Problem solved

[triplel43762]

I am trying to get heat on my 87 and it doesn’t seem to circulate correct no matter what I do! Should the 3/4 coming off the intake pump water while running? I would think so because it’s right beside the thermostat that flows when open. New thermostat and water pump. Everything motor wise works perfectly. I used a new mishimoto radiator I had for a Cummins but that should not affect the heater core/ system. With that intake line off I can blow coolant through the heater core!!!! Thanks in advance!! The black line from the intake to core and red from core to lower radiator hose.

 

[triplel43762]

According to this diagram, coolant should be flowing out of the intake hose ??! Any input????

 

[Itali83]

Only thing that comes to mind is that maybe the intake manifold gaskets are installed wrong and blocking the port to the intake. I didn’t think that could be done until the ‘87 Jimmy had them installed backwards by the previous owner. I had slightly different symptoms but something has to be blocking flow.

To be clear. With the engine running, to radiator hose hot and up to temp, the heater core hoses are cold? At idle and at cruising rpm? Blown head gaskets or cracked heads will put combustion gasses and pressure in the cooling system, which will collect in the heater core because it’s the highest part of the system. At idle, there isn’t enough flow to push past the vapor lock and you’ll loose heat. Until you gain rpm again to push it through.

Let us know.

Ben

 

[75gmck25]

My ‘75 GMC still has the original routing for the heater hoses. The hose off the intake manifold sends water to the heater core, and the return hose connects to a fitting on the radiator.

If I have the radiator cap off with engine running, there is a very steady stream of water gushing back into the radiator. In your setup it would be going back to the water pump.

 

[SquareRoot]

If I'm not mistaken, 87 would have a serpentine belt and reverse flow pump. Your pics show a v-belt. Not saying that's your problem, just an observation. Is that the original engine?

 

[triplel43762]

the motor is out of an 82 and worked fine in my old wood truck! I put it in this truck and No bueno. The heater worked before! I was thinking about the possibility of a blown head gasket, but really no reason for it just swapping motors. How coolant doesn’t come out of the black line coming off the intake while running really blows my mind.

 

[triplel43762]

how could you check for a blown head gasket? Seems to run fine and no water in the oil. I know that doesn’t rule it out though!?

 

[oldretiredafguy]

1987-first year of TBI. Not unusual to find early TBI vehicles with the old belt setup. I don't have a wiring diagram of my '88 V-Jimmy, but wasn't there a valve controlled by the heat/cool mix lever that shut off fluid flow to the heater core when not needed? I don't remember if it was electrical, strictly mechanical, or vaccum.

Most of the aftermarket parts suppliers tend to ignore the '87 and up TBI trucks, and their different requirements (Baffled fuel tanks, hi pressure fuel pumps, TBI, ECM/ECU). That's why this forum is so valuable because individuals discuss their unique problems with TBI trucks.

 

[Itali83]

‘87 and ‘88. Did NOT have reverse flow water pumps, period. They did have a serpentine belt for the water pump only but still had the alternator adjustment for that belt. It wasn’t until ‘89 that they went to the single serpentine belt for all accessories that the water pump is reverse flow. I have all three years in my yard, all stock and have run them all.

Ben

 

[triplel43762]

I took the hose off of the intake and coolant is sitting there at the top of the hose nipple! Why / how can it not spew out like a geyser while running?!?!?!‍wrong water pump direction? Any possible answer??

 

[SquareRoot]

I have never seen a heater hose t off off the lower radiator hose. Interesting.

 

[SquareRoot]

‘87 and ‘88. Did NOT have reverse flow water pumps, period.

Ben

Ok ok, take it easy big fella. lol

 

[Itali83]

It’s hard on the internet. But I wanted to really convey the point that for sure, no questions asked, without a doubt, reverse flow for square bodies wasn’t until ‘89. Period. Carry on, I’ll be here all night folks

Ben

 

[SquareRoot]

One thing is for sure. I would temporarily remove the t-stat, leave the hose off the nipple coming out of the manifold and start the engine. If nothing comes out, it can only be pump. However, remove the rad cap and see if coolant is actually circulating. If it is, your hose routing isn't correct. I would move the return hose from the heater to the return side of the radiator. There should be a nipple there. You may have caused a cavitation loop by t-ing it into the lower radiator hose.

 

[triplel43762]

no nipple on the radiator because I used the non stock radiator. I figured if it could go into the radiator- why not the lower radiator hose. Same route of circulation and it allowed me to used the bigger radiator that I already had vs buying a new one. Everything circulated through the motor fine and dandy even with the thermostat in place.