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Old 09-30-2012, 19:57   #1 (permalink)
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1998 Grand Marquis Overheating

Hi, I have a weird overheating problem on my 1998 Grand Marquis. After about 10 minutes of driving the engine overheats and coolant comes out of the reservoir. But after that initial overheating episode the car runs fine and doesnt continue to overheat. It acts as if the thermostat is opening to late. Once the therostat opens everything is fine. If I let the car idle in the driveway and wait for the thermostat to open before I start driving everything is fine. I replaced the water pump and replaced the thermostat twice. Anybody have any ideas what could be causing the overheating? I noticed that there doesnt seem to be much coolant going through the heater hoses. Im not sure if that is normal or how much coolant normally goes through in the first place.

Thanks for any help anyone can give.
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Old 09-30-2012, 20:18   #2 (permalink)
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Re: 1998 Grand Marquis Overheating

I can't say why it only runs hot if you drive off right away. Haven't seen that before. As for the heater hoses, they should get hot when the engine is hot. There's no shut off for the coolant. How does it do on a hill or hard pull?
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Old 09-30-2012, 20:23   #3 (permalink)
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Re: 1998 Grand Marquis Overheating

The heater hoses do get hot and the heat works fine. It just seemed like there wasnt much water flowing throgh the hoses. Also the car runs fine.
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Old 10-01-2012, 08:13   #4 (permalink)
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Re: 1998 Grand Marquis Overheating

Could try bleeding the heater hoses / cooling system if still not happy with flow , though not normally needed .
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Old 10-01-2012, 11:27   #5 (permalink)
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Re: 1998 Grand Marquis Overheating

Well, that is definitely weird.

You could remove the hoses from the heater core and loop them together. This would eliminate a heater core blockage. Although this seems unlikely, I have seen some really strange stuff.

Dunno. Good luck!
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Old 10-01-2012, 12:23   #6 (permalink)
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Re: 1998 Grand Marquis Overheating

I decided to try to bleed the cooling system. In the Ford technican service manual it tells to bleed the system by turning the heat on full, put the reservoir cap on and run the engine until the thermostat opens and the hot water fills the whole radiator. Then shut the car off, wait for it to cool down and then fill reservoir to proper level. I did that but mistakenly thought it said to keep the reservoir cap off. After about 7 minutes of the engine running coolant started to flow out the top of the reservoir before the thermostat had a chance to open up and I shut the car off. Should the coolant be coming out like that? I then put the cap on to do it the proper way. While the engine was running the fan came on and the thermostat(195 deg.) had not opened up yet. I thought I read somewhere that the fan doesnt come on until the engine temp reaches 195 degrees. The thermostat eventually opened but not until about 5 minutes after the fan went on. Also before the thermostat opened the heater hoses were warm not hot and the heater was blowing just barely warm air. As soon as the thermostat opened the heater started blow hot air, the heater hoses got hot, and the engine fan went off. So should there be more coolant flowing through the heater core than there is when the thermostat is closed? And should coolant be flowing out of the top of the reservoir when the cap is off and before the thermostat is open?
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Old 10-01-2012, 12:41   #7 (permalink)
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Re: 1998 Grand Marquis Overheating

Correct thermostat and not sticking ? there are some earlier opening ones here for example which might help .

Family Car - Auto Parts and Accessories Catalog - Mercury Grand Marquis Thermostat 1998
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Old 10-01-2012, 12:56   #8 (permalink)
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Re: 1998 Grand Marquis Overheating

195 degree thermostat and replaced it twice because I was concerned it wasnt opening at proper temp.

Last edited by alydar66; 10-01-2012 at 12:57. Reason: mispelled word
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Old 10-01-2012, 17:41   #9 (permalink)
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Re: 1998 Grand Marquis Overheating

try changing the radiator cap. The cap acts like a relief valve and if yours is too low of a pressure or it's failing, it might be dumping off at too low of a pressure.
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Old 10-02-2012, 21:22   #10 (permalink)
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Re: 1998 Grand Marquis Overheating

Bled the system and so far so good.
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