Dazed and Confused
Join Date: May 2012
Location: Upstate New York
Re: 05 Mariner Temp Controller
Not really a good test that I know of that's definitive, but, if you run the heat on high with the engine running, check the differential temperature across the heater hoses, there shouldn't be more than maybe 15 degrees. They used to have a weep hole that gave you a tattle tail if the seals were gone, but now they just port it internally. Play in the pump shaft would be a definitive indicator, but you would normally hear that. And last, pull the pump and see what the impeller looks like. Newer pumps seem to be more cheaply made than ever before. Being that it's 8 years old, if your mileage is high, pump could be a suspect. There are a few things to look at prior to pulling the pump, if the engine temperature is fluctuating with heater output, you may have a sticky thermostat. If you've never had the coolant system flushed, it might be fouling in the heater core, but this doesn't sound like it. Something else that occurred to me just as I was about to hit the reply button, The mariner has multiple fans, they could be causing a large swing in temperature if they aren't staging properly, but you should see this with the temperature gauge on the dash.
Chewing on asphalt and drinking dinosaur wine