I have a 1999 Montero that runs great except for a strange thing it's done the past week. About a week ago I was pulling out of my work parking lot and at the stoplight the idle dropped down to 250-500 RPM and the engine felt like it was starving for fuel or possibly one or two of the cylinders was not firing. I gave it some gas and it revved instantly with no hesitation whatsoever, but settled down to that same loping slow idle again. At this point the light went green so I decided to accelerate hard to see if I could clear it out and it did. At the next stop it idle fine and has for the last week. Then yesterday, leaving the parking lot my CEL comes on and the idle issue happened again about 5 minutes after that. I ran it hard again and again, it cleared the idle issue out completely. I went and had the code pulled and it's a P0400.

http://www.obd-codes.com/p0400

Causes

A code P0400 may mean that one or more of the following has happened:

Plugged EGR passage which restricts the flow of exhaust gasses
Faulty EGR solenoid
Faulty EGR solenoid wiring/harness
Vacuum lines damaged/disconnected to the EGR valve solenoid or to the EGR valve
Faulty EGR valve

A few months ago I got a CEL that had a P0403 code.

http://www.obd-codes.com/p0403

I first took off the EGR valve and cleaned it and when that did not work, I replace the EGR solenoid, which did the trick.

Based on the P0400 and my symptoms, I think there is a vacuum issue somewhere. My question is, did anyone else have this combination of code and symptoms, and if so, what ultimately fixed it? I'm just trying to get some ideas before I take the EGR valve off again.

Thanks!

I did some more research and my symptoms and the code pointed to a bad EGR valve. As it so happens I found an online coupon for advance auto that was $40 off for every $100 spent and they had the EGR valve in stock for $110, so I got it for $70. That was hard to beat, because the next best price I could find was $84 at rockauto.com (plus shipping).

If anyone else is interested in this deal, the coupon code is CCABIN, it's about a 1/4 down this page...

http://procouponcode.com/advance-auto-parts-coupon-code

So, I bought the EGR valve online, drove to the nearest AA that had it in stock, picked it up, went home, installed it and reset the CEL light. I immediately noticed a slightly smoother idle (I mean smoother than the already fairly smooth normal idle). No hint of the idle stumble on the way to work this AM either, so I'm pretty confident it was the EGR valve afterall. If anything changes, I'll update this thread.

wow, good information...strange, usually all you have to do is clean the egr out