Help! SBC overheating only with heater core connected.

[ssleeper]

Putting this in General car talk to hopefully get the most views. Sorry for the long post...I very much appreciate any help!

I'm at my wits end...maybe I'm overthinking this. I have a 357 small block (+.040") in my '61 which was rebuilt last year. The water pump is new (Proform aluminum) and is the correct rotation, the radiator is a new 4 core HD "409" from Cool Craft, the thermostat is a Mr. Gasket high flow style 180 degree that was tested in boiling water before it was installed, all hoses are new, radiator cap is a 13#, it does have an overflow tank hooked up, and it has Derale single electric fan (dual speed) with aluminum shroud that is sealed to the radiator and move a ton of air. Low fan set to come on at 170, high fan set to come on at 185, which they do. I have a dakota Digital fan controller which reports accurate temps and a mechanical Stewart Warner gauge which is within ~5 degrees of the Dakota Digital readout. Both gauge senders are in the same coolant port in the intake.

With the heater core bypassed (hoses looped together,) it runs at 180-185 on the gauge, even after I beat on it. All seems normal. But, when I hook up the heater core, the temps jump and swing wildly over 220 degrees. The odd thing is the temps come down when I turn the heater control valve off and then they go wild again when I turn the heater valve on. The heater core and water valve are originals. The 5/8" hose comes from the intake manifold to the 5/8" port on the control valve. I have a pre-moulded (reproduction) hose that goes from the valve to the bottom connection on the core, and then I have a 3/4" hose going from the top/outlet of the core to the port on the water pump. I used to have a crossover in the heater hoses but left it out this time as it doesn't seem to make any difference and the I6/small block cars did not come from the factory with one anyway.

The heater core does not leak and it is not restricted. I flushed it in both directions with it out of the car and pressure tested to 15psi which it held all day submerged in water. I've also used one of the Lisle coolant burp kits that go in place of the radiator cap when filling the system to let air purge out as the engine runs, but with and without the heater valve open. I've also jacked up the front of the car to make sure the radiator cap is the highest point (the car is lowered).

My gut says it's air in the system, but after running the engine at various RPMs for 10-15 minutes, I'm not seeing any air bubbles coming up into the Lisle funnel and then all of sudden I get a geyser of coolant boiling all over. Temps at this point are showing 210-230 on the gauge. If I close the heater valve, the temps will fall rapidly to ~170 or so...if I open the valve...they skyrocket again. I've tried purging the air a half dozen times and nothing I do seems to make any difference. Coolant in green 50/50 mix.

What gives!? I've never had this much trouble, especially since it's just a small block with the stock heater components.

 

[Fathead Racing]

You never mentioned, does the heater blow hot air when full on?

 

[1958 delivery]

Exactly which port on the intake is the sender in?

 

[ssleeper]

Heater does blow hot.

It's an aluminum Weiand stealth intake with a NPT port right next to the thermostat housing. There are two ports there actually, I'm using one for the mechanical gauge and one for the heater hose outlet fitting. The t-stat, heater hose fitting, and sender all share the same coolant passage in the intake. Same as this one...

 

[Don Jacks]

I think that it's air in the system.Attach heater hoses,remove the next to last bolt on the intake manifold,passenger side,fill radiator until you see coolant come out of the bolt hole,reinstall manifold bolt,finish filling radiator.Run test system,you should be good.

 

[MRHP]

I have had to backfill the heater hoses on Oldsmobile’s to help purge all the air out years ago. I had forgotten all about it until reading this post.

 

[61BUBBLE348]

I have a Buick V6 in my 38 Ford, I have a plugged hose at the highest point on the manifold, whenever I need to do any water hose work, I top the radiator up run the engine with the radiator cap off, I removed the plugged hose and it takes a little time to purge the air from the system, once I get a steady flow of water I re-install the plugged hose and off I go.

 

[Junky]

If you are running the hot water through the heater core, and there is no air moving across it, it is going to be retaining that heated water, and returning it to the engine, where it might be messing with your gauges. Also check the lower radiator hose for a intermittent collapsing condition. The days where the hose manufactures installed a wire in the lower hose are long gone. I take the old coils out, and push them into the replacement lower hoses anytime that have to change a hose. Only wish that I had saved more of them.

 

[ssleeper]

Update...sorry, it's a long rambling story, but I'm putting it in here in case anyone else has similar issues and does a search...

Thanks for all the suggestions and ideas everyone. Having the water returning from the heater core and messing with the gauge does't make sense to me because the water going out to the core comes from the area right where the gauge sender/sensor and it's all at the same temp right there. No hoses collapsing either. I did backfill the core to see if it would make a difference and it did not but I'm glad I did it anyway.

Side note, during this whole process I've checked the engine in various places, hoses, radiator, sender/sensor with an infrared temp gun and no areas were over 180 degrees no matter how erratic the temp gauge was reading at the time.

Anyhow, yesterday and today I took the car for three test drives. I actually put more miles on it than I ever have in 6 years. Yesterday I let the car come up to temp, 180 on the gauge (which took 8 minutes from completely cold, it was 80 deg outside). I jacked the front of the car up pretty high and let it run with the Lisle funnel attached. Can't say I got any air out of it but it idled in the garage for 20-25 minutes before the temp started to climb and go a bit crazy then came down to 170-185 depending on which gauge I checked. I let it cool off and thought maybe an air bubble worked itself out, and since temps were good, I decided to go for a drive....

About 2 miles down the road the temp started climbing somewhat erratically to 200...220..all the way to 240. I also checked the digital readout on the Dakota Digital fan controller and it was showing the same readings. After going another 2 miles, I pulled over and the temp came down to 180. I had a rag with me so I decided to crack open the rad cap and see what happens. Surprisingly, no coolant spray. It was down to about the top of the fins. Headed back home to top it off during which time the temp never went over 185. At home I cracked open the radiator cap to add some water and this time I got a spray of coolant. What! How is it low when I check it and now full to the top? I don't understand that, but decided to continue my drive anyway. I didn't add coolant or water and the overflow jug was up to the hot line. I ended up putting 60 miles on the car with the temp gauge steady and normal the whole time. Even gave it a few WOT runs and all temps were normal.

Today I went for another drive, expecting no temp issues based off how well it behaved yesterday. But after it came up to 180, about 2-3 miles from home, the temp started climbing erratically to 240 again, then settled back down to 180-185 on the gauge and stayed there for the rest of the trip. I stopped at a friends house for about 30 mins, then left and experienced the exact same pattern. It came back up to 180...then erratic to 220-230, then down to normal for the rest of the drive back home.

So..three trips...three of the exact same temp issues each time. Seems like air or a restriction to me.

I decided to take the thermostat out and see if it goes through the same pattern. I retested the tstat in water and verified it was opening once the water got to 190-195. It's a Mr. Gasket high flow 180 tstat. It does remain open as the water comes down to 185 and starts closing as it gets to 180. It seems like it takes more temp to initially get it to open. Interesting. I also noted that it has a small air bypass already stamped into the valve and you can actually see light around the edges of where the valve closes to the body so I know drilling holes in it won't make any difference, air can already bleed through it. Tstat left out, away we go on a test drive.

This time things were different! The temp came up as it normally did and topping out at 170 before I left (low speed fan is set to come on at 175). I put 40 miles on it and both temp gauges were rock steady at 180. Not a blip, not a swing, nothing out of the ordinary. The highest I ever saw the fan controller read was 183. I also cycled the water valve on and off several times and it made no change in temps at all. Finally some progress! When I pulled back in the garage and let it idle, it crept up to 195 and then 200...the high speed fan came on and the temp stayed steady and started to come down a couple degrees. Not fixed, but headed in the right direction and I'm breathing a sigh of relief. Dare I say I actually enjoyed driving the car for once?! Not checking the temp gauge ever 10 seconds!

I'm ruling out the heater core, seeping head gasket or small crack in a cylinder wall, or a restriction. All great things. And now searching for the most reliable thermostat.

 

[409newby]

Sure sounds like an erratic thermostat I would try a high flow 160.

 

[1958 delivery]

Just use a Stant 160 or 180 stay away from Mr Gasket and other aftermarket wiz bang parts

 

[ssleeper]

Another update...

Picked up a Stant Superstat 160 and 180, tested both in boiling water and both are working normally. Decided to put in the 160 since the car will run at 180 with no stat at all. I figured a 160 will give itself a bit more time to full open before the temps skyrocket, hopefully avoiding that.

When I took it for a drive the temps went back to erratic, exactly the same as when I has the full flow 180 stat in it. Both gauges reported the same temps, both going al the way up to 260 before dropping back to 180-185 for the rest of the 50 mile drive. It was never erratic after the first spike, just like before. I also tried letting the car warm up with the water control valve open...thinking maybe if the internal coolant bypass was restricted, then the heater core would act as the bypass and any coolant in the intake would have a path out and back into the engine, but it made no difference.

So the question is, why are all the stats (3 different ones, and all tested to be working normally) not opening correctly until the temp goes super high, then comes back to normal and never is erratic for the remaining drive...until the engine cools and goes through another warm up cycle?

Flow through the intake? I read two others reporting a high flow water pump was giving them the identical erratic temp issues, but I don't see how a high flow pump would affect that. Thoughts? I'd prefer not to leave the stat out but so far thats the only time the temp never spiked and the car warmed up normally. And since no stat is affecting flow in a big way on warm up, it has to be the reason. But why?

 

[La Hot Rods]

What year is this block, does it have the bypass hole below the water pump bolt hole on the passenger side?

 

[64ss409]

I'm ruling out the heater core, seeping head gasket or small crack in a cylinder wall, or a restriction. All great things. And now searching for the most reliable thermostat.

I suspect combustion gases getting in the coolant. A jeep years ago, same symptoms. I thought head gasket. Finally found a small crack in the head.

 

[ssleeper]

What year is this block, does it have the bypass hole below the water pump bolt hole on the passenger side?

'69 or '70. It has the bypass.

I suspect combustion gases getting in the coolant. A jeep years ago, same symptoms. I thought head gasket. Finally found a small crack in the head.

I thought so too, but I've run two combustion gas tests with the blue liquid and they did not change color either time. Plus, the heads are new AFRs and the block was zero decked last year, put together with cometic gaskets and arp hardware. I was thinking maybe a small crack in a cylinder wall but I should think it would be pressurizing/overheating the cooling system more than the one initial spike...or using a bit of coolant, which it's not.

I think a for sale ad might be the right choice

 

[La Hot Rods]

Does the water pump have the bypass hole?

 

[ssleeper]

I don't recall checking for it specifically before installing it (that was a year ago now) but I'll assume yes based on the pictures on the Summit site. It's Proform #67265.

Yesterday I let it run for almost an hour with the Lisle air bleed funnel, let it cool off completely, and went for a drive. The temp came up slow and steady for the first 5 miles/~10 minutes of driving, then both gauges started going erratic again. Mech gauge bounced around all the way up to 250 then back down to 180 over the course of about 30 seconds. After that it remained steady at 180 the whole way home. Side note, I did adjust my vacuum secondaries to open a bit faster and I got a chance to test that too...wow! Much better...really woke it up down around 3k rpm. If I didn't know the size and design of the stock driveshaft, I'd be banging gears with much more often and with much less care haha!

Once home I popped the radiator cap off and let the coolant spit out. There didn't seem to be any air 'gush' when I cracked the cap open, just started spitting coolant right away. I let it run with the cap off and temps stayed at 180 or less...even going as low as 160 on the Dakota Digital gauge.

The coolant was about 1/2" down from the top of the tank so I put the cap back on and revved the engine. To my surprise the upper radiator hose sucked shut each time I revved it. I guess that means the pump is sucking coolant out the bottom of the radiator as it should, and I assume this must mean the tstat was closed. Seems far fetched but is there a chance the high flow pump is flooding the engine with too much cold coolant causing the tstat to close when it shouldn't? This would be after 20-25 minutes of driving and idling. I'd think the whole system would be stabilized by that time.

I'm really dreading pulling the engine again but I have a feeling that is where this is headed.

 

[wristpin]

Could it simply be when the engine has warmed up the anti-freeze enough to open the theremostat, the anti-freeze suddenly reaching the temp sensors is a hotter temp from the heads than the temp of anti-freeze that was at temp sensors? Then as cooled water that was in the radiator flows up into and through the block the anti-freeze temp drops and stablizes?

 

[DonSSDD]

Has your car ever boiled over?

Does your IR readings on the thermostat housing agree with what you are seeing on your gauges?

 

[ssleeper]

Could it simply be when the engine has warmed up the anti-freeze enough to open the theremostat, the anti-freeze suddenly reaching the temp sensors is a hotter temp from the heads than the temp of anti-freeze that was at temp sensors?

I've considered it, but the temp swings so erratic and quickly, it almost seems like localized boiling in the sensor area, even though all surrounding areas test between 160 and 180 degrees at the same time. As I understand the flow path, the coolant goes in the block, up to the heads, then out the front of the heads to the crossover channel in the front of the intake where the thermostat sits. It's in this crossover chamber that both of my temp sensors are located. It's also where the fitting for the heater hose hookup is taken from which provides some circulation/bypass when the water valve is open..but having the valve open or closed now seems to make no difference with the temp being stable or erratic. All the coolant in the crossover should be the same temp so I would think when the gauges start rising quickly, the thermostat will also be in that same hot water and opening up.

Has your car ever boiled over?

Does your IR readings on the thermostat housing agree with what you are seeing on your gauges?

It has not boiled over, but it is a closed system. Overall, yes. When the temp sensors rear around 180, the housing reports about 160-165 on the IR. I thought maybe the mechanical sensor was on the fritz after it goes over 185 degrees, but the Dakota Digital temp sender is located about an inch away from the mechanical sensor and reports similar rise in temps at the same times. The mechanical sensor does read 10-15 degrees warmer than the electric sender but they are close enough, to me, to validate each other. Up to 180 degrees, they read nearly identical which also coincides with the IR gun on the heads/intake/pump/rad.


This car had the exact same temp issue before I took the engine out 3 years ago. On the highway it would peg the temp gauge at 260, yet it would still idle without stalling out. The only part of the whole system that is the same between then and now is the short block and intake manifold. Everything else has been changed...heads, cam, water pump, radiator, hoses, tstat, carb, headers, fan, etc. The engine was taken out because a roller lifter seized a wheel and damaged the cam. The short block was disassembled (by me) and taken to the machine shop for a zero deck mill and hot tank cleaning. They put in new freeze plugs and honed the cylinders. I reassembled it with new cam, main, and rod bearings...then added all the new parts. The person I sold the old heads to had them magnafluxed and he found they were not cracked (dart iron eagle). This makes me think maybe it could be a block problem, but it would be nice if it would just put some coolant in the oil or give me a stream of bubbles coming out of the radiator and make it easy to pinpoint! At least I would have a direction to go with it.