Postby smj999smj » Thu Nov 02, 2017 9:21 am
You changed the converters a few years ago, but since then, the engine has been burning a lot of oil and contaminating those converters. So, upstream converters will definitely be on the list and at this point, I would question the integrity of the rear converters, as well. If you go back to an earlier post, you said a second mechanic said it's not possible for converters to come apart and score the cylinder walls. Twenty years ago, I would have agreed, but that's not the case anymore. Many vehicles today, like your Pathfinder, don't have conventional EGR valves; they use the variable valve timing to accomplish exhaust gas recirculation. They do this by cracking open the exhaust valves during the intake stroke to pull exhaust gas back through the exhaust valve and into the combustion chamber. If the catalyst substrate has deteriorated, bits of it can get sucked into the combustion chamber, as well, which will score the cylinder walls. To replace or rebuild the engine without replacing the upstream converters would be foolish, IMO. As far as the rear converters, I would replace them with Doug Thorley mid-pipes, which deletes the rear converters altogether (this may not be an option if you live in a state with strict emission testing).
The VQ is not an easy engine to rebuild. Replacing valves seals is a lot harder than it is on, for example, a small block Ford or Chevy, because of the overhead cams and use of bucket tappets. Some have had issues with head gaskets failing after a rebuild which I assume has to do with the inability to get the deck surface of the head within tolerable limits. May of today's engines (Ford's modular engines are a good example) need a mirror-like finish on the head to properly seal. Ford got to the point where they didn't recommend sending the heads out to a machine shop for resurfacing, rather simply replace the head with a new (and expensive) one. I have a feeling the VQ is similar in that the head surface needs to be basically perfect and free of any kind of imperfections, no matter how small.
The block is another story as we don't know the condition of the cylinder walls. If you are lucky, they well just need to be honed. Worst case scenario, they are damaged beyond the point of honing, they will need to be bored and new pistons will be required, adding a lot of cost to the job. Then, most techs will tell you to rebuild the engine and not replace the timing set is not a good idea, add-in more cost. You mind as well install a new water pump and thermostat while you are there and don't forget the cost of a gasket set, RTV, etc.
Personally, if it was mine, I would trade it in on another vehicle; maybe, get me a 2011 or 2012 Pathy. If the only option was to repair the vehicle, I would look for a 2011 or 2012 used engine for it. Note I specifically say 2011 or 2012, because those are the years that didn't have any timing chain issues and as a whole vehicle, it was also the years that all of the other "bugs" were finally taken care of. Then, with the used engine, I would get a set of Walker upstream converters, NTK upstream air/fuel ratio sensors and the DT mid-pipes. It would also be a good time to install headers, as the heat shield bolts on the factory manifolds are probably going to break, anyway. But that's "me." Obviously, you have to do what's best for you.