Soil testing before putting lime on old lawn
Soil testing is available from commercial labs. In Portland we recommend A&L Labs. Contact them about sending a sample, and which tests to order for your situation.
Two publications from OSU Extension cover basics of lawn care and fertilization for our area. Appropriate fertilizing, mowing, and watering are basics to keep the turf as healthy as possible, limiting weeds and other problems.
Practical Lawn Care for Western Oregon: https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/ec1521.pdf
Fertilizing Lawns, EC1278: https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/ec1278.pdf
"If your soil test pH value falls below the optimum range for the grasses in your lawn, a lime application will help raise the pH. It often takes 2 or more years to significantly change turfgrass soil pH with lime, because lime is not very soluble...." (page 5 in EC1278)