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Discussion Starter #1
2004 Ford Focus (MK1 USA) 2.0l Zetec, 4-spd auto, w/ a/c, 112k miles. I know an overheating condition can be caused by multiple things, so I'm asking for assistance in narrowing down causes. Here are basic symptoms:
  • Engine overheats, worse at higher revs/speeds.
  • Turning on heater reduces coolant temp relatively quickly, and heat works well
  • No sign of coolant leaks or decrease in coolant levels in reservoir.
  • flushed/replaced coolant, but despite getting up to normal op temperature, system only accepted 4 quarts instead of the 6.1 stated capacity. Note idiot move: did not jack up front of car as I did this.
  • Confirmed that radiator fan does turn on as temp rises.
Thinking about the thermostat first, but, if the thermostat was stuck closed, then I would guess that the coolant temp wouldn't go down by turning on the heat, since that would indicate that the hot coolant isn't trapped in the engine by the stuck thermostat, if it's circulating to the heater core? Just a guess and I could be way off base.

If it's an air bubble, would the heat be working so well?

Finally, I should mention that I recently changed the PCV valve and hose. I don't think this would cause an overheat directly, possible I messed something else up doing that job.

I'm leaning towards replacing the thermostat first, since that's not too bad to do. Thanks for your help.
 

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It may/may not be the thermostat. Turning on the heater fan just acts as another heat exchanger (albiet a very inefficient one) to remove heat from the engine/coolant. Problem could be be a failing water pump, system clogged somewhere restricting coolant flow, or a cooling fan issue >> are you certain both fans are running and run in both low and high speed?

If you are going to replace the thermostat Ive done a few Zetec thermostats and offer some advice:
1. Buy a quality thermostat (ie Motorcraft) they are not that expensive
2. Carefully inspect the thermostat housing where its seal to the cylinder head is - the plastic groove for the seal is prone to failure. Replace the housing if the seal groove has failed and replace the seal either way.
2. Put effort into thoroughly cleaning the cylinder head where the housing seal sits - razor scraper, die grinder with abrasive pad, etc. Chase the threads in the mounting holes.

Good luck
Paul
 

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Discussion Starter #5 (Edited)
Thank you. Question: if a thermostat is stuck closed, shouldn't that trap the hot coolant in the engine, preventing it from circulating into the heater core, and thus prevent the HVAC from getting hot? Similarly, if the coolant temp sensor is in the block with the trapped hot coolant, should the coolant temp be going down when I turn on the heat and else the heater core? Just trying to piece together an understanding of the cooling system...
 

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The heater core ALWAYS circulates on these, the loop for it bypasses the stat. There is no water control valve on the heater loop either.

The stat has a little bleed hole that must face straight up to bleed engine correctly, do that and the engines self-bleed of air by the time the stat opens. There is NEVER any need to 'burp them' at all. Once the engine comes up close to temp then race the throttle at 2000 rpm for maybe a half minute and the agitation will bleed the system out. Done.
 

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It may/may not be the thermostat. Turning on the heater fan just acts as another heat exchanger (albiet a very inefficient one) to remove heat from the engine/coolant. Problem could be be a failing water pump, system clogged somewhere restricting coolant flow, or a cooling fan issue >> are you certain both fans are running and run in both low and high speed?

If you are going to replace the thermostat Ive done a few Zetec thermostats and offer some advice:
1. Buy a quality thermostat (ie Motorcraft) they are not that expensive
2. Carefully inspect the thermostat housing where its seal to the cylinder head is - the plastic groove for the seal is prone to failure. Replace the housing if the seal groove has failed and replace the seal either way.
2. Put effort into thoroughly cleaning the cylinder head where the housing seal sits - razor scraper, die grinder with abrasive pad, etc. Chase the threads in the mounting holes.

Good luck
Paul
It works. Thank you very much for the information.
 
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