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Discussion Starter #1
'97 GT with only 97K miles. Leaking heater core (fogged up windshield w/heater or def on and glycol smell in cabin) as well as fluid dumping out of rear of the engine necessitated trip to shop to replace. I didnt feel like pulling the dash myself to fix.

Turns out in addition to a leaking heater core one of the water manifold tubes on the block had corroded through causing the coolant leak. So we replaced that too.

Even with the non-pressurizing system the temp gage would never get past the 'O' in normal. Ever since the 'fix' the gage runs up to 'A' or 'L'. Way too hot.

Since then the shop has replaced the thermostat twice with no fix. The fan works properly.... Comes on when A/C is selected to "Max" and at the right temp.

When the temp is up the upper radiator hose is hot but you can squeeze the hose flat.... The fluid may be circulating but nowhere near where I think it should.

I've heard about trapped air issues in the sealed system. Let it run a bunch with the cap open but that didnt seem to affect anything.

Im reasonably handy with tools... Should I pull the thermostat and do the drill-the-hole thing or is there a better idea? Any hints I can give the shop next week to get this thing running cool again?

This car/engine has a lot of life in it but not running this hot. Any ideas appreciated. Other than a cold air induction kit I installed myself in 2000 it's completely stock.

Thanks
 

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it may be the water pump
 

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When was the cooling system last flushed? I would flush system and see what that does. Sounds like you are seeing some circulation but not enough. I would say clogged or bad radiator. The other two things that come to mind is Water pump and head gasket.
 

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Since then the shop has replaced the thermostat twice with no fix. The fan works properly.... Comes on when A/C is selected to "Max" and at the right temp.

When the temp is up the upper radiator hose is hot but you can squeeze the hose flat.... The fluid may be circulating but nowhere near where I think it should.
OK, I think you have two issues going on here based up the OP.

1. the LOW speed engine cooling fan is not working. The HIGH speed fan is working. But because the set point for the HS fan is higher than the LS fan, the temp gauge is reading higher. For all practical purposes, the engine temperature is now being limited by the HS fan.

Further evidence of this is the report of how the fan works on AC. The low speed cooling fan should come on when ever the AC is on. It does not matter what the AC has been set to. I suspect that the HS fan is cycling on/off due to the AC high pressure switch.

2. The cooling system is NOT holding pressure. If it were, the top hose would be hard. Look for leaks somewhere in the system. Start with the pressure cap.
 

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Discussion Starter #5
Thanks for the tips. I was under the impression that the fan had two operating modes: 1) if the A/C was set to max the fan runs all the time regardless of engine/coolant temp and 2) the fan comes on when coolant temp dictates based on a sensor. Is this not correct?

As far as leaks, it does not appear to be losing fluid anywhere and does seem to hold pressure as noted when (slowly) removing cap after it has been running.

Last radiator flush at 60K... Guess I will start there. The thing I dont get is that cooling works fine even with a system leak until the shop gets ahold of it then it fails to function properly.

Thanks again
 

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AC off: LS fan runs when the engine is above the LS fan set point. HS fan come on when the HS fan set point is reached. Fan does not run above 45 MPH unless the motor is overheating.

AC on (regardless of setting of the AC itself), LS fan runs at speeds below 45 MPH, fan off above 45 MPH.

LS fan runs as long as the AC is on. The HS fan comes on when the motor temp rises above the HS set point (below 45 MPH and not over heating).

The HS fan will override and run ANYTIME the motor is overheating OR the AC high head pressure limit is reached.

Again, as far as the AC is concerned, on or off is all that matters. High, med, or low AC plays no part. The engine cooling fan runs at LS.

As you can see, the logic is actually fairly complex. Tanking into account the AC (on/off), AC pressure (too high), engine temperature for 4 set points (cold, LS fan, HS fan, and overheating). And car's speed (off above 45 MPH).

So do you not see how if the LS fan we not working, then the motor's temperature would be mainly goverended by the HS set point?

For an additional cross check, what does the engine temperature do if driving at an extended distance above 45 MPH? If it drops into normal range, this favors the fan theory.

If the temperature remains elevated, then there's a problem with the base cooling system. Air flow over the radiator, radiator clogged, worn water pump bearings, incorrect anti-freeze percent, T-stat problem. clogged exhaust to name a few.
 

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Discussion Starter #7
Thanks for the explanation on the fan operation.

And I should have mentioned that the temp gage reacts the same whether I'm rolling down the freeway or sitting in the driveway. No ram air cooling effect when driving at high speeds.

The slack upper radiator hose even when the temp is maxed is a big clue there is not proper fluid circulation, no?
 

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I agree. The soft upper radiator hose is a real problem. Should be hard and under pressure.

What have you done to rule out the water pump?
 

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Water pump is fairly cheap and an easy install as so with the flush and new coolant that is what I would recommend first off. Also how does the radiator look you got alot dust in your area? I see you live in cali. I don't think that would cause poor flow but just cooling problems in general but I am no expert either!
 

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Discussion Starter #10
Thought about water pump but it worked great until i got heater core/manifold replaced.

How could that jack up the water pump? Plus in my past a bad water pump leaks.

No leaks at the moment. Is the lack of leak definitive that the pump is good?

Radiator is pretty clean. car has been garaged its whole life or parked in an airport parking lot under a car cover.

Shop agreed to take it back and troubleshoot for free. Taking it in tomorrow. Will advise.

In the meantime driving my '93 Ford F150. Wish I had the Mustang fixed.
 

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Something is obviouslly slowing down your flow bearings could be going bad in water pump who knows, honestly I would start with the flush but if you say you get this problem after getting your car back from the shop and they have no clue what is wrong with it I would stay away from them or find someone else.
 
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