold vacuum, there is clearly a reason for that.

**John Robinson** · 08-22-12, 01:23 PM

Thanks for the answears. I discovered that the FSM does designate which vacuum hose goes were. If you look at the code 32 chart in the 93 FSM it shows a vacuum line coming from the manifold to the solinoid and it has a littel dash mark that runs through the vacuum line in the drawing. That dash line indicates that there is an orifice in that vacuum feed line. When I had the solinoid off the car I did see an orifice in what I reffered to as port A on my drawing. That then is the port for the manifold vacuum line. There also is the same orifice reference on the EGR vacuum line and if you take the EGR off you will see that the vacuum port on it has an orifice in it. Untill I had that pointed out ot me by a GM mechanic I would not have known that was what it meant.