My 86 190e suffered a broken timing chain two months ago. I decided to put a used 70,000 mile engine in it, which was cheaper than replacing the chain. After getting my car back out of the shop, it first had idle surges ( up to 2000-2500 ), and then settling back down to 750. It also suffered slight hesitation. The hesitation is less apparant at highway speed. Later, it stopped the idle rise on it's own, but now doesn't idle smoothly like it did before. The hesitation is becoming very annoying and my mechanic couldn't remedy the problem. He even tried to charge me again for looking at it. After leaving it at the shop for two more days, they said they couldn't find the problem and it must be the ECU. It didn't need an ECU before I took it there, so I wasn't convinced, since there are no check engine lights coming on. All of the fuel system parts etc, were transferred from my old engine which was running fine until the chain snapped. Any ideas would be helpful, as I've already spent too much on this car.