Radiator cap / heater core

[Coadster32]

Update:

Went through the entire summer and winter with the 10lb cap, and had no real issues, (initial question asked). No overheating in the summer, and plenty of heat in the winter. Still had the annoying initial anti-freeze smell briefly upon warm-up, but maybe a bit longer...hard to gage it to be honest. No antifreeze in the cab, and will probably see if I can make it through another winter like that as well. The Jeep is garage kept, and I drive less than 1/4 mile to and from work. I don't bother with the heat for the most part anyhow. With the added expense of having to replace some A/C components as well, which still works perfectly, not in a big hurry to swap out the heater core. (Especially since most of the threads contradict which aftermarket Heater Core is actually not going be a head-ache, do-over job a couple of years down the line).

I did a loop bypass, not a "dead-head/end" system. I found that on chilly nights with the top off, I still wanted a little bit of heat, so I hooked it back up again. I did flush out the core/ back flush as well. Used really hot water from the stove, and also tried a couple of different leak-stop products just in the heater core alone, and to no avail.

 

[KennethS]

Update:

Went through the entire summer and winter with the 10lb cap, and had no real issues, (initial question asked). No overheating in the summer, and plenty of heat in the winter.

FYI, I installed a 7 pound radiator cap on my classic car many years ago to reduce stress on the radiator and the rest of the cooling system. No issues with cooling in 105 degree heat (I quit driving when it get above that) - the aftermarket temperature gauge shows the coolant stays at 180-190 degrees (this is with a 180 degree thermostat).

 

[Steel City 06]

I don't know how it works but it seems to me that if the heater core isn't the integral bypass, then the pump is deadheaded against the thermostat until it opens some. If the HC is, then that likely means the coolant in the block has to be circulating through the HC. Or, the pump can dead head against the thermostat with no ill effect. What am I missing?

I am not in any disagreement whatsover about connecting the heater hoses to each other to form the bypass. Just trying to understand the rest of it.

I suspected the confusion in this thread (from other posters) may have been partly from the difference between bypassing and straight up blocking the flow from the heater core.

One will have no I'll effect, the other will certainly have an ill effect if there is no other bypass. I did some more digging and I can find no evidence the TJ has any other bypass like except the heater core. Some newer or more complex cars have separate bypass lines or even use the turbocharger cooling circuits for the bypass flow.

The 2021+ F-150s are set up this way, at least the turbocharged ones. The turbocharger coolant lines serve as the primary bypass, and there is a simple heater control valve that can shut off the flow to the heater core. For example:

Older Fords that needed to block off heat in the heater core typically used a 4-way heater valve, like the one below:

These divert the flow past the heater core when in bypass mode so as to prevent deadheading the water pump.

In a normal TJ setup, there is always coolant coming through the heater core when the engine is on regardless of whether or not the thermostat opens. The water circulates through the block and then through the heater core, even if the heater isn't on.

This means that the engine can heat up uniformly and prevent hot spots and cold spots from developing. It also means the coolant has a more constant temperature, and since it is circulating past the thermostat, it allows the thermostat to open when the average coolant temperature reaches the setpoint.

Without active circulation, the coolant could actually start to boil in one part of the block before the thermostat ever opens. (I did accidentally do this once. The best part is that the engine temperature gauge will read stone cold until the steam hits it and then it goes straight to alarm temperature...)

 

[mrblaine]

Without active circulation, the coolant could actually start to boil in one part of the block before the thermostat ever opens. (I did accidentally do this once. The best part is that the engine temperature gauge will read stone cold until the steam hits it and then it goes straight to alarm temperature...)

All I know is the temp gauge does strange things. Guy called about a split radiator top and how to get home. I gave him the standard drywall mesh and JB Weld repair to get it home. He called the next day from the trail wanting to know what to do. He said the repair worked so well that they decided to go wheeling, on a hot day, with the doors off, AC on full blast to keep the girly friend happy. Engine pegged the temp gauge, Check Gauge light and warning and then went back to much cooler. He did mention lots of steam and coolant from under the hood and another split but that wasn't his question. His question was could he keep driving it since the temp gauge was no longer in the red.

I explained that the lost coolant, steam, and split tank probably meant that wouldn't be a good idea regardless of what the gauge said.

Best I could come up with since the gauge worked afterwards was it quit reading anything intelligently when it was not in coolant flow any longer. Purely guessing.

 

[Steel City 06]

You are correct in your thinking. Coolant is not being circulated until the thermostat opens.

Here is a small caveat. There are some thermostats that have a hole in the flange. This hole allows a small amount of coolant to always be moving. This is beneficial for a couple reasons. If you happen to have a small amount of ice in the lines or core this water may break it, melt it, or move it along to a larger cavity so when the thermostat opens there is not a lot jam with pressure being added possibly rupturing the system. Second, the hole cuts down on cavitation. Cavitation wears out pieces.

Coolant isn't being circulated through the radiator, just through the block (and the heater core). Some cars have a separate way to bypass the coolant directly back into the block, but most simpler cars simply just send 100% of flow through the heater core.

On a more fundamental level, what we call the "thermostat" (and its housing) is actually a thermostatic diverter valve. It requires flow through/past it to work. Below a certain setpoint, it diverts flow in one direction. Above a higher setpoint, it diverts flow in the other direction. Between those setpoints, it's a super crude form of proportional-integral control, diverting flow partly in each direction. (The thermostat we put in a car is only part of the thermostatic diverter valve assembly - the housing makes up the test of the part.)

Fun fact: a thermostatic mixing valve is just a thermostatic diverter valve run in the opposite direction.

 

[DuckNut]

Coolant isn't being circulated through the radiator, just through the block (and the heater core). Some cars have a separate way to bypass the coolant directly back into the block, but most simpler cars simply just send 100% of flow through the heater core.

How is this so?

Water is not flowing in any direction until the tstat opens.

Tstat closed and water is not circulating through the block because it cant get replacement fluid from rad until tstat opens.

 

[Steel City 06]

How is this so?

Water is not flowing in any direction until the tstat opens.

Tstat closed and water is not circulating through the block because it cant get replacement fluid from rad until tstat opens.

There is nothing blocking the flow from the heater core lines. The heater core supply is tapped in pre-thermostat. If the thermostat is closed, 100% of the coolant is forced through the heater core. The coolant returning from the heater core is what gets circulated back through the block.

Once the thermostat opens, it allows a portion of the flow going through the assembly to pass through the radiator. (Even if the thermostat is 100% open, you still have some bypass flow through the heater core.)

 

[DuckNut]

I am not at my jeep.

I will take your word on it, but i honestly thought it was the other way around.

Thank you for the clarification.

 

[Steel City 06]

All I know is the temp gauge does strange things. Guy called about a split radiator top and how to get home. I gave him the standard drywall mesh and JB Weld repair to get it home. He called the next day from the trail wanting to know what to do. He said the repair worked so well that they decided to go wheeling, on a hot day, with the doors off, AC on full blast to keep the girly friend happy. Engine pegged the temp gauge, Check Gauge light and warning and then went back to much cooler. He did mention lots of steam and coolant from under the hood and another split but that wasn't his question. His question was could he keep driving it since the temp gauge was no longer in the red.

I explained that the lost coolant, steam, and split tank probably meant that wouldn't be a good idea regardless of what the gauge said.

Best I could come up with since the gauge worked afterwards was it quit reading anything intelligently when it was not in coolant flow any longer. Purely guessing.

Sounds pretty close to what I did.

Was attempting to flush the block prior to replacing the radiator as there was some odd sludge left behind. Drained it, filled it with thermocure and water. Ran it, drained it, filled it with distilled. But I was lazy and left a fairly substantial air bubble somewhere in the system, apparently enough to stall the flow. (My best guess is the water pump impeller wasn't fully submerged.)

Idled for like 10 minutes and the coolant temperature gauge didn't move. Then I heard a fairly substantial gargling sound coming from the coolant expansion tank. Followed by the temperature gauge going straight from nothing to red and the alarm going off, followed by a lot of steam for 10+ minutes after shutting it off.

Fortunately nothing bad became of that, but definitely don't want to repeat it.

 

[mrblaine]

How is this so?

Water is not flowing in any direction until the tstat opens.

Tstat closed and water is not circulating through the block because it cant get replacement fluid from rad until tstat opens.

The thermostat housing has a bypass around the thermostat. Again guessing, but it appears that the water pump pushes fluid it picks up from the lower hose into the block and out the head where the thermostat lives. Beside the thermostat is another port for the heater hose so the fluid is circulating full time through the core.

 

[DuckNut]

The thermostat housing has a bypass around the thermostat. Again guessing, but it appears that the water pump pushes fluid it picks up from the lower hose into the block and out the head where the thermostat lives. Beside the thermostat is another port for the heater hose so the fluid is circulating full time through the core.

View attachment 614122

So the loop is lower hose to water pump.
Pump through the block to the tstat housing. (Blocked by tstat)
Through tstat housing bypass to heater core.
Core back to exterior tstat housing.
Tstat housing to upper rad hose to radiator.

Then tstat opens and loads of fluid goes through tstat to upper radiator.

Glad we got it solved.

Thanks Mr. Blaine.

 

[mrblaine]

So the loop is lower hose to water pump.
Pump through the block to the tstat housing. (Blocked by tstat)
Through tstat housing bypass to heater core.
Core back to exterior tstat housing.
Tstat housing to upper rad hose to radiator.

Then tstat opens and loads of fluid goes through tstat to upper radiator.

Glad we got it solved.

Thanks Mr. Blaine.

Purely observational guesses. Radiator has to flow out the bottom in the event of a loss of coolant.