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Discussion Starter #1
First off, the car is a '01 ford focus 2.0 doch zetech auto. The heater blew cold to slightly warm air, so I did this to hopefully fix it.
I flush the whole coolant system with radiator flush from napa and then flushed the whole system with water, i did this at home and back flushed everything,
then put new coolant in and bled the system.
Lastly, i figured i would just throw a new thermostat in as a last resort to see if that might help,plus it was the original t-stat so it was in need of a change anyway.

flushing the heater core and coolant system helped the most, i have done this many times on other vehicles with great success.

Well, the car has o.k. heat now, but when you rev the car past 2.5-3k the heat is great, its kinda odd as it acts like its on demand, once you let it back to idle the heat kinda sucks again. so with that being said what could it be?

I guess i was thinking maybe something with the blend door but i don't know much about them and how to test them, or maybe there is something else I'm over looking?
 

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Sounds like a coolant flow issue. When you backflushed the system, did you backflush the heater core itself or only the radiator? If the heater core is a bit blocked then at lower engine speeds may not be enough flow through the system to allow adequate heat.

Another possibility is the water pump may be on its way out. For some reason it may not be able to make enough coolant flow through the system at idle but when at speed there is enough flow through the system.
 

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^^^ good hints

Bleeding system (check small hoses for plugging), checking for air/leaks - basics come first.

Last possibility that's sometimes seen is a water pump issue.
 

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Discussion Starter #4
Yes, the heater core was back flushed also.
I flushed everything forward and back twice, and more or less isolated the heater core and flushed that separate and then the radiator, i made sure to do a extra good job.
i have a little adapter kit to this this where i end up hooking up a garden hose and flushing the shit out of each system separately, aka heater core, rad, hoses and then i also cleaned the overflow tank.
 

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Those small hoses leading to the tank are the critical ones for bleeding, tiny plastic hoses get overlooked & need to be clear.
 

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How much coolant did you put back in? Often you get an air bubble and the reservoir looks full but you don't get full heat because it's not quite bled all the way.

If you didn't put the full 6L in, you likely have air still in the system.

I had these exact symptoms after a flush and fill on an '03 Zetec, and I only put in 4L before it hit the MAX mark. I put the front up on jack stands, warmed the engine up, and loosened the cap just enough to allow the air out. The level went below MIN, I added the other 2L, and presto! full heat again.
 

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My money is on the water pump. I had all of the same symptoms and did the same thing as the OP stated.

Tstat didn't help. Fluid level is normal. Still blows luke warm air and takes a long time to get the needle to the halfway mark.

Mine is also a 2001 ztec auto.

Water pump is next on my list to replace.
 
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