Is it normal for a mechanic to ask for payment before telling how he plans on fixing car?

I have a 2001 Honda Civic Ex, and I just replaced the pcm, immobilizer, and got a new key (the correct part wasn't available through manufacturer or salvage, so I had to use a closely compatible part from Acura).

However, after 3 weeks of running perfectly, it started with issues again, where the car won't accelerate past 40mph. I unplugged the pcm and plugged it back in, and it worked perfectly well again. This has happened off and on for several days now.

I put an ad on Craigslist for a mechanic (the dealership won't work with me, and local repair shops don't know what to do after running about 4 scans), and one contacted me.

He said he could "disconnect something," and my car would run fine. Downside would be the car would burn more fuel. He won't tell me WHAT he is disconnecting, and said it is $100. I think he is afraid that if he tells me what it is, I will fix it myself. However, I don't want a random stranger unhooking things in my car, and not telling me upfront what he plans on disconnecting/replacing, or whatever he has to do.

Is it reasonable to expect him to disclose his repair plans before I hire him?

I wouldn't let them near my car if i were you. If they can't fix it right then don't let them near your car.

Since the car has to pass emissions by law.
The Emissions control testing people can send you to a shop who is monitored and certified by them to perform repairs that can address your problem lawfully.
Once they check your car and realize you have a problem that causes polluting or makes the car undriveable or not starting or otherwise affected by the engine management system, the Law requires they make repairs that only affect its passing and running right.
They can question the dealer and other shops about previous repairs and you can keep asking them to till your car runs and passes.
If you get overcharged, or unrelated repairs are performed, it won't count towards a possible waiver. If you qualify for a waiver they can grant you one. They can arrange a refund for repairs that are not helping but were said to be your fix.
They help with this kind of thing all the time on many years makes and models, they may know already what to do, they may know already who to call for the correction. A bulletin may exist that they have access to read.
They can be very conscientious about this kind of thing and minimize your difficulty.
Google your states emissions testing authority and present your info and ask what the steps are to get it done.

Do not trust craigslist, car mech or someone selling garbage that will break down in a day or 2