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1981 K20

6" lift. 3/4 ton axles.
Overview Gallery Discussion
I would think that you may be correct that it had an air bubble caught somewhere. I also don't believe that you should be able to leave the radiator cap off while it gets warmed up. It will cause the coolant to boil easier due to the lack of pressure. Then, since you gave it a direct route to the radiator with the hose route change, it causes a problem quickly. The heater core would act to slow this down some, more restriction between the engine and the radiator.
I don't remember seeing a gas engine that I could leave the cap off without it puking coolant. Maybe my memory is failing right now. I am always ready to put the cap on quickly when warming one up with an open radiator.
 
Don't put that hose in from intake to radiator. I would just cap off the port on the radiator and port on the intake with short hoses that are plugged or some of those heater bypass caps.
 
6872xtc said:
I would think that you may be correct that it had an air bubble caught somewhere. I also don't believe that you should be able to leave the radiator cap off while it gets warmed up. It will cause the coolant to boil easier due to the lack of pressure. Then, since you gave it a direct route to the radiator with the hose route change, it causes a problem quickly. The heater core would act to slow this down some, more restriction between the engine and the radiator.
I don't remember seeing a gas engine that I could leave the cap off without it puking coolant. Maybe my memory is failing right now. I am always ready to put the cap on quickly when warming one up with an open radiator.
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I guess I’ve never tried before. I know the whole point of the pressurized system is to raise boiling point. Since the upper hose is below the filler neck and liquid follows lowest least resistance shouldn’t the coolant flow into block vs overflowing unless there was a flow problem?
Never ducked with one so it may not work that way but I don’t see what would cause it to just bubble over with the cap off.

Truckman4life said:
Don't put that hose in from intake to radiator. I would just cap off the port on the radiator and port on the intake with short hoses that are plugged or some of those heater bypass caps.
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I was always under the impression if you just cap that port on the intake it will cause an air pocket that will give grief. I thought it had to flow somewhere.
 
Smokinthehippies said:
I guess I’ve never tried before. I know the whole point of the pressurized system is to raise boiling point. Since the upper hose is below the filler neck and liquid follows lowest least resistance shouldn’t the coolant flow into block vs overflowing unless there was a flow problem?
Never ducked with one so it may not work that way but I don’t see what would cause it to just bubble over with the cap off.



I was always under the impression if you just cap that port on the intake it will cause an air pocket that will give grief. I thought it had to flow somewhere.
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It does
You’ll do a heater core eventually. This was just to see if the issue stops.

Nice thing is it’s a fairly inexpensive test, the best kind
 
Before the thermostat gets time to open, coolant will be flowing through the port which normally goes to the heater. The smaller hose increases the velocity, and it doesn't have any restrictions you have it now. I think that is why part of why you are getting geyser action.
What is the boiling point of the coolant at atmospheric pressure? I know that it's less than under 12 psi. This can make coolant boil, in small amounts, at the cylinder walls or in the heads, even though there is more coolant on top of it. More contributors to the geyser.
I believe that it is fine, you just have to put the cap on it, run for a bit, which will let you check for flow through the upper hose, shut off, let it cool a little then check the level.
During initial filling, you would want the heater hose port flowing to keep from having an air pocket. Otherwise you can keep the thermostat from getting a good "signal " of coolant. Once properly purged and filled, you can shut the coolant flow off, but I like to have 2 small holes in the thermostat flange for flow around the copper bulb of the thermostat.
 
Good info! I didn’t know any coolant flowed at all until after the stat opened.

But that test is exactly my plan for later. Put the new cap on and get it up to temp and see what that gets me.

I honestly hope that the problem was the bad deal on the cap so it was bubbling and sucking air when it shouldn’t.
 
Oh shit I just realized. I have the new core sitting in a box. I can just plumb it in and leave it in the engine bay as an experiment if it doesn’t work the first time. That way we can see if the core being online makes the difference.
 
Smokinthehippies said:
Oh shit I just realized. I have the new core sitting in a box. I can just plumb it in and leave it in the engine bay as an experiment if it doesn’t work the first time. That way we can see if the core being online makes the difference.
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Oh, Santa came and brought you a present!
 
Let it come up to temp. I kept feeling the radiator so I could track how the coolant was progressing. Everything works fine. Let it idle for about ten minutes and shut it down since I don’t have fans right now (going to windstars)

After playing with it and seeing how long it takes the coolant to push thru the radiator, it’s not wonder it bubbles over. Basically without the pressure to force feed the coolant thru the passages of the radiator, all of the water pump power is pushing it right out the top.

See a little experiment and things make way more sense.

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That's a crazy variance give how close the sending units are. But I've read about the stock sensors that come in the kits being off.
 
Well let’s be honest here. That analog up was part of a 3 pack that cost $30. At this point I don’t think I trust either of those or the factory to be what I consider accurate. But if call it accurate enough to know it’s not overheating.
 
That looks like a capillary tube style gauge which usually are fairly accurate. You could pull the sensor and dunk it in pot of boiling water to check the accuracy.
 
I haven't read all the responses but I would replace the sensor that came with the fitech system. They're known to be off 15-20 degrees, not always out of the box either. Another option (@ZooMad75 mentioned) is to take a pot of boiling water and drop the sensors in it with a thermometer.
 
skunked said:
I haven't read all the responses but I would replace the sensor that came with the fitech system. They're known to be off 15-20 degrees, not always out of the box either. Another option (@ZooMad75 mentioned) is to take a pot of boiling water and drop the sensors in it with a thermometer.
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Do you know which sensor I should put in? Other than having it be accurate, you aren’t intended to use the display as a gauge/dash so I don’t plan on keeping it plugged in all the time.
 
Smokinthehippies said:
Do you know which sensor I should put in? Other than having it be accurate, you aren’t intended to use the display as a gauge/dash so I don’t plan on keeping it plugged in all the time.
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the temp sensor needs to be accurate for the engine to run properly with EFI. Temp sensor P/N is AC Delco 213-928

and my handheld is on all the time, it's more accurate than my factory gauges plus I can see AFR and IAC Steps......
 
I had planned to originally mount it permanently but there’s not a great way other than the windshield cup.
 
Accuracy is a must for the EFI. Otherwise like skunked said the ecm will change fuel strategy based on an incorrect temp value. If it is low, it will stick it cold enrichment until it sees the temp come up to whatever the target value is. Meaning it's going behave like a carb with a stuck choke and keep dumping fuel in order to get the engine up to temp.
 
Yeah I thought bout it more on way home from work and realized how important accurate temp is for the efi. I already got one in my amazon cart. I guess I was still thinking h about it more for the gauge not the actual efi
 
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