3406C engine is building too much cooling system psi check engine light on

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This engine is building excessive pressure in cooling system. It will not build pressure at idle or even high idle, but seems to as it is driven down the road. If the engine is run under a load with the radiator cap loose the check engine light will not come on. If you allow the system to build pressure by leaving cap tight the chek engine light comes on. No sign of water in the oil.