Overheating & Revving Problem…among others.

Vehicle: 91 Civic Si with a JDM B16A engine swap.

When I start the car it will idle high and then oscillate up and down. Rep up then drop, rev up then drop.
It over heats. I replaced the thermostat, but even if im moving down the highway it over heats. What is strange is that its worse at nigh when its cooler than in the afternoon or evening when its hot. On several occasions I have turned off the ignitin and coasted while the temp dropped, and this did something once the car was running again...it remain hot for a moment then started to drop. This works now and again, but not at night when its cool.

get a coolant flush...also check if your fans are working

this problem could be more serious than just the cooling system...when a car runs lean, it runs hotter than normal....the only reason i mention that is because of the weird oscillating idle you described. When that happened to me, i had a rather large vaccuum leak...

sounds like your idle control valve is failing. Here's an article on how to test/fix it:

http://www.findarticles.com/p/articles/mi_qa3828/is_200004/ai_n8883803

It's a swap, so it probably won't run right without the correct maps. What air/fuel control are you using? If you don't have VAFC or Hondata or some kind of fuel management, im sure it won't run totally sound. Have you tinkered with the FPR?

If oscillating idle goes away after warm-up, it is most probably improperly filled or low coolant. Had this on two civics and pretty much sure in it. Take you time and fiil it properly, that might fix your overheating, too.

it could be ur sensor underneath ur intake manifold why its reving up and down
,and try keepin ur overflow box full all the time.

but the reving is also accompanied by overheating. did you swap out the water pump when you swapped in the b16? hows the head gasket? comp test? leakdown test? oil leaks?