1991 TBI Overcooling

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Nick_R_23

I'm Awesome
Joined
Dec 13, 2020
Messages
153
Reaction score
292
Location
Alaska
Okay, so after installing the correct clutch fan, and sealing off the fan shroud temporarily, the issue persists.

I did notice at the start of this test drive, my temperature fluctuated for a minute or two, but settled in where it normally does.

Upon return, I popped the hood to check things. Upper radiator hose is firm and hot. Heater core lines are warm. Inlet side of radiator will burn your hand after a quick second. However, outlet side of the radiator is MUCH cooler. Radiator cap is cool as well.

I‘m lost. The firm/hot upper hose is telling me the thermostat is working. I can’t imagine having multiple new ones in a row cause the same issue. The radiator having such an extreme temperature difference and a cool cap concerns me a little bit. To me, that indicates a plugged radiator, but it also should be overheating, not overcooling. I’m half tempted to throw a new one in it anyway. Maybe replace the thermostat a 4th time. Ugh.

Any other suggestions?
 

studigggs

I'm Awesome
Joined
Feb 19, 2018
Messages
257
Reaction score
317
Location
CA
I didn't see in this thread where you verified coolant temperature independently from reading the dash gage. My '90 350 takes a while to come up to temp compared to a modern car. In these trucks, there is always coolant flow from the radiator, through the engine, out to the heater core and back to the radiator. You could use a scan tool to read ECT from the intake-mounted sensor, point an infrared thermometer at the upper rad hose, or even tape a manual thermometer to the upper hose to verify you actually have a problem.
 

Schurkey

Supporting Member
Supporting Member
Joined
Jan 14, 2018
Messages
10,969
Reaction score
13,751
Location
The Seasonally Frozen Wastelands
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.
Define "small bleed holes".

There's plenty of examples of foreign cars using a jiggle-valve in the 'stat. The thermostat engineers are smart enough to design a system where the vent hole CLOSES as soon as the water pump develops flow.

Robertshaw thermostats have a stamped air vent, but it's TINY compared to the usual drilled holes poked into thermostats by amateurs. Therefore, almost no coolant flow even though air could readily escape.
You must be registered for see images attach


You must be registered for see images attach



The coolant was also completely drained during the block heater change out, so the coolant is brand new and pre-mixed 50/50.
Not that this is part of the low-temperature problem, but HOW was the coolant drained? If you just popped out one core plug, you only drained that side of the block to the level of the core plug. The other side of the block would only drain to the level of the water pump openings. Draining the rest probably requires unscrewing the two block-drain plugs.

The typical SBC with computer control has a block-drain plug (iron or steel) on the left side, the block-drain hole on the right side is where the knock sensor is screwed in. Put another way--the knock sensor is the block drain plug.

The Vortec 7.4L engines have knock sensors on both sides, but they're not in the block drain holes. The block drains are plain iron/steel plugs. When I remove the plugs on engines I own, I replace 'em with brass draincocks.

I popped the hood to check things. Upper radiator hose is firm and hot. Heater core lines are warm. Inlet side of radiator will burn your hand after a quick second. However, outlet side of the radiator is MUCH cooler. Radiator cap is cool as well.
I'd expect the rear heater core hose to be as hot as the radiator hose/rad inlet.

The heater core outlet hose, going to the radiator "cool" tank may be lower temperature, but most folks would still call it "hot".

I‘m lost. The firm/hot upper hose is telling me the thermostat is working. I can’t imagine having multiple new ones in a row cause the same issue. The radiator having such an extreme temperature difference and a cool cap concerns me a little bit. To me, that indicates a plugged radiator, but it also should be overheating, not overcooling. I’m half tempted to throw a new one in it anyway. Maybe replace the thermostat a 4th time.
The OEM quick-coupler is deliberately restricted. This is intentional. The intake manifold gaskets are also deliberately restricted, so the heater uses hot coolant from the intake manifold passage instead of hot coolant coming directly from the cylinder head.

"I" would yank the quick-coupler out of the manifold for inspection. Perhaps it's got debris lodged in the restricted part of the coupler.

Did you use the special TBI intake gaskets?
You must be registered for see images attach
 

Nick_R_23

I'm Awesome
Joined
Dec 13, 2020
Messages
153
Reaction score
292
Location
Alaska
Thanks for the replies, guys. I'm about ready to pull my hair out over this.

I didn't see in this thread where you verified coolant temperature independently from reading the dash gage. My '90 350 takes a while to come up to temp compared to a modern car. In these trucks, there is always coolant flow from the radiator, through the engine, out to the heater core and back to the radiator. You could use a scan tool to read ECT from the intake-mounted sensor, point an infrared thermometer at the upper rad hose, or even tape a manual thermometer to the upper hose to verify you actually have a problem.

I may or may not have, I can't remember. But I have a Solus Pro at my other place, so I'll snag that next week and re-verify. However, I do believe the colder reading on the gauge to be accurate, as the heater isn't putting out the heat it should. I know it's capable of this heat, as during the initial bleeding, it will run up to the correct temp (195* area) and put out the heat it should. However, after that, it'll drop in temp, exactly like a thermostat opening, and I'll never see it climb that high again. I've owned at least 20 of these trucks, this one is definitely an anomaly.

Define "small bleed holes".

There's plenty of examples of foreign cars using a jiggle-valve in the 'stat. The thermostat engineers are smart enough to design a system where the vent hole CLOSES as soon as the water pump develops flow.

Robertshaw thermostats have a stamped air vent, but it's TINY compared to the usual drilled holes poked into thermostats by amateurs. Therefore, almost no coolant flow even though air could readily escape.
You must be registered for see images attach


You must be registered for see images attach




Not that this is part of the low-temperature problem, but HOW was the coolant drained? If you just popped out one core plug, you only drained that side of the block to the level of the core plug. The other side of the block would only drain to the level of the water pump openings. Draining the rest probably requires unscrewing the two block-drain plugs.

The typical SBC with computer control has a block-drain plug (iron or steel) on the left side, the block-drain hole on the right side is where the knock sensor is screwed in. Put another way--the knock sensor is the block drain plug.

The Vortec 7.4L engines have knock sensors on both sides, but they're not in the block drain holes. The block drains are plain iron/steel plugs. When I remove the plugs on engines I own, I replace 'em with brass draincocks.


I'd expect the rear heater core hose to be as hot as the radiator hose/rad inlet.

The heater core outlet hose, going to the radiator "cool" tank may be lower temperature, but most folks would still call it "hot".


The OEM quick-coupler is deliberately restricted. This is intentional. The intake manifold gaskets are also deliberately restricted, so the heater uses hot coolant from the intake manifold passage instead of hot coolant coming directly from the cylinder head.

"I" would yank the quick-coupler out of the manifold for inspection. Perhaps it's got debris lodged in the restricted part of the coupler.

Did you use the special TBI intake gaskets?'
You must be registered for see images attach

We're talking 1/16" bleed hole on this one, literally just enough to bleed air. However, I do have yet another brand new thermostat that I might install.

As far as coolant draining, I have a vacuum evacuator that I use. I'll remove the lower head bolts and use the tube to suck the coolant out from those holes. It drains the block really well. The system took about 4 gallons of coolant on this refill.

The heater core lines were hot ('warm' wasn't a great description), but I would say you could grab those for 2-3 seconds before they became uncomfortable to hold, where the upper radiator hose was within the first second. The lower hose/radiator cap/passenger side of the radiator you could've held your hand there indefinitely, while the driver's side was within the first second, just like the upper hose.

The OEM quick disconnect was in place initially, however it was in bad shape and for simplicity, I have now converted it to a regular 3/4" fitting with an insert I machined to restrict it to the factory 5/16". You can take a look at the fitting in the trucks' build thread, I have several pictures of it there.

I also do have the correct TBI-specific intake gaskets installed with the restricted ports to the rear.
 

Nad_Yvalhosert

I'm Awesome
Joined
Oct 11, 2016
Messages
1,666
Reaction score
2,085
Location
Rochester, NY
Have you verified the radiator cap is holding pressure? If its venting at single digits, you'll also never come up to temp.
 

evilunclegrimace

Does not always play well with others
Joined
Sep 18, 2017
Messages
2,288
Reaction score
2,332
Location
pennsylvaina
My '92 K15000 had this same problem when I got it up north to Erie from North Carolina. On the drive up it seemed like it got hot enough to keep you warm in the cab but that was in a hot climate.
The first winter up here it would only get luke warm heat into the cab and after going through all of the normal steps to remedy the problem including two rounds of flushing the entire cooling system I pulled the engine planing on a block swap.
After I had the block on a engine stand an fully disaasembled, I pulled all of the core plugs and found that the cooling passages surrounding the last two cylinders on each side of the block were full of casting sand. After two hours with a pressure washer I removed about a one pound coffee can full of sand out of the block. After a set of new bearings,seals and gasket the engine makes heat like it sould. Some times it is the out of the normal thing that is the cause of the problem.
 

Schurkey

Supporting Member
Supporting Member
Joined
Jan 14, 2018
Messages
10,969
Reaction score
13,751
Location
The Seasonally Frozen Wastelands
We're talking 1/16" bleed hole on this one, literally just enough to bleed air. However, I do have yet another brand new thermostat that I might install.
1/16 is better than 1/8, which is better than 3/16, which is better than multiple 3/16 holes.

"Literally just enough to bleed air"? 1/16 is 0.0625. Here's
the orifice stamped-into a Robertshaw thermostat, with a #72 (0.025 or 1/40) drill bit, and a 1/8 for comparison:
You must be registered for see images attach


You must be registered for see images attach


Really, the ONLY reason to have an air-vent in the thermostat at all is to purge air during the first warm-up after cooling system service. Which is why most thermostats don't have air-bleeds.

As far as coolant draining, I have a vacuum evacuator that I use.
I'll remove the lower head bolts and use the tube to suck the coolant out from those holes. It drains the block really well. The system took about 4 gallons of coolant on this refill.
Or just pull the block drain plugs. With any luck, you'll also drain the debris that's fallen to the bottom of the water jacket, too. On small-blocks with a knock sensor, the knock sensor is screwed-into the right side block drain hole, there's a plain plug in the left side.

The heater core lines were hot ('warm' wasn't a great description), but I would say you could grab those for 2-3 seconds before they became uncomfortable to hold, where the upper radiator hose was within the first second. The lower hose/radiator cap/passenger side of the radiator you could've held your hand there indefinitely, while the driver's side was within the first second, just like the upper hose.
The radiator is certainly working. Any chance the fan is not disengaging properly?

Are you using an OEM TBI intake manifold, or do you have some other intake on that truck?

Are the heater hoses connected backwards?
 
Last edited:

Schurkey

Supporting Member
Supporting Member
Joined
Jan 14, 2018
Messages
10,969
Reaction score
13,751
Location
The Seasonally Frozen Wastelands
Shouldn't be... Inlet is 5/8", outlet is 3/4"
O.P. verify please
I had that conversation on another thread some time ago. I expected the outlet hose to be larger than the inlet hose. It "usually" is, unless the hoses are the same size. Reduces overpressurization of the heater core.

I was told that's not true on the GMT400s. The heater core inlet is bigger than the heater core outlet.
 

L31MaxExpress

I'm Awesome
Joined
Apr 21, 2018
Messages
5,891
Reaction score
7,540
Location
DFW, TX
May be a dumb question, but have you reverse flushed the heater core? The heater core can plug up with scale and rust and other gunk (dexcool gunk on trucks equipped with it) and really block the flow in it. Turn a water hose on at limited pressure and reverse flush the core. Might be suprising what blows out of it. Do not overpressurize the core though as it can develop a leak if you do.
 
Top