1991 TBI Overcooling

[Nick_R_23]

I’m having to daily my 1991 C1500 while switching out some vehicles. This was intended as a project truck so it really hasn’t been gone through as well as I would like up to this point.

It has a TBI 350 that is overcooling to the point that I’m getting lukewarm air from the heater at best. All of these TBI trucks I’ve owned up to this point, the gauge sits just below/to the left of the 210* mark. This truck is sitting around 150*-170*, and although I haven’t thrown the scanner on to verify, I’m definitely getting poor fuel mileage from the truck from it being stuck in open loop, so I feel that the gauge is correct.

I replaced the thermostat twice with 195* units - once because the gasket was leaking, and again because I thought the first one had possibly stuck open. Both are quality parts. I have also drilled a small (1/16”) hole to eliminate the possibility of air bubbles stuck under the thermostat.

I drained the block to install a new block heater. Used new 50/50 coolant. Upper radiator hose feels warm and has pressure after a drive. I have verified that the blend doors and actuators are working, and even physically removed the actuators and manually operated the doors. I’ve blocked off half of the radiator with cardboard since it’s been cold lately (-10*F), which made a very small improvement.

Radiator appears to have been replaced in the past, water pump looks much more recent. Heater core cooling lines are hooked up as factory (rear intake/upper radiator). Fan clutch seems to be working properly, not seized, spins for a few rotations when engine is shut down. Hell, even the fan shroud has a large gap on one side from unrepaired previous accident damage, and the engine still will not come up to operating temp. I have not used a temp gun on any of the hoses yet. Timing was set recently and is correct (0* with wire disconnected).

I‘m about out of ideas aside from removing the radiator and running only the heater core. Even that may not be enough!

 

[Schurkey]

It has a TBI 350 that is overcooling to the point that I’m getting lukewarm air from the heater at best. All of these TBI trucks I’ve owned up to this point, the gauge sits just below/to the left of the 210* mark. This truck is sitting around 150*-170*, and although I haven’t thrown the scanner on to verify, I’m definitely getting poor fuel mileage from the truck from it being stuck in open loop, so I feel that the gauge is correct.

While I'd want to confirm via scan tool and the "other" coolant temp sensor, it sure sounds like the truck is genuinely not getting up to proper temp.

I replaced the thermostat twice with 195* units - once because the gasket was leaking, and again because I thought the first one had possibly stuck open. Both are quality parts.

Correct part number for the application? Failed thermostat is still the most-likely cause. Perhaps a "batch" of defective 'stats, and you happened to get two from the same batch?

I'd try another thermostat from a different supplier, perhaps even a different brand.

I have also drilled a small (1/16”) hole to eliminate the possibility of air bubbles stuck under the thermostat.

No such thing. The instant the thermostat opens, any air is forced out and into the radiator where it's seen as "low coolant level" until it's then pushed into the "overflow" reservoir.

No holes needed. Wasted effort. Counter-productive to getting the engine up to proper temperature quickly.

I drained the block to install a new block heater. Used new 50/50 coolant. Upper radiator hose feels warm and has pressure after a drive.

Upper hose should be "hot" not "warm". More confirmation that the engine really is not getting up to temp.

I have verified that the blend doors and actuators are working, and even physically removed the actuators and manually operated the doors. I’ve blocked off half of the radiator with cardboard since it’s been cold lately (-10*F), which made a very small improvement. Radiator appears to have been replaced in the past, water pump looks much more recent. Heater core cooling lines are hooked up as factory (rear intake/upper radiator). Fan clutch seems to be working properly, not seized, spins for a few rotations when engine is shut down. Hell, even the fan shroud has a large gap on one side from unrepaired previous accident damage,

Good diagnosis work.

 

[GoToGuy]

Your sure your using correct thermostat and debis in the cooling system isn't keeping thermostat open. And you need to verify your temp gauge. As to the heater if your water temp is that low so is heater core. If block water temp is normal and no heat ventilation doors are problem or heater core full of crude need pull heater core hoses and give good fushing. If your temp senders are bad giving bad info to pcm. Or wire problem ( ?) giving bad info?
Your sure you didn't slip a 160 thermostat in? Just a thought.... Goid luck!

 

[0xDEADBEEF]

You're in Alaska ... maybe run a blanket on the nose like a diesel? No idea.

Another thing is maybe to try more coolant and less water in the system. That should make it run hotter.

 

[Schurkey]

You're in Alaska ... maybe run a blanket on the nose like a diesel? No idea.

The populated parts of Alaska (coastline) tend to be warmer than The Seasonally-Frozen Wastelands. But my TBI truck warms-up just fine. It's not likely the ambient temp that's the problem.

Anyway, he's already tried blocking half the radiator with cardboard. (Not recommended long-term, as it produces an uneven loading of the fan.) I've owned cars that needed cardboard in front of the radiator, but block the radiator completely except for a ~6- or 9-inch circle in line with the fan clutch or center of the fan. Larger open circle if the temp gets too high.

Another thing is maybe to try more coolant and less water in the system. That should make it run hotter.

Not if the thermostat is actually controlling the temperature. And, of course, given the cold weather, he's probably got at least 50/50 coolant, maybe stronger (I go stronger than 50/50)

 

[0xDEADBEEF]

Not if the thermostat is actually controlling the temperature. And, of course, given the cold weather, he's probably got at least 50/50 coolant, maybe stronger (I go stronger than 50/50)

Yeah that's true. I'm always thinking of tracking cars where the thermostat opens on the warm up lap and stays open.

My 96 had some kind of solenoid controlling flow to the heater core, maybe it's stuck closed or barely open.

 

[Nick_R_23]

The two thermostats I’ve tried are two different brands and both 195*. The initial one was clean when I took it out (it was only in ~200 miles).

And yes, air locks do occur in the cooling systems so small bleed holes are beneficial - both for filling and draining the system. I’ve ran into this plenty of times before. It’s best to start out with coolant completely surrounding the thermostat.

Yes, the radiator is currently partially blocked off. I haven’t noticed much improvement from this.

The coolant was also completely drained during the block heater change out, so the coolant is brand new and pre-mixed 50/50.

The heater core and lines are completely factory, including the quick disconnect at the rear corner of the intake manifold.

 

[hobbit319]

I have a '98 C2500 with a 454. When I am not pulling a trailer, its temp runs at the mid mark between cold and 210 degrees and my heater puts out hot air. So, as long as your truck warms up, enough to open up your thermostat, you should be getting hot air. So, if you believe that everything is fine in regards to your radiator temp and thermostat, then that leaves your heater core being the problem. I had the same issue with my GMC S-15 after I did a motor swap, what I found out was that the heater core coolant flow ls "directional" and that I had the coolant lines connected reversed. When I switched them, I had heat. So, either your lines are reversed or your core has blockage. At least, that is my 2 cents.

 

[Nick_R_23]

The engine itself is not coming up to temp. Lines are 100% stock.

 

[Nick_R_23]

Update on this thread. Engine now has new heads/headgaskets due to an unrelated issue. Block and heater core flushed - found no issues there. Also now has a new water pump and all new lines. Coolant is new again as well.

Engine still overcools, but I noticed during the head replacement that the fan is labeled “1996 S10” from a junkyard. I will be installing the correct TBI fan and repairing the shroud issue correctly and update from there. I’m thinking that the fan clutch may be either incorrect enough to cause issues, or faulty.