1991 K1500 no heat issue

[Red350]

Hi guys I’m new to the forum since I bought my first gmt400 !

I’ve been trying to figure out a no heat issue since I bought my 1991 sierra k1500 350 in last October preparing myself for the winter. Before my only solution to have heat in my truck is to cover the truck with fuel and strike a match I figured I would ask you if someone else got the same issue.

The only warm air I got its when my blower is at low. If I turn it to medium or high the small heat I got disappear. The engine temperature runs good right around the middle or less. So no overheating.

The inlet heater core hose is hot, outlet is cold so I figured the core was blocked… so I backed flush it. Everything is fine no problem. I change the heater core anyway with a gm genuine one because I like to know everything that have been done to my truck.


-Next I change my thermostat with a gates 195… no change, it seems open fine

- I thought maybe there was air bubbles or bad coolant so I change the entire coolant, let the air out with a leak free funnel…. No change

- I unplugged the inlet heater hose (the one with the fitting going to the back of the intake to see if there is pressure coming out with the engine running but only a small trickle of coolant was coming out so I thought maybe the water pump impeller is worn out. Change the water pump and clutch. Top the coolant bleed the air. No change + same amount of coolant coming in the heater core.

So next I remove the inlet heater hose again and blow through it to see if it clogged. Except a pretty bad taste in my mouth it flow well and even come back through the heater core inlet.

The 3 blend door actuator works fine …

It’s about it, I’m pretty stuck there

Thank you for your answer

Julien

 

[Schurkey]

The inlet heater core hose is hot, outlet is cold

That's the problem. Or, at least "a" problem. Apparently, the outlet hose and it's plumbing is blocked.

Where does the outlet hose connect? Radiator?

The quick-coupler in the intake manifold is heavily restricted, by design. The fluid passage is about a quarter-inch in diameter.

 

[Red350]

Thanks for your reply,

Yes the outlet connect to the radiator. And there’s no valve or anything, it’s pretty simple.

That’s what I thought too but I blow through it and the coolant started exiting the inlet heater tube…

 

[Schurkey]

Is the nipple on the radiator blocked--perhaps with a plastic plug used during shipping to keep junk from falling into the rad tank?

 

[Red350]

No…. Radiator seems to flow fine also.

Maybe like you said the quick fitting that goes into the intake doesn’t let enough coolant go through the hose

 

[GoToGuy]

It sounds like you gave flow checked everything except the the hose that connects to the intake manifold near the egr valve.
Mine was leaking at that rear QD and I had a leak at front left intake head area.
Removed the QD piece, replaced with stainless hose end. A couple new hose pieces and complete.
You may have blockage, restriction where that rear QD enters the manifold like a water pipe with age corrosion blocking water flow.
Both QD have been replaced with stainless hose ends on my '95 K2500.
My thoughts are with an ageing GMT400 world those QD fittings are more of an accident problem waiting to happen. And the replacement QD are not that high of a quality. Good luck.

 

[Schurkey]

No…. Radiator seems to flow fine also.

Does it "flow fine" from that hose nipple into the radiator tank? Or just through the cooling tubes into that tank?

Maybe like you said the quick fitting that goes into the intake doesn’t let enough coolant go through the hose

Not at all likely. The quick-connect fitting is deliberately restricted; you'd have to have a rust flake or some other fairly-large contamination plugging the passage.

Mine was leaking at that rear QD and I had a leak at front left intake head area.
Removed the QD piece, replaced with stainless hose end.

The Quick Disconnect is intentionally restricted to prevent over-pressurizing the heater core during high-rpm engine operation (high dynamic water pump pressure) and also reduces noise/gurgling in the heater core.

NOT using a restriction in the heater supply hose/plumbing circuit is a poor idea.

My thoughts are with an ageing GMT400 world those QD fittings are more of an accident problem waiting to happen. And the replacement QD are not that high of a quality.

The original GM quick disconnect screwed into the manifold is a terrible, corrosion-prone piece of crap.

While I am not, in general, a fan of Dorman products, in many cases they're the only reasonable solution to failed OEM parts because the OEM parts were horrible to begin with, or they're discontinued, or both.

I put a Dorman quick-coupler into the intake manifold when I installed a GM crate-engine in my K1500 back in...1999-ish. The original was corroded and unfit for re-use. That Dorman quick coupler has survived beautifully; when I re-engined that truck again in '19, I re-used the Dorman quick-coupler because although it was twenty years old, it looked like new.

Is Dorman still building the quick-coupler to the same standards as 20+ years ago? I don't know. But I'd be glad to take a chance on one now, based on my experience with them then.

Dorman's product line pretty-much has to be evaluated on a piece-by-piece basis. Some of it is total junk. Some is Communist Crap with design upgrades from OEM that may result in an equal-or-better part. Some of it is a knockoff of an item that's absolutely not available anywhere else, so for good or bad, that's the only option there is for a new piece.

 

[GoToGuy]

Does that include the manifold gasket restrictor hole in the water passage at right rear head to intake manifold?

 

[Schurkey]

IF (big IF) you're using a TBI intake manifold, there's no shortage of coolant in the flow path ahead of the quick-coupler.

The manifold gasket may be restricted, but there's the whole coolant passage cast into the underside of the manifold itself. The quick-coupler--and then the heater core--is fed by coolant at the front of the manifold that doesn't go through the thermostat--it's routed under the throttle body and fuel/air runners to keep the manifold at a semi-uniform, warm temperature to make the fuel in the fuel/air mixture hot enough to vaporize properly. The heater is supposed to be fed by the outflow of that under-manifold coolant.

 

[Red350]

I received the dorman fitting (two in case one broke) and the gm hose that go with it. I will try this and let you know!