Loosing coolant only when driving

[motomutt]

My 73 Bronco has had a '68 289 in it for 20 years. Only about 40K miles on it. I recently replaced both head gaskets, exhaust gaskets, new edelbrock manifold, new 450 cfm 4bbl carb and cleaned up a lot of the hardware and wiring. My radiator, water pump, high output mechanical fan are all recent. I fill the radiator with a gallon of antifreeze and the rest water. new 16lb. radiator cap (I tested it). I'm running a 160 thermostat here in Sacramento. The gauge will go as high as 190 but no further. I can let it sit in my driveway, in park, idling and let it get to 190. Turn her off and this is when I would expect a little expansion into the recovery tank. Nope. Everything's good. I drive it about a mile, pull in, pop the hood and my recovery tank, which was at the "cold" level is now above the "high" level. I wouldn't be too alarmed but it never gets sucked back into the radiator even if left overnight. I tested the cooling system with my pump tester and no leaks whatsoever. There's no water in the oil and no funky smell from the radiator fluid. What gives? I'm usually pretty good at diagnostics but I'm stumped. Your $.02 is greatly appreciated in advance. Mutt

 

[Broncobowsher]

Had this in the past. Found it with a leak down test. There was a sub-pin hole head gasket leak. It took the high pressure of a cylinder firing under load to leak a little gas into the coolant. Idle didn't have enough cylinder pressure to leak pressure. The leak down was enough to find it. It was a slight rise in the coolant level during the leak down test.

 

[motomutt]

I guess that's what I'm doing tomorrow. PITA. I just finished this major overhaul. If the leak down test proves negative I'm still at a loss

 

[phred]

The edelbrock manifolds will often not seal properly. I’ve had them leak water at the rear crossover. The engine just sucks it in and pumps it out the exhaust

 

[motomutt]

The rear cross over is blocked on the stock 2bbl manifold and on the edelbrock. If what you're saying was correct I'd be losing coolant volume. I've still got the same amount I started with. I used a good amount of Right Stuff on both sides of the water ports. That would be an easier fix than head gasket(s).

 

[motomutt]

Leak down test shows number 2 cylinder makes coolant over flow. All the other numbers were strong. How am I supposed to tell if I have a defective felpro head gasket? I've never had anything like this happen. I'm a good spanner turner. Made sure the install was sano. I made 2 guides for the bottom corners ( 2 long bolts with heads ground off) so the heads just slid right on. Ill have the head magnafluxed when I get it off. At least its on the passenger side. Thanks for the input. Any suggestions going forward are welcome. Mutt

 

[Broncobowsher]

Don't have any more advise.

The good thing is you found that it is a compression leak into the cooling system, and you know what cylinder it is on. So you are not looking everywhere for the problem, it is in a single cylinder. It could be something as simple as something was dropped on the gasket and dimpled the fire ring and it just won't seal. Having that one head checked, and have them look at that one cylinder extra hard, is a good idea.

 

[Rustytruck]

you should not of needed guide studs with the head dowels in place. Were your head gaskets perma torq with the orange printed silicone on them" I use to really like the perma tork blue teflon coating they use to sell. did you put the head on with out the gasket in place and make sure the head bolts have enough clear threads to go all the way down and then take it apart and put the head gasket on and torque her down.
did you check the head and block with a strait edge. I have had new head gasket issues twice on the small block ford and they were on the passengers side at the back of the head. both times the bolts stopped before full torque each time I replaced the head gasket chased the threads and for good measure sprayed the gaskets with copper head gasket spray which defeated the purpose of the fancy head gaskets coatings. in the few engines I have done I have had thread issues head dowels too long erosion around the water jackets and wrong water ports for the gasket used. never had a crack or warped cast iron head but every aluminium head on imports were warped.

 

[motomutt]

The bolts are all fine. My bronco is lifted and the dowels made it super easy. Line the dowels up with the holes and the head just slid down perfectly into position. No wrestling or wondering if I bumped the gasket. My machinist checked for flatness but I think I'll have it shaved. Btw, I bolted on the quick fuel 450cfm 4bbl carb. Great little carb. After a few adjustments it is really responsive and my plugs look great.

 

[Broncobowsher]

Quick question, do you have the alignment dowels? There should be 2 basic sheet metal rings that act as alignment guides to keep the gasket and head properly located on the block.

 

[motomutt]

Yes. They're in place

 

[motomutt]

Thanks for your direction o the leak down test. It was definitive.