You may be able to rejuvenate them by pruning out some of the old thick branches and letting them send up new growth. Reference:

http://www.finegardening.com/how-to/articles/pruning-lilacs.aspx

Thank you so much!!! I shall study the article and take it to heart. I really want to try to save them if I can. Lilacs are special :) (roses are too but the deer eat all of them so I have to hang on to the lilacs LOL)

I also have some old lilacs that do not bloom well. I have pruned them down as the article Ken linked instructs, but the pruned branches do not bloom until they've grown quite tall again.

I understand that lilacs need LOTS of sun to bloom well, and mine are on the north side of the house. I wonder if mine ever looked that great. Perhaps an 80-year-old planting mistake?

Actually, lilac rejuvenation is a no-brainer.

Cut EVERYTHING off, right to the ground level.

It will grow back vigorously from the roots, and will make a full sized shrub, full and beautiful, in 2 to 3 growing seasons.

I too had several geriatric lilacs, and did this to them about 15 years ago, made them good as new. A couple of them got ratty and I did them again last year, and, while only about 4 feet tall this year, are full of buds. By the end of this summer, they'll be 6 to 8 feet tall again.

No need to be delicate, I used a chain saw.

LOL I've never cut anything with a chain saw...we may have to film this heh heh

Thanks all for so much help!!! This garden is a huge challenge...too much sun LOL too much growth.

I had some lilacs growing behind my garage. They had flowers only on top-about 15 feet off the ground- and were looking spindly . I "rejuvenated" over three years by cutting out one-third of the trunks every spring for three years straight. , cutting right down to the ground. Done in late March just as soon as yhe last snows melted. This method really works. I now have plenty of blossoms for picking!