Hello All! I am new to posting to this site but have looked around for awhile. Great Forum. So I have a 1990 V6 Mitsu Montero (I dont know if it is a sport) that I just bought from an auction. Everything is clean inside and out besides normal wear and tear. The only history I know of this vehicle is that it was taken care of and recently had a new crate motor installed. Seems like the whole package (new battery, engine, wiring, oil change, sensors, etc.).

My issue is it runs fine at idle, and at slow acceleration til about 2500 to 3000 rpms. Then it bogs down and I hear popping coming from the exhaust on the passenger side. Also just doesn't seem to have any power when I floor it. It has new air filter, just replaced the spark plugs, wires and rotor. Cleaned the throttle body which had black soot on it. Cleaned the MAF sensor and checked pressure before and after catalytic converter, all good. All wiring looks good and I have gone through two tanks of 91 octane gas with fuel injector cleaner added. Just did a sea foam treatment to the intake and still bogs and pops at higher rpms. I just don't know what to do next??? Any suggestions would be great, THANKS!

 

Also it seems to be running a little rich too! Any Help?

 

I had similar issues with my 90, especially the popping at 2500-3000 rpm. Mine turned out to be warped heads. Spend $85-$100 to get a diagnostic on it. That's how I found out my head gasket was blown. Another thing is to make sure that the timing isn't off, especially if the new motor was recently installed. If your timing is off by just a couple of degrees, it can cause problems.
You'll end up spending more on fixes in bottles than the diagnostic. At least the diagnostic can give you real answers (which I think is what we all really fear anyway).
Good luck! It'll be worth it.

 

soot in TB?
Im with ND on a timing issue.
Also check your valve train.