302w losing coolant

[scumdog]

My ‘new’ ride is a ‘65 F100 Styleside running three speed manual transmission and 302 Windsor with headers, Edelbrock manifold and 600 Holley.
Trouble is it’s going through coolant, it’s not leaking from any of the places like radiator, heater core, core plugs etc and isn’t dripping from anywhere.
But after a 150 mile trip (for example) on checking the coolant after the motor has cooled the coolant could be up to two quarts low.
I suspect a head gasket (or two?) but there’s always the possibility of a cracked head or block? Any ideas or thoughts welcome!
(the F100 is going into get some other issues sorted but the main issue to be attended to is the coolant disappearing)

[simplyconnected]

I would do a pressure test on the cooling system. Take the radiator cap off and charge each cylinder, one at a time, with 100-PSI of compressed air through the spark plug holes to see if any bubbles appear in your radiator. If you have a head gasket problem it will show up.

SBF engines are exceptional in durability and performance. The only reason Ford changed was to get the cams in the heads for improved valve train performance. The 'new' modular engines are about the same size, 4.6L instead of 5L, performance is about the same with an increase in mileage because they use more aluminum to make the engine lighter. My favorite SBF engine is the 351W but I really like the 390 FE the most. Ford used both engines in all their car and truck lines so parts are readily available. (As an example, SBF water pumps are available with inlets on either side and HO engines have hydraulic roller cams.) - Dave

[OX1]

Hard to believe, using that much antifreeze, you don't have some white exhaust smoke and/or some in the oil.

[Eric S]

Dumb things first, are you sure you're not loosing coolant out of the tank's overfilling. Is the cap of the correct pressure and how full is the tank when you say "it's 2 quarts low"?

[scumdog]

Hi all, thanks for the advice!
Dave I’m leaving it to my favourite repair shop to test and find where the coolant is going, they have the equipment and expertise.

OX1 I haven’t noticed any white smoke at all but having said that I’m suspicious about the volume of ‘water’ coming from out of the tailpipes when the motor is first started and before it warms up.

ERIC S At least once I couldn’t see the coolant ie it was lower than the top tank, it took three litres to top up, other times I could just see the coolant level, then it took roughly two quarts.

NB: the ‘water/vapour’ seems to eminate from both tailpipes equally.

[scumdog]

Update:
I've fitted (or had fitted!) a new thermostat (it never had one) and new top, bottom and thermostat by-pass hose, had the radiator flushed by a radiator shop as the core was partially clogged, they supplied a 16lb cap but I may replace it with a 10lb cap to ease the load on the original heater core and I've also had the cooling system pressure tested.
So far I've not taken the truck on a lengthy trip so unable to confirm if the coolant loss problem has been fixed.

[OX1]

Something still not right. Can't lose 2 quarts antifreeze, internally, if it's not going somewhere (which it can't if pressure tests good).

[simplyconnected]

Just a thought... Your intake manifold may be sucking coolant in. You wouldn't notice a small leak. Eventually the engine will overheat. It's a good thing you are on top of it. - Dave

[scumdog]

Well still a mystery as to where all the coolant went but I'm just back from a 1000+ miles trip and didn't need to add coolant and even going over a couple of mountain passes the temp never got higher than 175F.
Plus I was getting around 22mpg a fair bit of the trip and the oil level never changed.

Now I did end up with a coolant leak AFTER I got home - I noticed a small coolant puddle under the truck, a quick look showed the manifold to water pump rubber elbow needed the upper hose clip needed tightening, problem solved! Goodness knows why the leak started after I got home..

Thanks for all the advice and suggestions guys.