A couple different idle problems (sorry)
 

I know there is a post for an idle problem on every different page of threads but I read a couple and I still have a few more things that need answering. The car is a 1991 Honda civic hatch with a 1.6L SOHC ZC motor. Converted from a 1.5L.

The problems - 1 the idle drops and then goes back up, not up and down. With this problem ive replaced the TPS sensor and calibrated it perfectly no CEL. I bought a used IACV off ebay cleaned it and put it on. As well as the other sensor to the right of the IACV I think its the EGR valve? I got another used one cleaned it up took it apart and made sure nothing was sticking. I cleaned off the IAT sensor. I cleaned out the throttle body and couldn't find a gasket so I made my own with gasket paper and used some sealant as well so I dont think it would leak. Vacuum lines are pretty old and I am not 100% sure if they are connected correctly... Any ideas anything I can do to check? I have also replaced the ECU and it made no difference.

The second problem I will be sitting at a light and the idel will just drop below 500rpm and the car will shake. Today the car stalled at a light. So its time to figure it out, so for this problem, this might be the problem but havent had time to get to it, when the idel drops to 200 sometimes if I quickly rev it up the belt will squeak and it will run normally for a little bit. So could it maybe be as simple as tightening the belt, possibly the alt not giving enough power for spark??..

Think thats all there is

I would recommend that you install a new alternator belt and properly tension it.

If the idle problem persists, then use the troubleshooting guide to suggest other possible causes. For example, I think that adjusting the idle speed might be a good idea.

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Also I am getting code 20 which is electrical overload? Any ideas? Its been there since I got the car.. Cant figure out what it is and my hanyes book says see dealership...

Try resetting the ECU... I bet that's why it says see dealership (to get the ECU reset)
Remove under-hood ECU fuse for 5 minutes, put back in, start engine and idle for 5 minutes (don't touch gas pedal). You're then good to go. See if the code stays away.