I hope someone can help with this mystifying cooling system problem I have.
1976 F100, 302, with factory AC. I recently installed some 351W heads on it because the stock heads had a leaky valve. I know my head gaskets have the L shaped hole for the water port in the front and back. The instake I installed is a 1984 factory Ford 4V aluminum intake.
I also has a new 180 degree thermostat, new heater control valve all new hoses, and I made sure the spring is in the lower radiator hose.

I have put a piece of clear vinyl hose from the hater hose port in the intake to the heater hose going through the heater control valve. All testing talked about here was done in Park, in the yard. Coolant level in the radiator is about half way filling the top tank.
When I start the engine the coolant flows through the heater hose for about 2 minutes, or tries, depending on the engine speed. At idle you can see it bouncing around in the clear hose, but not actually flowing. If I rev it a little it flows through the heater core and I feel faint heat from the vents.
After two minutes the coolant drains from the clear test hose and does not flow at all, no matter how fast I rev the engine. During this time the temperature gauge rises slowly until it gets to the H. At that time the thermostat seems to open and suddenly coolant begins to flow through the clear test hose like crazy, heat in cab is great, and the temperature gauge drops like a rock to the bottom of the normal range.

What I have been thinking:
This system design should flow some coolant at all times, especially since the heater hose and bypass hose go AROUND the thermostat, unless the water pump is not pumping (impeller or belt problem). Since coolant flows fine when I first start it, and after it almost overheats, I think the water pump is fine.
As the heat is always on during this test the heater control valve is open. I have no touched the heater controls during all of the above test.
If there was a problem with the ports in the heads wouldn't I have flow problems and/or overheating all of the time? Therefore I don't think it is a flow problem in the heads.

Note: during other engine runs, when I have driven it, the thermostat has opened after hitting the H, and temp normalized, then I have cut it off for hours and restarted, with no overheating again that day.





I am open to ideas.

Thanks,
Chris

 

In the spirit "easy things first" it is fairly common to have an air pocket in the cooling system after first fill. This can cause temp spikes and no heat to the heater. Usually one full cycle of warm up, cool down and re-fill the raditor as needed takes care of it.

That may be why your system behaved OK after the first spike. It burped its air out when the T stat opened. I would suggest re-check coolant level and repeat the test.

 

It has been doing this for many cycles and driving about 50 miles since I did the heads. I have repeatedly attempted to get air out, up to and including changing this over to an overflow tank system, as this was not done in 76. I have a lever cap on the radiator and have removed the cap when hot to see the coolant level in there. I do not know what else I can do to bleed air out of the system. I have been messing with 302's for almost 30 years and have never encountered this before.
I will check it again tomorrow, by running it until the thermostat opens and then fill the radiator.

Thanks,
Chris

 

Just some ideas...

1. Verify the heater control is opening the valve and the valve gate has not separated from the housing.

2. Verify that the water pump is not a reverse rotation pump?... Dunno when it changed direction.

3. Drill a 1/8-inch hole on the thermostat's flange and position it at 12 o'clock if a vertical face. Otherwise, just install it. The hole will allow trapped air to escape without unduly affecting the warm-up cycle.

4. 'Could be a water pump that flows at idle but the impeller freewheels at higher RPM or when hot.

5. Clogged heater core? Has it been flushed forward and reverse direction?

 

Since I get heat before and after the overheating event, without any interaction with the heater controls, I believe the new heater control valve is fine and the heater core is not clogged.
Since I get coolant flow before and after the overheat event, and it does not overheat at any RPM after the thermostat opens, I can't say that there is a problem with the water pump, but it is one of my suspects at this time.

This began happening after I installed the 351W heads and factory Aluminum 4v intake. This is why I put a new thermostat in. I am at a loss for what could be wrong or clogged with this head/intake setup to cause flow to stop happening until the thermostat opens then flowing fine after that.

The water pump pumps all the time. If the thermostat is closed then coolant does not flow through upper radiator hose but should still flow through the heater hoses and the bypass hose at the water neck. Since both of these hoses get their coolant flow from the same place as the thermostat, the front intake crossover coolant channel between the heads, I don't see how either could stop flowing at any point, regardless of the thermostat status.

Thanks,
Chris

 

I'm not a cooling system expert, but are the head gaskets on right? Backwards installation would cause a hot running scenario, but you would have noticed the "hot" heads a lot earlier.

Sounds like a thermostat problem, as you have alleviated the water pump and clogged heater core as symptoms. I would re-check the thermostat....have you the right one for yer 302?

 

Btw, using 351W heads on a SBF it is usually recommended that the intake gasket's dog-leg portion over one of the coolant passages be trimmed.


Good call on the head gasket reversal. Top means top and front means front.

 

Whenever I have found head gaskets reversed, the thing just goes from cold to boiling hot in about 10 minutes. There is no cool-down event like you describe here. Not saying that is NOT the problem, but they don't act like this in my admittedly limited experience. The 351W heads on a 302 is a wildcard I have not ever dealt with.

Reverse-flow water pump usually means it works fine until the heat-load gets up there, like in the summer, then it just doesn't cool as well as it should. Again, not usually going to see the hot-cold thing you describe IMO.

What you describe sounds to me like the thermostat is not opening when it should. The first thing I would do it test the thermostat by putting it in a pot of water under heat and a temp gauge in the water. See what temp it actually opens at. While you have it out, you might try running it without the thermostat to see how it acts.

 

One other little thing to try is the radiator cap. Those lever top type can be sketchy... sometimes do not hold pressure May be worth trying a known good standard one.

 

Jas88 - Mebbe only one gasket is somehow reversed. That might explain the delay in getting too hot.....I hear you on the point you made, though.

Take a gander at his # 1 picture. Could be my eyeballs, but it looks like one head gasket is different than the other. Could be the cause of this illness.

Thanks, HIO.

 

I found I do have pics of the new head gaskets in position. I can see the "Front" printed on the driver side gasket.







But, if both gaskets have "Front" printed on them, on the end that does not have coolant holes in it, that would suggest that I have the passenger side head gasket on backwards. Weird since the gasket lines up with the holes in the block, as you can clearly see in the pic.

Thanks,
Chris

 

Every beast has their own peculiarities. Let us know what you come up with, yes?

 

From the pictures it appears the passenger side head gasket is installed backwards.
The driver side is correct.

The coolant passages on the ends of the gasket need to be at the rear of the engine.
The water pump pushes coolant through the block, to the rear then up into the heads. The coolant travels from the rear of the heads forward into the crossover at the front of the manifold and to the t-stat.

With the passenger side gasket the way you have it the passenger side of the block and head aren't in the coolant flow, the coolant goes into the block, directly into the head and to the t-stat.

Good luck.