Vehicle has 166K, seller says the ECM is throwing PO400 EGR, as well as 4x4 light and anti-lock brake light being lit. He said a mechanic checked the usual suspects, found no issues, so seller replaced the EGR sensor himself which cleared the code for two days, but has since returned.

He listed on CL in another area because we have emissions here, asking $2400. What are some possible causes and what should I be looking for? Once the seller returns to town, I will check the vehicle out, perform a compression test etc.

 

I own a 2001 Montero Limited that has encountered the po400 CEL code a few times. My diagnosis process is to pinch closed the vacuum hose that has a yellow trace (line), that runs from the manifold to the EGR vacuum solenoid. If the engine stumbles when this line is pinched, this indicates that the vacuum lines and EGR valve are are probably good mechanically. I have corrected the problem twice by replacing the EGR Vacuum Solenoid. I purchased the part at Autozone, Dorman Part Number: 911-850 for $45.99. This includes a lifetime replacement warranty, and I have swapped the defective part for a free replacement. This part seems to be failure prone, but easily replaced at Autozone. The solenoid can be tested by applying 12 volts to the terminals while testing with a vacuum pump/gauge.
If the engine doesn't stumble when the vacuum line with the yellow line is pinched, check the line that runs from the EGR vacuum regulator (brown plastic with green dot) to a tee fitting. This line contains a brass restrictor that clogs. I used a few blasts of intake cleaner to flush the buildup. I cleared the CEL and drove enough miles to complete the cycle.
I hope this information is helpful. The po400 code is frustrating, but easily diagnosed. Don't replace the EGR valve until the lines, solenoid and intake vacuum ports have been checked. I'm not a professional, but I enjoy the problem solving process and saving a few $$.

Edit - This is the process for testing the solenoid
http://www.justanswer.com/mitsubishi...ange-test.html

 

100% agree with this guy. Thanks for the tip on the brown "mystery" doohickey

 

LOL, 2 years later, same issue. P0400. I work from home so I'll fix the problem this week. I hope its just a clogged restrrictor in the line that runs from the EGR regulator to the tee. Easier fix and no trip to Autozone. This problem took me a year and a few bucks to figure out initially so I hope the original post has helped people solve the same issue.

 

LOL, I was so desperate the first time I had this issue that I replaced this device also. It was listed for Chrysler cars as a EGR vacuum regulator. I worked on the PO400 for months until I found the solenoid and clogged vacuum line issue. Now I'm a Montero EGR expert

 

 

I recently had a EGR problem in a 2001 Sport--it ended up being a failed solenoid, a corroded wire in the EGR solenoid electrical clip, and a stuck EGR valve. Go figure. I cleaned the valve and got it to move smoothly and responding correctly to vacuum, and nothing improved. I cleaned the hose with the restrictor (which did not seem clogged) and nothing improved. I made sure the vacuum passages were all OK, and the system was not leaking vacuum anywhere. And nothing improved. I diagnosed the EGR solenoid and replaced it, confirming that the old one acted badly when compared with the replacement. Still nothing. Then I found no power from the computer. I found the green crusties in the plastic fitting for the wiring, cleaned it all out, soldered a new connection etc., and all of a sudden, _everything_ worked properly. So starting out, you never know whether you have one problem or three.

While you are cleaning stuff, I think it pays to also clean your PCV valve, your idle control valve and passages, and the rest of the throttle body. They all gum up with time, and if you (or the previous owner) let the EGR get gummed up, the other three are likely gummed up as well. They are all easy to take care of at the same time.

In my case, I bought my '01 for US $350 with a wicked amount of blue smoke pouring out the tailpipe, a badly tapping valve train, a p0400 code, and oil all over everything. I cleaned the PCV valve immediately when I got it home, which removed 1/2 of the blue smoke. I then changed the filthy oil (which I found was overfilled by at least 1 quart) and put in the correct weight (what was in it was too heavy) and the correct amount, and the smoke declined to say, 1/4 of the original. I then replaced 5/6 of the valve seals using a rope down the spark plug holes (I couldn't get to the #6 cylinder) and that took care of the rest of the smoke problem. By then, I couldn't see any visual difference in the smoke when I finally fixed EGR system, just that the code went away and the engine ran smoother. The hydraulic lifters rattled like little demons before the oil change too, and went quiet after I ran the revving cycle to purge the air from them two days in a row with the right oil in the engine. I also changed the timing belt and tensioner etc. which quieted things down as well--I suspect the old timing belt tensioner was bleeding down and rattling sometimes as well, although I never confirmed it. I found a lot of gunk under the US passenger side valve cover in the valve train area which I cleaned out, but that was after the lifters quieted down.

Good hunting!