Additional issue discovered on the Montero Sport that I bought a week ago. CEL lit up while I was test driving the vehicle for the weekend (Yah, Uncle's Dealership).
Initial Symptoms:
-Rough idle (Idle RPM seems slower than normal)
-Audible exhaust leak (CAT)
-Found CAT leaking exhaust, fixed leak, reset CEL, lit up again within 20 minutes
-P0170 and P0400 codes
-Did the EGR vacuum test where you apply a vacuum to the EGR valve. Engine RPM increased (bad egr valve would stall engine)
-Found additional exhaust leak, welded split pipe, reset CEL, lit up again as before.

Has anyone else experience this dual code issue? Looking for any advice as I intend to resolve this myself.

 

That's opposite - a properly functioning EGR when open would stall engine at idle. You may have a build up of crud in the EGR tube or in the EGR valve housing blocking the valve operation. Start there.

 

Update: I read a few articles on the interweb and found that the MAF sensor could cause this problem. Removed airfilter (also problematic, will replace) and found the MAF honeycomb element was covered with a layer of dirt. I sprayed it clean with carb cleaner. As I was putting it back together I noticed a faint hissing sound. Upon closer examination, I determined that it was coming from somewhere between the intake manifold and the firewall. I moved around the vacuum lines in that area but could not effect a change in pitch. I grabbed a spray bottle and started spraying the vacuum hoses but found no evidence of a leak. By then the intake was too hot to be sticking my hand back there. I will try again with a cold engine. I am also going to spend more time under the vehicle to see if there are anymore exhaust leaks. Maybe look at the O2 sensors for obvious issues.
HunterD, Yah, not an expert here by any stretch of the imagination. Just following what I read. If it is on the internet, it must be true...Right?

 

Did you check EGR passages and tubing for any crud build up?

 

I have not. Will add that to my list of items to check. However, I have removed the source vacuum line to do the vacuum test. Hose seemed clear. What other EGR plumbing do you recommend that I check. Appreciate your assistance!

 

Pull the EGR valve off the manifold (I'd imagine its two bolts). You can then see if the passage inside is clear or not. Also, at that point you can apply vacuum to the EGR vacuum port and you'll see if the plunger actually moves or not. That will give you a clear indication if your EGR valve is good or bad. After that you check the EGR pipe that goes from the valve to the exhaust manifold. It might be plugged up. Since you said you did not stall the engine when applied vacuum directly to ERG valve, I'd focus on those two items right away.