Re: 89 XR4 150 poppet valve over heat
You found it!!!
When the engine is going slow, it's not making much heat, and the thermostats allow what is needed for cooling to escape the block.
When you rev it up, the pressure builds up as does the heat. The poppit valve allows some water to flow out of the bottom of the block adding to the cooling.
The flow is from within, out through the round grommet as allowed by the sprung disc valve, and down the triangular shaped passage to the mid. The membranes or diaphragms are used for metering control from various places, depending on the engine, and need to be in pretty good shape to function.
The valve should come out easily. It might be stuck together by some trash. you shouldn't be able to see past it without pulling it out. Just pull/tear it out, clean up and flush it out, which might be tricky, and reassemble it, probably with new parts to replace those damaged by the trash and/or the forced dissambly.
If your water pressure wasn't pretty high, considering the plugged poppet, I'd look into the pump also. I also would pull and check the thermostats. Maybe you can use the thermostat holes for flushing.
hope it helps
John
