I just bought this 1996 Z28 and it was shooting 4 codes out. I fixed 3 but P0400 is still there. so far I have changed out the EGR valve, EGR Solenoid, ran a seafoam treatment (it really made a big difference) and ran q-tips and canned air through all my disconnected vacuum lines. I also made sure that every line was in the right place. I haven't taken off the EGR feed pipe yet but that is next. When I bought it, it came with an aftermarket 3" Cat back muffler. This 96 shows that it was made in Aug '95 but comes up a '96. I noticed that the y-pipe comes before the Cat. Shouldn't it come after. Thats what I keep seeing in diagrams. Anyways, I'm running out of things to check. My next option is to just delete the whole EGR all together. I just don't have the money do get it adjusted PCM wise just yet.

 

Did you see if the vacuum line is broken or even connected? Also try doing some circuit testing. See if you have a good power and ground at the connector.

 

The P0400 code could pertain the whole EGR system, but I don't think the pipe is your problem. If it was the pipe you'd be getting a code for low EGR backpressure. It's probably the valve, solenoid, or harness.

To test the valve just hook a mityvac up to the inlet and apply some vacuum. If the engine runs rough then your valve holds vacuum and the rubber diaphragm isn't cracked (which is very common).

To test the solenoid you should unplug the pigtail and hook the mityvac up to the inlet that faces the firewall and passenger side. You could also remove the solenoid and try to suck air through one side but it's much easier just to use the vac. The solenoid should hold a decent amount of vacuum when there's no power.

Just cap off one end of the vacuum harness and try to blow air through it while listening for leaks. It's made up of 2 independent tubes so test both.

If all of those check out I'd look for a vacuum leak somewhere.

 

As for the cats, your car should have one for each bank before the Y pipe.

 

I have a guy changing out my clutch right now but as soon as I get it back on Sunday I'm checking these out.

CATMAIGNE I already replaced the solenoid and valve so I wouldn't think testing these would be much of an issue. I have traced every line out for cracks and nothing yet. my only thing left really is the Harness. I'm going to meter that one out though before I change that one out.

one thing I caught today was a faint hint of gas so I'm assuming there is a leak I just haven't found yet.

 

test the evap system. look for any exhaust leaks before the 02 sensors.

 

It would also be a good idea to take a multimeter to the EGR solenoid's pigtail and see if you're getting 12v when you should be.