Heater core replaced, unfortunately customer ran 2 types of stop leak in truck

[Stuart Paton]

Hey everyone 1st post. 1998 chevy 5.7L vortec, Customer had a leaking heater core(puddle inside cab, foggy window, smells) found he has used 2 (that I know of) types of stop leak. After replacing core, flushing system, t-stat new, only get heat at idle, and while revving until tstat opens. Hitthe highway and loses heat. Never over heats. Used vacuum fill tool to get all air out of system, filled to proper level and strength. My experience is stop leak is only to be used in emergency situations and then immediately removed. Also be prepared for issues after, like rad, core, water pump, tstat, etc..

Also the head gasket and intake gaskets are fine. Blend door functions properly. Any thoughts? 

[davester]

It sounds like the engine itself isn't getting hot, not that the HVAC part isn't working right.

 

And for that, the primary thing to check is the thermostat.  Pop it out, examine it for debris, test it for what temp it opens at and if it checks out, pop it back in.

[Stuart Paton]

Tstat opens properly. I had the same thought. Pulled it out and checked it. Put back in, no improvement. No external leaks, did find a plugged line from rad to reservoir. Clean it out and still no change. Heater hoses are both hot. Flushed new core again. Still nothing.

[davester]

If both heater hoses get hot, that means the heater core isn't plugged.  The only part of the system doing any significant cooling is the radiator.  Could the thermostat be installed backwards?

[silveradosid]

having heat at idle and then loosing it at highway speeds, tells me you have a head gasket problem and it fills the heater core with air

[Stuart Paton]

You figure it fills the core with air through the intake, from head gasket issues? I'm also leaning that way, but the typical signs of head gasket are not there, also pressure tested fine, and doesnt seem to be losing any coolant. I've heard of upper rad hose collapse when hot, and restricting flow, or water pump not supplying enough flow.

6 hours ago, silveradosid said:

having heat at idle and then loosing it at highway speeds, tells me you have a head gasket problem and it fills the heater core with air

Also after getting best heat I've had at idle, road tested and lost some heat, found rad cap had no pressure at it. Cap is new btw

[Stuart Paton]

10 hours ago, davester said:

If both heater hoses get hot, that means the heater core isn't plugged.  The only part of the system doing any significant cooling is the radiator.  Could the thermostat be installed backwards?

Nope the tstat opens at 190ish like it should. 

[davester]

Then could the fan be engaged too much?

[Stuart Paton]

3 hours ago, davester said:

Then could the fan be engaged too much?

Meaning the fan clutch? 

[davester]

Yeah, if it has one.

[silveradosid]

did you replace the heater hoses that go to the heater core, could be one of the hoses is failing on the inside, the higher the rpm gives more flow and this could cause a bad hose to cause the restriction