I ran without a cooler for a while in my jet boat and always had the same problem. Sometimes it eaven lost pressure at high rpm so I would slow way down or just stop and let it cool. When I put it in the stern drive and added the cooler ( I know different situations) the motor NEVER looses pressure and the temp on the oil and water is always real cold. On the jet the water temp was 0 all the time but not at idle.Hope that helps a bit. OH and I will NEVER run without the oil cooler again.