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Old 05-19-2011, 12:59 AM   #1
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Default 1998 4.6liter temp gauge issue

I have a new to me 1998 Expy XLT 4x4 with 4.6 liter. My temp gauge alway goes right to high when started and doesn't move until key is turned off. I get no codes, current or history. I changed the grey sensor and no change, this always fixed the problem in my 91 explorer. If I disconnect the sensor the gauge behaves the same. The heater always blows cold. The coolant reservoir has fluid and the level changes. Cap hisses when opened. Any suggestions? I'd really like to fix this. I take it off road and don't want to burn the beast down. Also gets colder than a witches @*^ during the winter so heat would be nice.
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Old 05-19-2011, 01:40 AM   #2
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Have you checked the thermostat
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Old 05-19-2011, 01:48 AM   #3
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I have a new one but it's been raining the last few days. I am hoping its the thermostat BUT the upper hose does get warm. I can hold it and squeeze it without burning my hand. It seems fairly flaccid for lack of a better word. I would expect it to be firm when running and water is going through it.
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Old 05-19-2011, 07:00 PM   #4
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bump
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Old 05-22-2011, 09:39 PM   #5
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OK, I changed the thermostat and topped off the coolant in the reservoir. Temp guage still pegs to high when keyn it turned. I'm out driving today and oil temp light goes on, check engine lightgoes on and starts running weak and kinda rough. I pull over at the gas station I was headed to. While getting gas I remember I have my harbor freight OBD2 code reader. I plug it in and pull code P1299 CHT over heat and no other codes. I clear the code and leave the reader attached. I set it to read live temperature output. Temp ranged from 170 parked, 195 to 205 with a peak of 212 going up hill. No further codes and no leaks. I'd love to know what is going on. the prior owner said it likely had a blown headgasket. No visible leaks, now exhaust cload of any color. There is a noise from the passenger side, that sounds like an exhaust leak to me. Looking for pointers of dirrection to persue. Truck is a 1998 XLT with 4.6 liter and already changed the temp sensor(furthest frontward on passenger side with the black 2 wire connector

edit. since purcase the oil pressure reads just below to slightly over the high line.

Last edited by freeulster; 05-22-2011 at 09:41 PM. Reason: added detail
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Old 05-22-2011, 10:51 PM   #6
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Default cht sensor failure vs. blown headgasket

A ford tech on answer.com suggests a faulty cht causing my problems, any feedback.

Last edited by freeulster; 05-22-2011 at 11:23 PM.
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Old 05-22-2011, 11:27 PM   #7
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There's a lot going on here. First the oil temp gauge is just a switch not a pressure reading so as long as it's not bottomed out or bouncing it's fine. For your heat blowing cold that is most likely your heater core. Take both of the heater hoses off and spray water with a hose into one of the heater core tubes and see if you get any to flow out the other one. A blown head gasket could cause the CHT over temp but you should find a low coolant condition in that case. Either change and look at the oil or usually you can see a milky nasty substance on the dipstick when you check the oil if there is coolant leaking into the oil passages. I want to say that the CHT is in between cylinders 3 and 4 on the passenger side head so it's probably pretty difficult to change, might have to pull the intake. If it's not there it's between 5 and 6 on the drivers side which might be easier to get at.

BUT there may be a simpler answer which would be the water pump. And since they're pretty easy to get off I'd recommend at least pulling it off and looking at the impeller in there to see if it's gummed up or rusted out or anything. If there's no coolant flowing you're going to overheat AND there wont be coolant flowing through the heater core so it's definitely a possibility.

PS that's a Ford tech answer from f150forum.com
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Old 05-22-2011, 11:32 PM   #8
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good luck changing the heater core on that truck.. if the water pump was bad itd get a lot hotter than 212 which is completely normal.

when the truck is at operating temp are the radiator hoses firm? if they arent you need to check for leaks, or get a new rad cap.

it could be that theres not enough pressure getting built up *(faulty rad cap*)to push the fluid through the heater core but there is enough pressure to where enough fluid flows so it doesnt overheat.
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Old 05-23-2011, 01:08 PM   #9
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I've seen a lot of water pumps that the impeller comes apart and pushes some coolant but not enough, plus generally if it hisses when you take the cap off there was pressure built up and he did have a CHT over temp code is why I went with the bad water pump idea.

That is a good point though and I think you can borrow the cap tester and cooling system tester from Autozone so it wouldn't hurt to check them out.

Edit: The heater cores aren't THAT bad, but you do have to pull the steering column and entire dash. Should be able to do it on a saturday, just keep all the screws and bolts organized so you know where they go back.
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Old 06-13-2011, 01:16 AM   #10
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It's been a while since I checked in. Thanks for the answers guys. FWIW the upper radiator hose is NOT firm when running and at operating temps. I did use a NAPA block test kit to check for a blown head gasket and it didn't indicate a leak into the coolant. No milkiness or froth in the oil. Coolant reservoir cap is brand new, it was replaced when the original one blew off while driving.
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Old 06-13-2011, 01:16 AM
 
 
 
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