AT HIGHWAY SPEEDS ENGINE RUNS COOL AND HEATER BLOWS WARM

  • Tiny
  • MichaelKline
  • 1981 Toyota Pickup
  • 246 miles

(not hot but warm enough). At idle engine overheats and heater blows cold. 4 cyl 22R 2.4L 5sp with 246k miles. Recent tune-up, O2 sensor, and complete exhaust with new cat. Replaced radiator, cap, hoses, thermostat and water pump. Thermostat opens, coolant circulates, and coolant level stays full. Fan appears to be working properly, and the fan clutch looks new from previous owner(shiny & I can still read the codes printed on it). I'm stumped- any ideas?

Sunday, April 29th, 2012 AT 12:38 AM

5 Answers

  • Tiny
  • Jacobandnickolas
  • Expert
  • 68,410 posts

Make sure there are no hoses collapsing. Make sure the radiator isn't plugged.

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Sunday, April 29th, 2012 AT 1:52 AM
  • Tiny
  • MichaelKline
  • Member

I flushed the entire system yesterday, took the (new) radiator out and flushed it as well. All the hoses are new and none are collapsing. I put it all back together and tested the system from cold start. Same results- coolant starts circulating at 195 degrees, fan clutch engaged, engine overheated as heat blew cold. The only other thing I can imagine to be the problem is the head gasket, but I don't want to go tearing into the engine if that isn't the issue.

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Sunday, April 29th, 2012 AT 5:30 PM
  • Tiny
  • Jacobandnickolas
  • Expert
  • 68,410 posts

If you feel it is a gasket, check compression to see if there is low compression in one of the cylinders.

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Sunday, April 29th, 2012 AT 6:22 PM
  • Tiny
  • MichaelKline
  • Member

A bad head gasket (or cracked head) will show up in a compression check? Sorry, I don't have experience in this.

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Monday, April 30th, 2012 AT 1:21 AM
  • Tiny
  • Chopper
  • Member

No it won't show up in comp test. If your losing heat out of vents means air is building in heater core. Assuming you burped the system correctly you have a blown head gasket.

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Tuesday, May 1st, 2012 AT 5:30 AM

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