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Discussion Starter #1 (Edited)
I'll start at the beginning:

As I was driving my 2003 Dad-Mobile (wagon) last night in the cold, the heater was only blowing warm air when I had the car at high speeds and after I increased my speed suddenly. Sitting at idle, the air was cold. The engine temp was normal so I figured that maybe it was a little low on coolant.

This morning, I topped up the coolant and the heater was still blowing cold, sometimes warm, sometimes hot air.

Here's a quirk- I left the heat adjusting knob set to max hot and as soon as I switched on the AC? Hot air came out. [8]

So far, I've only added coolant and operated the hot/cool adjustment knob. I can hear a clunk noise as I do that and it seems to be operating with the usual pressure and with no new "feelings."

I'm not sure yet how the system operates and haven't found anything on the net yet. Some education or outright guidance would help.
 

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Discussion Starter #2
So, I went back outside to take a look at things again and the expansion tank had settled further. I started the car and held a hand by the vents and little by little heat started. I topped the expansion tank up again and this time noticed a leak near the thermostat? Drivers side, upper, big hose and expansion hose attached?
I took the car for a spin to loosen any air in the cooling system and the heat was unbearable- back to normal. The leak isn't happening so it's a "cold leak."

Now I have the root cause of my heat problem. Leak made the coolant level dip below the suction line, introduced air into the system, blah blah blah.

But. Why would the AC being on help the poor thing make heat?
 

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Discussion Starter #3
It's a cracked thermo housing. I had the same thing wrong with my 2000 Wagon. Memory is returning.

The 70's were hard on me.

So it turns out that I solved this myself but aren't you glad that I rely on your vastly superior experience with Focuses when I think I'm in a rut?
 

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Thermostat housing, the black boxy thing that the hoses go into? Yes, first-gen Foci are prone to that. Easy fix, might want to replace the thermostat while you're in there if it hasn't been done in awhile. I didn't, but I'm cheap and hate replacing stuff that's still working.

Make sure you clean off the block surface before installing the new one. Mine had pitted a bit, so I smeared a very thin film of RTV sealant over it just to get it flush. Do not use more than a tiny bit or it'll end up breaking off and jamming up a coolant port or worse.

Also, keep an eye on the expansion tanks. Both of mine have ended up cracking on the inside, closest to the engine and near the bottom. They started as small leaks but turned into large ones in a hurry.

The AC thing is a bit of a puzzler for me. Hoping a smarter poster can figure it out for you.
 

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Discussion Starter #5
I found the part I need. It comes with the housing (of course) and the thermostat. The sensor is probably still good because the engine is operating normally otherwise. This won't be a show stopper. It waited until 150K miles to happen when most others crack at about 100K. I was lucky?
 

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My original housing from my '01 had 394k kms on it. When the one on my '03 started leaking at 350k kms, I just installed the old housing. Just rolled 486k kms, and it's still going strong...with the equivalent of 350k miles on it. Not too shabby!

Must have been a really good day on the production line...
 

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A/C shouldn't make a difference, unless you hit 'recirc" as well to use inside air through the system. When not rolling the slight engine speed changes might make a difference when it was low on coolant, gas pedal changes would do more.
 
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