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ok, so, my trucks been overheating. once on the way from the shop, and to the shop in the morning. like clockwork. same 1 mile stretch bothways. (i was told to take a diiferent way home) at almost the halfway point. bout 5-10 mins into the drive. temp goes up, i get the idiot lite, then it cools off to under 140 the rest of the drive. did the thermostat a week ago already. water pump within the last year or so. i cant beieve its some kind of sender. it seems to like doing it only when i hop on the highway almost right off. but then i can drive it for like 5 hours straight wit no problems.

any ideas? 94 chevy 5.7

jc
 

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If you're running Dex-cool (the orange crap) you may need to flush and fill the system. Also, if you've mixed the orange crap with green anti-freeze, you'll need to flush and fill with one or the other as they don't mix. The orange crap goes bad after a while and gums everything up. I figured that out on my 94 Blazer after it kept overheating sporadically.....

Jim
 

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Discussion Starter #4
ok, ill give it a try tomorrow.

thanks!

jc
 

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and only the green crap. i already replaced the intake from the shitty antifreeze destroying the intake and leaking coolant into the rear drivers side cyl. it destroyed alot of stuff in my motor. so im pretty good at paying attention to it.

thanks again.

ill bleed it, top it and see what happens.

jc
 

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Alright, I deal with alot of overheating trucks at work. So it doesn't overheat on the freeway is what I understand. In stop and go traffic will it over heat? Do you have electric fans or clutch driven fans? First thing I would do really is start it up in the driveway and let it run for awhile and heat up. When it starts overheating do the fans come on? Both electrical and clutch driven fans can go bad. If they do come on, next feel the top and bottom radiator hoses. They should both be hot and about the same temp each. If not, you have no or very little coolant flow. Which could result from just a bad thermostat.

I'm assuming also that the truck is running good and the radiator+resevoir are full up on water or coolant. Also, check for any leaks on the radiator itself. A leak would cause poor pressure build up and thus lower the boiling point. Hope I was of some service.
 

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Discussion Starter #7
actually what happens is, after a given time, like, say 15 mins, of jumping right on the freeway, it overheats, then the thermostat opens, and it cools right back down. only does it once, at about the same exact point in my trip. then it wont do it again. i tried an experiment this morning and while driving at full speed, i shut it off when i saw it really start to go up. then turne it back on. it went right down when i id that. i put in a new radiator about 2 winters ago. its good. wouldnt the overflow prevent any kind of "bubble"

jc
 

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Discussion Starter #8
oh, and ckutch fan. otherwise its running great.

jc
 

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Sounds almost like the thermostat is sticking shut. Then gets hot enough to open up.

Could also be a bad sending unit just making you think it's overheating.
JohnnyB
 

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just did the thermostat. its doing the same exact thing it was before. thats why i tried the shut it off then restart it thing. really weird. sending unit....wonder where that is. maybe top of the block...nah, probably not.



jc
 

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The sending unit is usually either back by the distributor, or somewhere below the exhaust manifolds (V8). Sometimes if you are really lucky it's on the intake in the water crossover.

Didn't put the thermostate in upside down did you? Yes I've seen it done quite a few times. When that's done the heat sensing part is away from the hot water and it takes a while for the whole thing to heat up enough for it to open.

How cold it is outside? And....is the heat in the truck working consistantly the whole time?
JohnnyB
 

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Discussion Starter #12
see, like i said, i replaced the thermostat for this exact reason, so nothing changed. and no, its not in upside down. been there, done that, as a kid. the heat is perfect. the usual scorching 150 degrees all older trucks have. it was 70 here thursday and monday. 32 today. nothings changed. might be time to look into the sensor. other wise the thing runs great! and like i said, it only seems to do it once durring a trip. then its happy.

jc
 

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thermostat</u>- make sure it is in right, even so, sometimes they sell crappy ones so it might be worth it to get another. you can check it by pulling it and heating it up and seeing if the spring pops open. your haynes manual should tell the temp it is fully open

Temp sending unit</u>your car might not be over heating, just could be a bad sending unit. Hanyes manual has a way to check it i believe. A sign of when a car overheats along with temp gauge, the oil pressure starts to drop also. When car/truck whatever over heats, i have seen/heard of sending units going bad. My moms bug, the coolant/oil pressure sending unit went out after it overheated.

radiator</u>- drain it and flush it. Put the hose in one end and see the flow come out the other. if it about the same coming out of the hose it should be good. one thing people like to do is put tap water in it and just a little bit of hardness will build up over time. Also check the fins and make sure there isn't any mud in them or something blocking them. when you dump old fluid take a small sample and hold it up to the light and check for oil in water. Since there is oil pressure and coolant normally isnt under as much pressure, oil tends to leak into the coolant. And like soem have said, make sure the system is bleed correctly

water pump</u>dont think it would be it, only ran into one case where the water pump went out and no leaks...but the pulley was doing some serious wobbling so that was bad guy in that case.

Fans</u>if one is mechanical and is spinning ok with no play in the pulley/shaft, wouldnt worry about the mechanical fan. If electrical, get it up to temp. and check to see if they go on. if they dont, get the volt meter out and check the voltage going to the fan(s), if no voltage, check the fan relay's and fan switch.

Plugs</u>If you have done a tune up lately, pull plugs and check the gapping. Even if they come pre-gapped, i ALWAYS double check. Make sure they are the right ones.

-now i have heard of a case where there was a blockage in heater core that took the flow of coolant down, so that is possible also.

You might have known some of this already, so not to offend, i summed up what i would have done/looked for. If i think of anything else ill post but thats just off the top of my head. do post when you figure out what it is!
 

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with all that work to do on his truck, joe won't be able to figure out how to bend the rules anymore before racing season begins.
-parks
ps he won't even be able to get them changed either.
pps maybe his kick starter plug is plugging up his coolant system. that must be it.
 

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Those trucks are pretty popular for overheating because of an air bubble. Bleed the system and you'll be good. Same thing happened with mine
 

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air bubble.i fourth that,also check the fan by.cold it shouldnt spin more than 3 times/but no more than 5times, should have resistance. hot it should spin more freely but with some resistance.fan shroud in place as well,helps pull air past the radiator.speaking of radiators check for bugs and bent fins,also check between condensor and radiator for newspaper.it tends to find it self in that spot.
 
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