SO my 1996 tracker which I had bought just recently also throws a p0400 egr malfunction code just like everyone else who has the 96 or newer trackers. It also throws a p0171 system lean code. Now, the guy I bought it from obviously tried and tried and tried to get these codes to go away, with no success.. he replaced the oxygen sensors, not sure if he replaced the cat or not, but it looks fairly new, he claims he cleaned the egr and what not, and he also claims he replaced the ecu.
Well, I figured I'd check everything for vaccuum leaks, there are none, I took off the egr it was fairly clean but I went ahead and cleaned it a bit more, as well as the tube, and the port on the intake manifold that everyone gets clogged or is only the size of a pinhole. I also went ahead and cleaned the MAF sensor, reset the computer by disconnecting the battery, and the check engine light goes away, like it should, but after driving for a good 100 miles or so it always comes back on.
Now I think I have narrowed the problem down, the egr solenoid valve which has one line coming out one end, and two lines coming off the side on the opposite end and it it has a two wires going into it which are blue black and green red. I have checked the wires with my multimeter and get nothing. Heres my first question, since this is a solenoid its only gonna get electricity sent to it by the ecu when it feels it is the correct time to do so.. right? Or is it supposed to be constantly fed with 12 volts and then it drops when the ecu feels its the right time?
Next question, if something in the egr system isnt right can that be what is setting of the system lean code too? Or could it be the fuel regulator and how would I check that?
Lastly and the reason why I call this thread wiring from hell, is because this guy also f*cked with the wiring like you wouldn't believe, probably to see if there where any shorts anywhere causing these problems, since replacing the oxygen sensors didnt solve the problem like he thought it would. He obviously didn't do any research before attempting to tackle these codes.
So here are some pics of the wiring from hell.. the guy obviously unravled all the wiring harnesses, and he taped together some relay (red arrow in picture) and also wrapped tape all around some white plug (green arrow) that also has a few random wires taped into the mess that arent attached to anything! so what is this relay and what is this white plug that isn't attached to anything if anyone knows?
I will be adding more details and pics to this adventure tommorow when I have more time to look at it.. oh yeah, I almost forgot to mention, amidst these codes and terrible wiring, the car runs great, starts right up plenty of power no hesitation at all, but it gets around 17-18mpg and thats if I drive it nicely...

This looks fun..

You have 3 connectors to the ECU so I guess that thats a Sidekick Sport? The EGR solenoid should be supplied with a 12V feed from the main relay (the Blue/Black). The ECU grounds the other side to activate it.

One test would be to bypass the electric solenoid and go directly to the vacuum point.  That way you can rev up your engine and make sure the egr is automatically opening and closing like it should be.

You might need to replace the solenoid, that one went bad on mine as well.

so which tubes should i connect to what, in order to bypass the egr solenoid? and just so everyone knows, it doesn't move while i rev it, at all, i did take it off and the stuff inside it does move freely, nothing is stuck, and there are no vaccuum leaks.
What does the main relay look like and where is it located?, so I can locate the origin of the blue/black wire. Also I just happened to notice that one of the three wires on the MAF sensor is also blue/black, so is that also fed 12V from the same source (the main relay)?
And here is a picture of two little relay lookin things over on the battery side of the engine, the one on the left has a blue/black wire going into it (red arrow), anyone know what it is? or is that the main relay? If it is I thought it would've been a bit bigger or something..
By the way I've come to the conclusion that the unknown white plug I mentioned in a picture earlier on in this thread is just part of a remote start/alarm system that used to be in this vehicle. Definately a relief, since I'm pretty much whistling in the dark with this whole thing...

oh yeah about the ecu having three plugs in it, all i know is this is a 1996 geo tracker with the 1.6L 16 valve engine.. if that is not the ecu that this model normally would have, ..greeeaaat.... i wouldnt doubt it if the guy just put all the wiring and electrical compenents from something else in here to try and make it work, believe it or not he goes on this site sometimes.. hehehe i hope he sees this and is embarrassed 

You could show a pic of the Engine it could be a 1.8 if it is a newer Engine.

Under your hood should be a vacuum diagram showing how the system works.  There are two electric operated relays on the front of your engine.  The left one is the one that opens up and allows your EGR valve to open and close properly.  I forget if it's closed by default or open to allow vacuum through.

There is a line coming from your intake into that.  Then from there it runs into a vacuum operated relay.  The vacuum operated relay opens up and allows a larger amount of vacuum to open your EGR valve.  To test that the vacuum relay thing is working, disconnect the line going into the left electrical relay on the front of your engine while it is running, then breath in on the tube as hard as possible while checking the back of the EGR valve.  It should automatically pull itself in.

To test the vacuum solenoid on the front of the engine itself just disconnect the tube running to the vacuum relay on the back of the engine and see if you can pull air through it via breathing in on the tube.  Then hit it with 12V, you should have the opposite effect.  I can't remember if it's 12v on or 12v off, but it should change.  The computer will switch it automatically when the engine warms up.

As for the EGR unit itself, that isn't what plugs up.  It is the tube connecting it to your head.  You also need to use a wire to break the carbon loose up inside the head where the tube connects.  Then squirt a bunch of cleaner up there.

Here are some pictures:

Full Size Pic Here
You can see the two tubes going into the left electrical relay.  One comes from the intake, the other goes to the vacuum based relay thing.  If you simply bypass this relay your EGR should open and close with your engine RPM regardless of operating temperature.


Full size Pic Here
The black circle thing with the silver label in the top center is what I refer to as the vacuum relay.  Not sure what it's really called, but the label under the hood gives the correct name.  Basically it opens up with a light vacuum, then allows a stronger vacuum to open up the EGR valve.  You can use a vacuum pump or just disconnect the line running out of it and give a good pull of breath in order to manually kick it into action.

That was a lot longer post then I was planning on...

oh yeah about the ecu having three plugs in it, all i know is this is a 1996 geo tracker with the 1.6L 16 valve engine.. if that is not the ecu that this model normally would have, ..greeeaaat.... i wouldnt doubt it if the guy just put all the wiring and electrical compenents from something else in here to

I haven't seen a 1.6 with the 3 connectors before but we never got any OBD2 models here in the UK so thats probably what the difference is.

Ok well there are two egr solenoid vaccuum valves the one that Drone637 is talking about towards the front of the engine and then the one towards the back (thats the one with three hoses and blue/black green/red wires) So the one that Drone637 is talking about has a hose coming out its side that connects to the throttle body, I can disconnect that and there is no air coming in or out of it, but I can blow air into it and nothing happens. Then there is the hose coming out the bottom, I can blow air into that and nothing happens but if I try to suck air out of it it makes the egr move making the engine want to die. I also checked this for 12V and it has it, and it is grounded.
Now the other egr vaccuum solenoid that I mentioned earlier on is getting 12V but the green/red wire which is supposed to be grounded at the ecu, isn't.. and when I rev the engine I'd assume that the ecu is supposed to ground it to activate that solenoid making the egr do what it does, but nothing changes. SO I have a feeling that the wire is bad or something of that sort, because I know that solenoid itself isn't bad since I gave it 12V and grounded it to the engine, it clicks and makes the egr move so much that the engine dies before I unground the whole thing. But what is it that initiates the whole process from the begining? Cause if that green/red wire IS fine all the way back to the ecu, and the ecu was replaced.. something tells the ecu when to ground it.. and what could that be??

if I try to suck air out of it it makes the egr move making the engine want to die.

That means the EGR valve works.  The metal line going into your head might be filled up, but the EGR valve itself works properly.

Everything works via negative air pressure.  So you have to suck on all the hoses for testing.  I was just trying to avoid telling you to suck on a bunch of hoses. 

You won't have the solenoid open up until the engine warms up.  That way it doesn't kill your engine while it is cold.

There was a really good article I found once on troubleshooting the EGR problems.  Don't forget, after cleaning the metal tube out you have to unplug your battery, leave it off for an hour or two to reset your computer (Or use a ODBC2 computer to do it).  Once the computer is reset you just have to drive it for a day or two and see if the light comes back on.

Well I did clean out the metal egr tube and I cleaned the port in the intake manifold that it connects to from both ends(egr tube side, and from inside the intake manifold) Fortunately the guy who had it before me put a little quick battery disconnect that has a red plastic key so that makes it easy to turn off all power and reset the computer, I had done that too, several times while checking stuff through this process, and yes the check engine light goes away but it comes back before I can finish a tank of gas, or I guess after all the readiness monitors have gone through their drive cycle.
I gotta take a close look again at all the wiring when I get home from work...

OK still havn't had much time to look at things real closely but when I unplug (cutting 12v power) the solenoid that Drone637 was talking about (green arrow), the egr valve moves! even at the slightest touch of the throttle. I'm assuming the ecu controls this but how does it know when??? There must be something that happens first which tells the ecu to cut the power to that solenoid allow the egr to open at the right time, otherwise with it being unplugged it works ...but even right at start up with a completely cold engine, and still gives me the p0400 code.

The EGR valve is activated by the ECU when the engine is under mid load. It won't activate unless there is a reasonable load and it is closed over aprox. 70% throttle. It won't be activated if the engine is below normal operating conditions or if the vehicle is above a certain altitude.

Rhinoman - I was wondering when that would turn on.

9000rpm - Well, at least you know the EGR is working properly.  Sounds like it might be a sensor error at this point.