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Discussion Starter · #1 ·
I've got a 2000, 24-valve, 150k that randomly overheats. I have seen absolutely no correlation between outside temperature, engine load, rpm, or speed. Sometimes it runs hot, other times it runs fine. Replaced water pump and thermostat. Cooling system holds pressure. Stumped, I took it to a mechanic who claimed to detect hydrocarbons in the coolant. He says this indicates a blown head gasket. Would a cooling system still hold pressure with a blown head gasket? Thanks!
 

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A head gasket can fail in many ways..

- Gasket fails between the cylinder bores
Symptoms: Gradual and steady overheating, no outward signs of smoking or coolant loss or oil loss, may stall upon starting when engine is cold.

Diagnosing: Use a compression tester and take readings. This happened on the Vulcan in my Mazda B3000, I had 155-150-154 on the passenger side and 90-102-95 on the driver side.

- Gasket fails at oil supply/drain back into cylinder bore
Symptoms: Oil loss, greyish smoke with a blue tint coming out of tail pipe.

Diagnosing: Look at the tailpipe, spark plug may also give you some indication and the obvious, check your oil.

- Gasket fails at the water jacket into cylinder bore
Symptoms: Coolant loss, white smoke coming out of tail pipe.

Diagnosing: Smell the tailpipe, spark plug will also give indications of contact with antifreeze and, again, the obvious, check your coolant levels often. Combustion gasses will now be present in the cooling system, you can purchase the detection tool from NAPA for around $30, looks like a giant crayon you fill with blue liquid and jam into the neck of the radiator/coolant bottle. It changes to yellow when combustion gasses react with the fluid.

- Gasket fails at the water jacket into oil supply/drain back
Symptoms: Coolant and oil mix.

Diagnosing: Check your oil, it should look like a thicker version of chocolate milk.

But do I feel it is a head gasket? Not so much. The better question is, is it actually overheating? I would remove the radiator cap and allow the car to get to the temperature at which you think it is overheating. A 50/50 mix of antifreeze and water should boil at 257 °F (With the cap on it will push it to just over 300 °F). Been doing this for roughly 10 years now and number one thing I see, at least here in the south where the winters are tame, that no one maintains their cooling system properly. Coolant doesn't get flushed until something needs replacing and distilled or de-ionized water is replaced with tap water. Given that, I see A LOT of corroded up temperature sending units, which adds resistance to the sensor, which gives it a false higher reading.
 

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Discussion Starter · #3 ·
When it's hot, it will boil the coolant in the overflow bottle. I've seen nothing in the exhaust, and the oil looks fine. Guess it's time to do a compression test. It just puzzles me that the problem is so inconsistent.
 

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Discussion Starter · #4 ·
Ran a compression test. It's got 127-123-128 on the left and 136-135-130 on the right. I'm not convinced it's a head gasket. Any other ideas? Thanks!
 

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I'm going to go out on a limb and say, if you temperature sending unit is on the edge of going bad this is what may be giving you intermittent readings.

Another item that may produce these intermittent readings is a thermostat that is also on the edge of going bad. For example, the car has a normal temperature when cruising but may overheat while idling at a stop light.

Just my 2¢.

Monsoon
 

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Are the cooling fans running when it overheats?
 

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I would check on those fans. Otherwise, I would replace the radiator (it may become partially restricted and cause issues) with a Performance Radiator brand, they have more fins and slightly better cooling that stock, and at the same time replace the thermostat with a new Motorcraft one. I wasn't having random overheats, but I was running hot on longer trips, new radiator and thermostat fixed the problem.
 

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Discussion Starter · #8 ·
I was finally able to get a good temperature measurement when the car temp gauge was reading "Hot". It was 215 degrees. So I figure the temp sender needed replaced. I was intrigued because the fans were running on high. I went ahead and replaced both the temp sensor and sender at the same time. Time to drive it a few days and see if it is better.

Does anyone know the temps at which the fans are supposed run?
 

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Anything between 190F and 205F I would consider in the "normal" range.

Did you consider a partially clogged radiator might be one of you problems? I change my coolant every 33K miles religiously. I've never had overheating issues.

Monsoon.
 

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Discussion Starter · #10 ·
I change coolant every 30 to 40k. The majority of the time the car runs right about 190 regardless of speed, outside temp, etc... I seem to get these random spikes of heat. Could it be possible that something is floating around in the cooling system and periodically blocking flow somewhere?
 

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Health and Status Check....

I would check the health and status on the following items:
* water pump, some of the Duratec pumps been known to deteriorate over time, this is a defect with specific pumps. My original pump lasted 120K miles.
* thermostat, if your thermostat has over 100K miles it I would suggest changing it
* pressure test the cooling system for leaks
* belt tension, old belt tends to glaze/crack overtime
* proper fan operation

With the fan operation from my experience and I could be wrong but the driver's side fan seems to be the primary fan for cooling. When engine temp get to be about 195F this fan turns on and stays on until the engine is about 190F then shuts off, I can't remember if the passenger fan cycles as well. You can verify this operation if you have a laptop computer and autoenginuity software. Monitor the engine temp with the fan cycling on and off.

Autoenginuity software may be your best tool to diagnose your overheat problem, Windows and PDA compatible only.

Of course both fans come on when the A/C is engaged.

Monsoon
 

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Discussion Starter · #12 ·
The further I dig into this problem, the strange it gets. I've used two different thermometers and verified that normal coolant temp runs around 190 degrees. On a very hot day with AC and a heavy load, the coolant can run between 210 and 215. When the actual temp is 190, the car computer thinks the temperature is around 225 and turns the fans on high. Around 210 actual degrees, the car thinks it's running around 240 and the temp gauge will be reading hot.

Okay, so I replaced both the coolant temp sensor and sender. Neither one looked to be very corroded. No change in symptoms. The car still thinks it running hotter than it really is according to both the gauge and computer.

So, any ideas what can cause higher than normal readings in both the temp sensor and sender? Do they share a ground somewhere that could be faulty?
 
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