I have been working on my 95 Montero SR 3.5 and now it has 2 Check Engine Light codes. I just replaced the plugs and wires because it was idling rough and in the process i did not get the correct plenum gasket or the gasket that goes from the EGR pipe from exhaust to intake ( NAPA doesn't seem to have any) but i did replace the throttle body gasket. I reused both gaskets and it seems the engine is idling fine for the most part and runs more like it did when it was new. After replacing the parts, the check engine light has come on and i jumped the diagnostics port to ground to read the codes and got a 43 (EGR) and 11 (O2 sensor). I used the old carb cleaner trick and the propane trick to try to find a vacuum leak with no luck. I also removed the EGR valve and cleaned it well. One other thing i did prior to spark plug replacement was suck a 1/3 can of seafoam through the vacuum hose just in back of the EGR valve in the intake to try to clean some of the carbon before the plug change. My question is would a vacuum leak at the upper plenum gasket make the engine throw these codes? How likely is it that both the O2 sensor and EGR valve went out at the same time? I will take it apart again and replace the plenum gasket if that could be the problem but i didn't sense any idle change when i sprayed the base with carb cleaner or when i placed the propane tube underneath of the intake plenum. I do have a new plenum gasket for 96 montero 3.5 if anyone lives on the North Oregon Coast, i will give it to you. Any recommendations would be very appreciated.

 

I found the vacuum leak just in back of the coolant hose coming in to the upper plenum. I guess i will be taking apart again once i get a new (correct) gasket.