1994 E420 EGR Valve
 

The CE light came on with code 5. I clean the EGR valve and tested it to make sure it works when vacuum is applied and it does. The code always returns after a few cycles. I cleaned the passages as best as I can using brake cleaner and speedo cable.

The only thing I can't figure out is when is there suppose to be vacuum at the EGR valve?

How long does it keep the EGR valve open?

What is basic sequence of EGR operation?

Under light cruise or decelleration, never at idle or high load conditions.
The vacuum hose at the egr valve runs to a switch-over valve, there should be 2 switch over valves, one for egr and the other, right next to it, for the air injection valve, so you have to be sure someone didn't swap hoses around.
The ecm decides when to switch it on and for how long, but is driven by driving conditions, not any preset sort of time element. If you are coasting along and the egr valve is open and you suddenly accelerate, the ecm will close the egr valve, for example.

As a test you can apply vacuum to the egr valve with a hand vacuum pump and the engine will idle roughly. If it doesn't then you know the problem is the valve or passages and you are on the right track. If it DOES then the operational side of the egr system is fine, the problem is in the control, either what I mentioned, the vacuum hose is going to the wrong switch over valve, or the (switch over) valve is bad or the wiring is bad. I "think" the wiring can not be reversed between the AIR s/o valve and the EGR s/o valve, the connectors are different to prevent this from happening, but the vacuum hoses CAN, damhik!
Gilly

PS the switch over valve can simply be jumped power and ground and listen for the clicking, ou can also do this with the hose hooked up and again the idle should go rough. Another thing to check of course is the SUPPLY of vacuum to the switch over valve, could it be kinked or disconnected? Then of course the wiring to the ecm. I believe, but not 100% sure, that the valve is supplied with power all the time, like from the BASE module or OVP, and the ecm controls the S/O valve by providing the ground.

And also, when the egr is applied at idle, it should be quite noticeable, almost kills the engine, if you can just barely tell the difference, I'd blame plugged passages, not cleaned enough yet.

I'll learn how to get everything in one post yet!
Also, to check to make sure everything is routed correctly, you need to fnd out the wiring color for the EGR switch over valve and make sure the hose for the egr valve is running to the correct switch over valve. Not sure on your particular car, but typically they have these 2 identical switch over valves side by side and it's easy if major work is done to get it wrong, although usually you will also have codes for the other switchover valve, the AIR (air injection) system