no heat in 94 chevy

[chevy_4x4_trucks]

alright so this is my dads truck that is having the problem. its a 94 with the 5.7 in it. he says that it heats up to about 200 then drops down to about 120 and stays there. he has put 3 different thermostats in it all of which were different brands. he has also checked the thermostats outside of the truck to make sure they are working properly and all 3 are. the coolant is full and has no air in the system... any ideas what his problem could be?

[black07ss]

I'm curious on this cause my 89 does the same thing.  I've got a 192 t-stat maybe go up to 195.   What degree is he runnin?

[chevy_4x4_trucks]

im not exactly sure but i thought he said something about a 190 but i may be wrong

[old school truck dude]

Maybe has air in the system somewhere or heater hoses backwards..? Sometimes the guage reads funny and dont get very warm inside when theyre a little low - Id take cap off, speed motor up and hold that rpm and watch the flow, top off coolant, put cap back on and squeeze the top hose a few good times, kinda burp it? Make sure the recovery tank is in good shape and line to it not plugged also.. When its real cold you might need to cover part of the radiator, had to on mine before.. You want a 195 stat for sure..I had a door in the box messed up too behind the radio made a BIG difference cuz all the heat was blowin out under the dash instead of the upper or lower vents.   Does the motor run ok like that

[ToughBowtieTruck]

Do a pressure test, make sure the cooling system is sealed.

Then do a pressure test on the radiator cap. Radiator cap might be junk. These systems have to have pressure to heat up to 190 degrees and maintain it. Make sure its a 14-16lb cap.

The engine temperature seems to the be the issue, the coolant is cold before it gets to the heater system, eliminating the heater components of fault . . .

[justin0421]

Just had this problem on my brothers the other day put a new water pump on it and took the heater core lines off blew threw them and the heater core with a blow gun and it works great now. But as mentioned above check the radiator cap u can get a new one for like 5 bucks

[buddman (AKA Mick)]

Back flush the heater core . Very comon!!!!

[chevy_4x4_trucks]

the problem isnt that its not blowing heat its that the engine isnt running at the correct temp. so its not a problem inside the truck it has to be something in or on the engine

[Mudbuster]

are you reading it off the factory guage? if so try puting an aftermarket guage on the factory guages arn't always accurate.

[Captn. J]

temp sensor? or whatever u call it?

[Ranger Dave]

thurmostat  or  rad cap   

[old school truck dude]

Along with what Bowtie says up there, ya mite ought to figure out if the guage is readin right and go from there. Ive had a few that both didnt read read right on the guage and also had messed up doors in the box, so ya gotta look at all that. It could be gettin good n hot but the door not openin, ya know? This aint hard, youll figure it out 

[chevy_4x4_trucks]

ok we still havnt figured this out yet...i talked to him more tonight about it and he says the engine just  isnt getting up to temp... he said it was almost acting like there was no thermostat in it at all and its just running cold the whole time. I told him to check the heater door and to double check the gauge. he also said that if he puts cardboard in front of the radiator he can get heat but then when we get these warmer days the truck wants to over heat because it. and he doesnt want to keep taking the cardboard out and putting it back in all the time

[old school truck dude]

Ok it needs a new 195 thermostat. Dont matter if it already has a new one, just cuz its new dont mean its good. If he can get heat with cardboard in front of the radiator, thats whats goin on. I dont really see what else it could be other than a radiator thats way too big, a water pump that pumps too much, or maybe a fan clutch thats stuck.

[ToughBowtieTruck]

Ok it needs a new 195 thermostat. Dont matter if it already has a new one, just cuz its new dont mean its good. If he can get heat with cardboard in front of the radiator, thats whats goin on. I dont really see what else it could be other than a radiator thats way too big, a water pump that pumps too much, or maybe a fan clutch thats stuck.

Bad radiator cap. Without pressure to help maintain the temperature the thermostat has a hard time keeping the engine in range. They work together.

[old school truck dude]

I agree it has to have the pressure, thats for sure.. I guess I was also thinkin that he had replaced this already and it was still actin up so all I could think of was the thermostat.. What things have you actually done to try and cure this problem?

[chevy_4x4_trucks]

like i said in the first post he has tried 3 different stats all of different brands and he has checked all three stats outside of the truck (stuck them in boiling water to see if they open) and they all did so it is not a stat problem, im hoping toughbowtietruck is right and its the cap. i told him to check it but he only uses the truck to plow and with all the nice weather we have been having he hasnt done anything with it, to many other projects so i am probably going to have to get out there and dink around with it.

[buddman (AKA Mick)]

Old school mite be on to somthing, stuck fan clutch???

[chevy_4x4_trucks]

yeah i thought that too when I read that i planned on checking it out when I swapped out the caps

[old school truck dude]

Old school mite be on to somthing, stuck fan clutch???

That would sure make it run cold