Emission problems

:banghead: I apologize first if this is wrong. I have an 1991 Toyota 4Runner(2WD),took it in for an inspection and it failed due to high NOx reading of almost 3100. The mechanic of the establishment told me short of a new motor it wasn't gonna happen, we have only 2 shop that I know of here.It's got an 3.0 V6 engine that was replaced back in 2000 by a Toyota dealer in it now,the original mileage is around 300,000 it doesn't use or burn oil but it did idle rough. Thats when I changed some vacuam lines which helped,which brings me here is there anything else I can look for or do to before returning to get it rechecked .I'm on a fixed income with medical problems but can do some mechanical work on it.

NOx is produced by a high temps in the combustion chamber. This can be caused by a faulty EGR system, lean air/fuel ratio, too much carbon build up on the piston, creating artificially high compression ratio, just to name a couple. None of those problems require a engine rebuild

If you were messing with vacuum lines, then the odds are your EGR system isn't working properly. Thats where I would start. Sounds like that particular shop doesn't know much, I would look for someone that is intimate with toyota's

Things to check are EGR (as Bob said) and catalytic converters.

On many vehicles the EGR is operated by vacuum actuators. A kinked or split vacuum hose can stop things working.