I just replaced the exhaust manifold on my 99 XJ with a dorman flex type after market exhaust manifold. I think I did it right, the car is quieter and engine smooth. But, the city gas mileage still sucks, which is the primary reason I replaced the cracked manifold because my XJ get 20 mpg on the freeway, but only 11-12 in city. So should I be concerned that there may be a small exhaust leak around the new manifold, which kills the mpg? If so, how should I check it out?

Thanks.