Cooling system pressure tested

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peejay
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Cooling system pressure tested

Post by peejay » Wed Oct 17, 2018 7:52 pm

Years and years back there was a thread here about cooling system pressure. Someone commented that cooling system pressure in the block spikes to about 100psi with the thermostat shut but flatlines at radiator cap pressure with it open.

I had a "problem child" at work. The only specifics I will give is that it is a 60's era Ford in good condition (new everything, no combustion leaks). It kept blowing things apart. Core plugs, heater hose hardware, etc. Frustrated, I put a pressure gauge on the heater hose coming from the intake manifold. Water pressure was around nothing at idle, and it would ramp up to over 120psi over 3500rpm or so. I did not dare check higher.

That is interesting but not the part that made my brain hurt. I was sort-of expecting that.

I had the radiator cap off so I could watch coolant flow through the tubes to see when the thermostat opened so I could check pressure then. After the thermostat opened, I revved the engine up... and flow STOPPED, and pressure in the block ramped up just the same as before. Over about 1500rpm the flow would stop! I have never seen this happen before! Return to idle, flow came back.

I am thinking that the water pump pressure was actually closing the thermostat!


As a test I cut the legs off of the thermostat so it was just a restrictor. Never more than 30psi in the block, and a torrent of flow coming through the radiator.

Well, what the hell???

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Re: Cooling system pressure tested

Post by ijames » Wed Oct 17, 2018 9:01 pm

Starting with everything cold and running the engine until the thermostat opens, the first block full of water that goes into the engine will still be dead cold from the radiator so it is normal for the thermostat to slam back shut when that cold water reaches it. Then after this water warms up, the thermostat opens again and another block full of water enters. Depending on how big the radiator is this water may be cold enough to repeat the thermostat cycling or it may stay mostly open from this point on. I bet you revved the engine just after seeing the flow start in the radiator, just in time to have the thermostat close so you saw the pressure spike again. Back when I was tuning and making chips for my 86 turbo Regal I could clearly see the water temperature rise, then fall back, then rise again, then maybe fall again, then slowly stabilize, in the data logs. Of course the first time I really noticed it was just after adjusting the fan on and off temp settings to match my new 160F thermostat, so I had to think about it for a bit to be sure I hadn't foobared the decimal to hex conversion since this was before I found any editing software #-o .
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MadBill
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Re: Cooling system pressure tested

Post by MadBill » Wed Oct 17, 2018 10:56 pm

Sounds like the common fix of three 1/8" bypass holes drilled in the periphery of the stat might be just the ticket to ease such a cold slug issue...
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Re: Cooling system pressure tested

Post by Geoff2 » Thu Oct 18, 2018 4:22 am

Bill,
The 'three holes in the stat' originally came from stewartcomponents.com. Stewart recommended the t'stat with holes when used with their race water pumps, which eliminated the internal bypass that stock pumps use. The holes were for pressure relief in case the stat jammed shut. Warm up actually takes longer with a drilled stat.

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Re: Cooling system pressure tested

Post by Kevin Johnson » Thu Oct 18, 2018 5:30 am

WAG Try making a bypass loop for the heater core circuit. With such an old vehicle it might be blocked and that could interrupt the coolant flow path that dominates when the thermostat is closed.

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Re: Cooling system pressure tested

Post by Circlotron » Thu Oct 18, 2018 6:35 am

peejay wrote:
Wed Oct 17, 2018 7:52 pm
Years and years back there was a thread here about cooling system pressure. Someone commented that cooling system pressure in the block spikes to about 100psi with the thermostat shut but flatlines at radiator cap pressure with it open.
One of the advantages of a cross flow radiator is that the cap can be on the suction side. Radiators with the tanks on the top and bottom naturally have the cap on the top, and this limits the pressure available to push water downwards through the core to the cap pressure. The pressure upstream of the thermostat can be way more, regardless of the radiator type.

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Re: Cooling system pressure tested

Post by peejay » Thu Oct 18, 2018 7:24 am

Kevin Johnson wrote:
Thu Oct 18, 2018 5:30 am
WAG Try making a bypass loop for the heater core circuit. With such an old vehicle it might be blocked and that could interrupt the coolant flow path that dominates when the thermostat is closed.

New everything, nothing is plugged. Can blow through the water bypass in the intake manifold just fine.

The "water slug" theory is a nice theory except I was able to open and close the thermostat by revving the engine. Drop to idle, water flows. Pull revs up to 1500 or so, water stops flowing. Water temp as measured just before the thermostat, in the intake manifold, was cosistently 10+ degrees hotter than the stat opening temp.

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Re: Cooling system pressure tested

Post by Newold1 » Thu Oct 18, 2018 10:43 am

If the thermostat on the engine has a bypass on the engine the operating water pressure on the running engine cold or hot should not reach those kind of super high pressures unless either the radiator cap pressure opening is bad. Most radiator pressure caps are designed and supposed to open with pressures of 11-15lb. range,so if your cooling system (radiator + engine) is going up to the high pressures you are seeing during operation and not just momentary spike pressures then something is bad with radiator cap period.

I am not sure what water pressures you should be measuring for thermostat cold-hot or by-pass cold pressures with the spikes that might occur between those states.

How are you measuring these pressures and what type of pressure gauge are you using. Is the engine being operated with a conventional cap and is it opening at all?

Based on your description another possibility where you describe the start-stop of the high pressure readings could be possibly a thermostat being installed upside down ( reversal against regular path flow.) I suppose a thermostat installed this way could stay open at low rpms and then be forced shut with the pumps higher pressure generated at higher rpms?

Obviously if the pressures you are measuring on your engine operating for any normal period are as high as you are indicating, this will create all kinds of potential damage, especially head gasket sealing.

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Re: Cooling system pressure tested

Post by bill jones » Thu Oct 18, 2018 11:21 am

-what 1960's engine is this?

-are the head gaskets installed with the large water holes to the rear of the block?

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Re: Cooling system pressure tested

Post by allencr267 » Thu Oct 18, 2018 12:24 pm

Has it got a brass bellows type thermostat, I don't know that the other type would be susceptible to high pressure like a bellows sure would be?

Image

De-clearance the waterpump so it's not so efficient. :wink:

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Re: Cooling system pressure tested

Post by Newold1 » Thu Oct 18, 2018 2:43 pm

I am obviously not sure of what you are measuring these high pressures with, I hope the pressure is being measured with a gauge designed and calibrated for water and not some other fluid type (like oil). In any event pressures this high that will blow out freeze plug and such will not be helped by clearancing a standard type automotive water pump! An engine water pumps are not really designed to create pressures that high but to move certain volumes of coolant and not create a lot of pressure.
Pressure is created as restriction to free unrestricted flow and the only way these pressures are created means that something is creating a substantial restriction to the normal coolant flow in this engine and its attached cooling system. The pressure that builds up in a closed cooling system of engines is created from the expansion of the fluids and air in the system and is normally kept at a 20 psi max. Something is amiss in this system!

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Re: Cooling system pressure tested

Post by grandsport51 » Thu Oct 18, 2018 3:50 pm

I know that water pump pressure can build up behind a Tstat but I always Installed Robertshaw Series 330
They don’t open against the water flow.
Speedway sells em cheap and they have the 3 bypass
Holes pre drilled!!
PS I don’t believe any engine that doesn’t have screw in Core Plugs could withstand 150 Psig of coolant
Pressure.
Dave B.
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Re: Cooling system pressure tested

Post by fastblackracing » Thu Oct 18, 2018 4:59 pm

The T-stat that grandsport51 posted up should fix your condition.....or make the pump less efficient.

For all of you that think a cooling system loop will never go above Cap pressure need to get a better
understanding of how a seemingly simple cooling system should work.

I have seen as much as 65# block pressure....but you must have 1 very good pump with tight impellor
clearances.....car water pumps usually flatten out pressure wise around 4000 shaft rpms, but there
are differences.

The highest pressure will be between the pump and the T-stat which is the first major restriction
the water sees so there is usually a pressure drop across the stat, and the next restriction is the
radiator core entrance which depending on how much flow is coming from the stat vs. how much the
core can flow can either result in no additional pressure being generated or it can become the main
restriction......

The outlet of the core to the pump inlet can be either below 0 psig or it just be less than the system
pressure before the core entrance....as long as its lower there will still be some flow.....The bottom
hoses had springs in them to resist collapse when that part of the system goes below 0psig but it they
normally run somewhere around a few psi at that point vs. 20+psi at the pump discharge.

Its all about pressure differentials in the system loop, not as simple as one would think at a quick glance.

With your pump hitting 100+psi I could see it pushing the stat closed, Do you have an idea of what
your pump pulley drive ratio is? Is it being over driven a good bit? One of my old Chrysler service manuals
even called out all of the pump pulley drive ratios along with a ton of other great info......I will never
get of that manual, I have learned a lot of good stuff from it.

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Re: Cooling system pressure tested

Post by peejay » Thu Oct 18, 2018 9:37 pm

Engine runs 30psi pressure max with the thermostat gutted so that it is just a restrictor.


All pressures measured with the cap off so I could watch coolant flow. Block pressure was measured at the intake manifold near the thermostat, and was directly correlated with RPM. You could use the pressure gauge as a tach...


This was a very interesting problem that I had to think around. More specifically, engine is a 385 series Ford.

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Re: Cooling system pressure tested

Post by cjperformance » Thu Oct 18, 2018 10:24 pm

peejay wrote:
Thu Oct 18, 2018 9:37 pm
Engine runs 30psi pressure max with the thermostat gutted so that it is just a restrictor.


All pressures measured with the cap off so I could watch coolant flow. Block pressure was measured at the intake manifold near the thermostat, and was directly correlated with RPM. You could use the pressure gauge as a tach...


This was a very interesting problem that I had to think around. More specifically, engine is a 385 series Ford.
You should have said that to start with. Slow the pump down and fit a normal shaw or pellet t stat.
385's make a ton of flow at low rpm, designed to keep a low rpm engine in a chock a block engine bay cool.
I have seen these pop/split new heater hoses when revved up. Just slow the pump down relevent to what rpm the engine will see
Craig.

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