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coolant into the cylinders is a good way to see if there is a blown headgasket between the combustion chamber and a coolant galley. We see this a lot and verify though this test after a car has been overheated. Doesn't sound like you have a breach that involves the combustion chamber though (a breached the fire ring). It seems like you have a breach between oil and coolant galleys that is allowing coolant to mix in with the oil. This can happen at the headgasket but unlikely without the gasket being first breached around the combustion chamber (fire ring) which can reseat but often destroys the gasket further out as combustion gases try to find their way out. The pressure and stress on the gasket around the combustion chamber is much higher around the combustion chamber than around the oil or coolant galleys. From past experience I still say an internal crack between an oil and coolant galley may be your problem. To find the problem and prove it to myself, considering I was never able to actually see the internal crack, we took the short block and blocked off all the coolant passages. We first put a water pump on it. We have flat solid aluminum plates (that we use to check leakdown past valves on fresh heads) that we bolted on to the block with headgaskets sandwiched in between. We have some other plates that were made to block of the remaining passages. We filled the coolant paasages in the block with water and then introduced pressurized air into the coolant galleys though a hole tapped into one of the plates with a schrader valve threaded in. A few seconds later water started running on to the floor flowing from the oil galley where the oil filter tree bolts up. Water was running out of a port that carries oil from the filter towards the main galley. There was an internal crack in this block that there was no way to see or find without destoying the block. This was years ago. We went through everything a couple times on the car (as you have) and lost several thousand dollars on the job. I've seen it several times since, but now we test for it we are suspicious.
Russell Floyd Z1 Motorsports
www.300zx.com
1990+ Nissan Z specialists Used Parts, New parts, Service/Repair and Performance
Z1 Motorsports Exclusive Parts Line 2877 Carrollton Villa Rica Hwy
Carrollton, Ga 30116 770-838-7777
770-832-9253 (fax) 
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20:20:13 01/07/10
