SLC Cooling Questions

A month ago I had a leak in a heater core tube. That prompted me to replace all the coolant lines from the radiator back.

Problem is with refiling coolant and burping the system.

1) I am using the HD Airlift Evacuator System - recommended to my by Fran
2) I can hold 25lbs vacuum and the system pumps the antifreeze into the car - but only uses 5 gallons.

3) I have subsequently added another three gallons and the system is still not full.

4) The coolant return hose gets warm but isn't full - I can collapse the hose
5) I have raised the rear 6 inches to get the header take higher
6) I get to 200 and hold for a bit and then the temperature starts rising
7) I am getting a lot of air (burps, splashes) from the header tank

Anybody have tricks and techniques they have used to successfully fill and burp the system?


Ken Roberts

The air bleed lines from the radiator and cylinder heads should enter the fill tank at the top in the air stream and not at the bottom in the coolant stream. Is yours like this?
Cooling should be easy - fill remote tank, let it get up to temperature (e.g., fans on). let it cool down, fill back up. rinse and repeat 3 or 4 times and you should be good (i.e., coolant level stable).

The remote fill tank needs to be located above the engine. Engine air bleed ports and radiator air bleeds run to a high point on the remote fill tank. The remote fill tank should dump from the bottom and connect into a hose on the passenger side leading to the engine (i.e., around the waterneck)
Thanks to all for the suggestions. Here are some pictures that might help.

#1 is header tank with connections
#2 is overflow tank
#3 fans installation

I went with Mishimoto 11" Race Fans as they provide 1700 CFM per fan at only 12 amps. Given the size of the fans I don't have a shroud currently installed.

I wired 8 gauge from the battery to the relays. Then 12 Gauge to the fans.

I also have a Mezeire Electric Water Pump directly off the radiator out.


#1 Is 16lb okay for a pressure cap?
#2 Any value in removing return hose and adding coolant directly to radiator?



Howard Jones

Header tank is far too low. See my picture below. I can't see engine bleed lines but I will assume they are there. And finally heaters will hold air. Make sure the heater water lines are full open (max cabin heat) when you bleed system.

I am not sure what you mean by Q#2 but the diagram will work for sure. Hot water from engine to radiator, cool water from radiator back to engine. The coolant expansion hose goes to the cool side and as close to the pump impeller low pressure side as possible. The bleed lines all go as high as possible from their source, (top of radiator, top of engine coolant volume) and back to the highest possible point in the expansion tank in the air pocket below the radiator cap.

I am not sure how a LS Chev wants the heater plumbed in but I would try it like a corvette does it or other production GM cars do it first. I am sure other here with a heater will chime in.

3 degrees increase in boiling point for each pound of increase in pressure in a sealed system above 1 BAR (sea level). 16 pound cap = 212F + (16 X 3) = 48+212= 260F at sea level.

You should be able to get your car to run at 200-210F at low power demand and around 220-230 full power on the track. Lower is better as long as you don't run it very low like 150-170F. I would be very happy with a solid 190F.

Also check oil temp. Oil should be run above 212F to boil off water (condensation in sump) and below 275F where oil begins to break down. I like to see oil temp at about 230-240F on the track and hold solid at in that range.

You will need a oil cooler.


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Are you running the electric water pump in addition to or instead of the factory water pump?

What is your routing between the water pump heater core inlet and outlet? What heater control valve are you running and is it fully open while you are trying to bleed?
Yes. I am running both the electric pump and engine pump
Not sure on the heater valves. I might have to talk to Fran since he built the car.

Howard Jones

Lets get some clear data into this thread.

1. What model number/type electric pump are you using?

2. What mechanical pump are you using, OEM GM?

3. Diameter of both mech. pump pulleys, pump and crank.

4. Location of electrical pump. Rear at engine hot side, cold side, or front near radiator, hot or cold side?

5. Radiator type? RCR OEM? other?

6. Thermostat installed? temp setting of thermostat?

7. Oil cooler? pic or describe.

8. Oil cooler thermostat?

9. Radiator fans model numbers. Are they turned on automatically, if so what temp?

This will help all of us help you and this thread will help others in the future when you get it sorted. If you want to talk on the phone email me a phone number and I'll give you a call. Don't post # here.
Got some info to report.

The car is one of the two rapier models that was built in 2011 by RCR, so I don't have all the info. Here is what I verified.

Lets get some clear data into this thread.

1. What model number/type electric pump are you using?
Meziere High Flow 55 GPM remote pump

2. What mechanical pump are you using, OEM GM?
Yes - factory I believe

3. Diameter of both mech. pump pulleys, pump and crank.
No idea - working on this one

4. Location of electrical pump. Rear at engine hot side, cold side, or front near radiator, hot or cold side?
Electric pump is directly off the radiator out (right side) heading to 1.5" stainless steel tube

5. Radiator type? RCR OEM? other?
RCR - with Vintage Air A/C radiator mounted in front

6. Thermostat installed? temp setting of thermostat?
The sensor for the digi-dash system is reading the from the return (left) pipe before collecting in the radiator. Additionally, I have an ODBC reader on my iPad where I can read the engine temperature from the ECU. I also added a sensor directly off the electric water pump to send to my Dakota Digital fan controller.

7. Oil cooler? pic or describe.
Non installed. I am running the Ricardo transaxle. Temperature settles around 210. ** Note before 205 I cannot effectively shift the car into 2nd.

8. Oil cooler thermostat?
Mounted directly into the transaxle

9. Radiator fans model numbers. Are they turned on automatically, if so what temp?
I removed the small fans that were installed originally and related with Mishimoto 11" race fans. Rough estimate is 3600 cfm at 22 amps.

I turn the heater on full and I can feel that the heater lines are warm; however I am not getting hot air from the vents.

Additionally I am hopeful that I was able to get it fully purged yesterday. Jack up the rear and let it run for an hour. Holds between 190 and 200.

How full should the header tank be - seems that if I fill it up, it just pukes it out

Howard Jones

Second round of questions:

Question #6: I was asking if you have a thermostat and if so what is the opening temp: I.E. 185F, 195F. If you do use a thermostat then the coolant temp should be stable at the thermostat temp rating within reason.

Question #7: Do you have a engine oil cooler installed? If so describe.

Note: good choice on the electrical water pump. That one is used in the 25Hr blue race car if I am not mistaken. AND I found those fans a while back and I was going to use them if I replaced the OEM RCR fans. Another good choice I believe.

Most high HP output mid engine cars will need to cool the engine oil with a remote cooler IMHO. I have asked a few people I respect that build racecar/engines and the they come up with approx. 30% when asked how much of the total heat dumped into the combination of oil and water goes into the oil. That's quite a bit if the car is run at high power for extended lengths of time like track events. Secondarily the closed nature of mid engine motor compartments prevents a lot of heat dissipation due to airflow unlike a front engine car............So you might add a oil cooler later if you think it is needed as you gain experience with your car and get an idea what your engine oil temp is running at. It will remove quite a lot of heat from the overall cooling system. If the water /radiator system is near its limit then cooling the oil independently might just help enough to make it all work.

Another idea I have on my list is to insulate the headers to prevent as much radiant heat as possible form being added to the engine room by using header rap.

At this point I think you still may have a air in the water/coolant system issue to work out. Be sure the hose that feeds the expansion tank is coming from the low pressure cool side of the system and the air pocket in the expansion tank is the highest point in the entire coolant system. Higher is better, mine is as high as I can mount it and still close the engine room rear body work.

You might try bypassing the heater for now with a short length of hose just to make trouble shooting easier. Do this at the engine end of the heater water circuit.

The header tank, I have been calling it the expansion tank, needs some air in it to allow for expansion of the coolant. It will usually find its own level after puking some water/coolant out. If the volume of air in it is the highest air volume in the system and any other air volume is allowed to bleed into it with bleed hoses then you have a self bleeding system. This is the goal and why the header tank needs to be as high as possible but not lower than the top of the engine or the top of the radiator where air will be trapped if not vented back to the header tank.

As you can see from the diagram above GT40s engines are vented to the header tank from the top front of the intake manifold, the radiator top and I added the back of both heads, all the high points that trap air in the engine and create hot spots. When I did this to mine years ago it forever fixed "air bleeding issues". I just fill it up and run it now. It pumps out all the air on it own without any further effort.

Take the car out and drive it and see where you are now as far as water temp goes. You will want to know: temp at steady 60-70mph for 15 mins or so, fans off. Then same run with fans on. Next: Try low speed drive under 35mph with stops, fans off and if necessary then add fans on and see if fans pull the water temp back down. If you still can hold 190-200F for all tests great.

Next would be power runs but I can't see doing an effective test without a track. You might be able to do a few onramp power pulls with steady 70mph freeway run between and see what happens. Be legal and safe.
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Thanks Howard.

I'm not sure of the thermostat temp - I thought it was 190 - but see below - I was able to stabilize yesterday around 210

A little update

Yesterday afternoon I ran the car for 1 hour with the cap off and it stabilized around 210. Outside temperate was 80 and intake temperature was 88

This morning I put the 16lb cap on tried two runs. Within 5 minutes, both went above 235 before I shut the car down. The temperature was still climbing.
No venting or purging from the tank, but the temperature was much higher than I have ever encountered. Outside temperature was 65.

The return hose to the radiator (left side) is still not full - however the radiator is.

I am in the middle of installing a Dakota Digital fan controller - so I am running the fans manually (via a switch) - in both cases this morning they were ineffective at reducing the temperature.

I have wrapped the headers and mufflers in teflon/heat tape.

I must still have air somewhere - I've put in almost 9 gallons now - must have burped out 2 at least.

I am going to take your advice and bypass the heater and see if I have any better luck with purging.


Lifetime Supporter

The best article I have read on cooling systems is here:
Pirate4x4.Com - The largest off roading and 4x4 website in the world.

According to the article, 2% air in the system results in 8% less heat transfer, but 4% air results in a whopping 38% less!

Like others have said, you need the header/expansion tank to be as high as possible. If that's a lot of work to do right, take the tail off and hack together a temporary bracket. You can extend the lines with couplers if you need to. The radiator and steam bleed lines should be in an air pocket in that tank (i.e., above the coolant line). Use compressed air to ensure that the bleed lines aren't clogged.

You might also consider installing a sight glass into that tank so you can see what's going on with the coolant level or buy a cheap one that already has a sight glass/level indicator.

If that doesn't work, you might also consider installing a coolant swirl tank between the water pump outlet and the radiator. They are common on race cars and are found on some OEM cars. According to Carroll Smith:

"A de-aerating swirl pot between the pump and the radiator which is bled to the header tank is an absolute necessity on any race car"

They're pretty small and are easy to plumb if you have room. You could temporarily connect one (even in the nose) if you're experimenting with the car parked. I can send you some more info if you're interested in going down that path.


Howard Jones

I want you know that I have made dumber mistakes myself so don't take this wrong. Are you sure both pumps are pumping the water in the same direction? The fact that hot water seams to get to the radiator but not back is telling you something. Maybe the electric pump is in backwards?

You might temporarily put a straight piece of pipe in place of the electric pump and see what happens. Free test and might be very valuable information.

I would do the following:

Temporarily move the header tank higher, temporarily bypass heater, and replace the electric pump with a piece of pipe. While you are at it take the thermostat out. That comprehensive test will tell you a lot. As you can see all you will have then is a motor, water pump, radiator, and plumbing.

Don't get bummed out. You will get it in the end. All race cars have teething issues. Mid engine cars often have cooling issues. That the nature of the beast.


Lifetime Supporter
I agree with Howard that you should bypass the heater. You might have a heater shutoff valve that pinches off all flow through the heater circuit. This is known to cause LS engines to overheat. You can buy a simple bypass hose that's 3/4" on one end and 5/8" on the other here.

LS1 LQ4 LS2 LS3 LS7 Swap Water Pump Heater Core Bypass Loop Hose GATES | eBay

I that fixes the problem, you can install a heater bypass valve. The Run about $100 but will require a little plumbing.
Good afternoon.

Some updates. I had a great call with Scott Swartz yesterday that was most enlightening. Speaking with him about the heater bypass, I located a vacuum line that was off my heater control valve. Specifically I was able to find a disconnected vacuum line to the heater control valve. Plugging that back in had no change to the tests and did not result in heated air in the cockpit.

The car is overheating quite rapidly now. From 150 - to 240 in less than 3 minutes. The return pipes and header tank feel cool to the touch. I am beginning to suspect the thermostat

To be fair to the above heater test - it's possible the coolant lines didn't have time to heat up before the engine overheated - especially if the thermostat is closed.

I can shut off the car and restart it within 20 minutes and the temperature is back down to 160.

So I think there are two issues at least.
#1 - Fault heater control valve - or bad wiring to solenoid. I am testing this now
#2 - Bad thermostat - I am going to replace just to eliminate

Seems strange that the thermostat failed - I upgraded to a 16lb cap from a 13lb. Seems strange - but probably unrelated.

I've owned the car 4 years and had my fair share of challenges. I agree with Howard - it's totally worth it - Just frustrating.

Thanks again - I really appreciate everyone's help.

Howard Jones

Leave out the thermostat for now until you have a "it won't warm up" problem. If your cars "fast" to overheat turns into a "slow" to overheat you want to know that it's not the thermostat because it's not in the car.

The idea is to eliminate one thing at a time until it passes the drive it around for a few hours test. Then add things like the thermostat and heater system.
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A friend today suggested that perhaps I have a small air pocket blocking the thermostat. He recommended that as i see the temperature rising I should blip the throttle to speed up the water pump eliminating any air voids present. His argument being that at idle the blade is barely turning.

Any thoughts?
I believe the vacuum actuated heater valve is a fully closed type valve. So it's very possible - depending on your line routing - that you are not getting any recirculating hot water behind your thermostat. The LS engines have an odd thermostat design in that you have to have flow going through your heater core in order for the thermostat to open. So if you've got a line that runs from the water pump out to the valve, then through your heater core and back with nothing linking the two prior to the valve I suspect this is your issue; same issue that Scott is referencing and explained on that 4x4 website.

This is the issue that integrating the Chevs of the 40s heater bypass valve addresses.

Edit: Finished my post on this very subject. Take a look, hopefully it helps.

19. ‘Cause you’re hot then you’re cold – Cam's Superlite SLC
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I think it was an air pocket after all.

1) I removed any hose connection I could find and tilted the car is various angles to drain the existing coolant. I recovered almost 5 gallons.

2) I replaced the thermostat and heater control valve since the coolant was out.

3) I used by HD /AirVac system to create a vacuum and verified it held 25 HG for 30 minutes. I then flipped the valve and it sucked in 5 gallons of antifreeze.

4) Let the car run up to temperature - goes to 202 then settles to 194 and will hold for at least 30 minutes.

5) Rinse and repeated this a few times - replaced cap. Will take care out tomorrow for a more thorough road test