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I have a 98 EX that has a D16Z6? block swapped in from the previous owner. I don't really know what the head is, but the TB looks like a 96+. It has a very frustrating RPM flutter that starts around 1500. Adding throttle at that point makes the fluttering worse. I'd say it wavers by about 100 RPM very rapidly, so the car bucks (it's a 5spd) when you are in that RPM range. If you press on the pedal more it shoots up way past 2000RPM, but it is smooth. Makes it very irritating to drive.

I just replaced the TPS and there has been no change. When I cleaned out the IACV I think it actually made it worse. If I disconnect the IACV it makes no difference. I also cleaned up the TB.

I know the block is not right for the ECU, could that be the problem? I'm wondering if I should change the CTS and thermostat to match one for a 98, because I know the thermostat in the Z6 are 10 degrees cooler.

 
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IACV stock open probably.