94 chevy 5.7 egr question

[jremington59]

I pulled the vacuum line off my egr valve and there is absolutely no vacuum draw there. So I pulled the vac line off under the injectors and there is plenty of vacuum. From there it runs to a little box that has a red and grey wire. There is vacuum in the line that plugs into the front of this box, but none on the back side of this box where the vac line slides on that goes to the egr.

Can someone explain what this box does?

[Case 7230]

Not an expert but that sounds like the solenoid valve that is operated by the computer when it wants the EGR valve to open. You will only get vacuum to the EGR valve when the computer energises that solenoid. This will not happen with the engine at idle. It may happen if you give the throttle a quick blip.

[jremington59]

That makes sense. I did, for the **** of it, run a vac line right from the front of the throttle body to the egr and it didn't care for it, lol. It sputtered, run rough and eventually stalled....

[Case 7230]

Well now you know that the EGR valve is good, that is the proper test for it.

[jremington59]

Thx for the help case. And sorry to everyone else for using a bad word.

[crabtruck]

Case was right on again in his answer
won't stop me from kibitzing and expanding though.

You said the wires were red and grey, yet if you look closer
you'll probably see it's pink w/blk. Comes from ign fuse.
Grey wire is used by ECM to send "gnd" to turn on (open).
The fact that is HAS 2 wires is a tip off that it's solenoid.
Used on all 5.0s, LD 5.7s, and all 4.3s except AT w/NRQ.
NRQ is close coupled exhaust. Regular EGR solenoid.

If it had 3 wires, as above plus Blk/Wht going to ground,
then it would be an EVRV solenoid, which combines an
EGR solenoid with a much more accurate control circuit.
This is only found on the HD 5.7s.

The third main EGR set up that year was Linear EGR valve.
Recognizable with 5 wires running right into the EGR proper.
That was used on alll 7.4s and 4.3s with AT and NRQ

Diesel setups vary the design farther because an EGR
won't get it done by itself so they add a BPR, etc to mix.

EGR is normally closed at idle, on deceleration and WOT.
The first two cases is to prevent rough idle, at wide open
throttle it is closed to prevent power loss.

The rest of the time ECM is getting feedback from MAP
and elsewhere to figure how much EGR is needed at time
to control the amount of nitrogen oxides in emmissions.
Too much EGR will cause it to die at idle like for you,
or same deal at cruise, or surge under acceleration.
Too little EGR will cause heat build up in combustion
leading to overheat, detonation or spark knock.
This will also cause you to fail a smog check.

Course I'm just having fun here - the quick answer
would have been "what does your emission label say?"
Here's a cool picture of mine.
.

[jremington59]

Thanks for the explanation and the diagram crabtruck.