Tuning issue, or head gasket?
My 94 NT to Turbo conversion runs like crap. It's driveable, but at about 4500 to 5500 rpm, it stutters, bucks and backfires badly. It even seems like it's hitting fuel cut, but there are no boost leaks. After 5500 it will rev to redline smoothly. This is all the same whether it is under boost or not. After running it hard, coolant temps approach 250 degrees, the coolant bottle overflows, and the coolant system doesn't suck the coolant back in after it cools.
The car has a stock 6-bolt, a 92 MT Turbo ECU, hi-Z RX-7 480cc injectors, a bastard 16G with boost set to about 12psi, and a 2.5-to-3" turbo-back exhaust. Other than that, no mods.
I've swapped thermostats, and just about every other engine control piece. Fuel trims are all coming down under 100%, and I haven't seen knock over 5 counts, and that was only once.
I'm wondering if I don't have a head gasket leaking between a cylinder and a coolant passage. There is no coolant in the oil.