I had a very nice, long drive last week up to the mountains just north of me. We’ve had a cold snap here and it didn’t take long to realize that my high flow thermostat has bought the farm. Through most of the run including hill climbs, it was running between 120 and 140.

What was odd was that durring long down hill coasts, and I’m talking miles at a time, the motor would heat up substantially. I’m talking over 220. Is this unusual? Could it be a lean condition? It certainly went back to running cold as soon as I got back on the throttle.

Any thoughts. Unusual or I just never noticed before? Should I do anything about it?

 

Seems odd. I suggest one of these inexpensive remote temp detectors before starting any work. Just to verify that it is real

https://www.harborfreight.com/121-in...ter-63985.html

 

If you have a clutch fan, check to see that the clutch hasnt failed.

 

Hadn't thought of that. Where do you think I should aim the thing to get a meaningful reading?

 

Nope, dual Spaals. Good thought though.

 

You need to diagnose and repair your cooling system, A-Z. Start by installing a 180 degree thermostat like it's supposed to have. Running at 120-140 degrees you're fouling plugs, wasting gas, making less power, and creating lots of unnecessary pollution. And your heater/defrost won't work worth beans. Either someone's removed the t-stat from your car or it's stuck open if you're seeing 120-140 degrees.

Next let it idle in the driveway and use a laser thermometer on the thermostat housing, top of the radiator, etc to confirm your gauge readings. It could be your gauge is reading off because of a poor ground or B+ supply, and when you rev the motor, vs. coasting, the reading is following the charging voltage instead of the actual temperature. Have someone rev the motor and see if the gauge moves over a period of like 30 seconds.

Or maybe someone replaced the sender in your car with the wrong one and your readings are completely off.

Next you need to pressure test your cooling system. It needs to hold pressure, the needle on the gauge not move for 30 minutes or more. If not you've got a leaking radiator, dripping hose, heater core failure, whatever. You need to find it, fix it, re-test, and THEN put on a new Stant radiator cap.

I don't know how fast you're coasting down those hills, and what the outside air temp was, so it's rather hard to reach a conclusion, but, idling, doing at least 40 mph, outside temp of 70 or below you really shouldn't need fans running to stay below thermostat temperature. Do you have your fans on a switch, a thermoswitch or what?

220 degrees indicates a problem, MOST LIKELY loss of pressure. Without pressure coolant will boil inside the head, and bubbles of air/steam won't carry heat away from the head like liquid coolant does. It may be when the water pump is running @ higher rpm's it's able to keep enough coolant in the head to keep it from REALLY overheating.

I hate to even mention it but there's also the possibility of a head gasket leak. Let's rule out all the cooling system snafus first. Also, I assume you've got a 50/50 coolant mix.

With a working cooling system, a 180 thermostat, my vehicle NEVER goes above 190 even doing 0 mph in traffic on a hot day. Most days after it warms up ONCE it sits right at 180, just like it's supposed to. Reading 220 indicates something not moving heat, from the head to the radiator, or the radiator to the surrounding environment. Or a gauge problem. Let's get it working right so you never have to stare at the temp gauge while enjoying this fun car.

Cheers!

 

Perhaps hooking up a switch to run the fans while coasting would help?

 

Next to the thermostat housing where the water heads to the thermostat would be a good spot.

 

You built the car upside down backwards.

 

Overheating issues should happen regardless if the vehicle is going uphill, downhill or sideways. So what changes when coasting? What is happening different about coming down a long hill?
The vacuum is at its highest point.
So is the vacuum advance timing.

 

Carb is leaned out, being cold it’s a little leaner.
Mechanical advance may be retarded depending on the rpm and springs being used.

is it popping on decel?

 

Lots of really good suggestions. I will test the actual temperature at the thermostat housing on my next run, but I tend to think the issue is real as opposed to low voltage, though I’m not willing to discount that possibility out of hand.

i run an MSD.distributor with vacuum can. I need to verify but it was around 36 degrees all in plus vacuum advance. No popping on decel per se, but I do have sidepipes that pop and cough constantly so it’s kind of hard to tell for sure.

Speed while coasting downhill was 70-75. Moving right along.

Thanks everyone for your ideas. I’m going to do a little sleuthing and report back.

 

Vacuum advance follows load. No load downhill.

If sender is wrong, it is wrong consistent. Higher is higher.

While coasting were the RPM'S up?

If so, alternator and water flow is consistent.

Air pocket tripping on the sender? Maybe.

It's inverted from what you would expect.

 

Where is the water temperature sender located? If it is in the head and you were coasting down hill with no load the wind over the engine could have been cooling the actual thermostat housing faster then the engine heat from the cylinder heads heated the thermostat causing it to close. With no water flow to the thermostat since it was closed and no load very little heat is created.This causes the thermostat to stay closed but the water in the heads starts to heat up and read hot. Once you get back on the throttle you have enough water flow from higher rpm and higher heat output to cause the thermostat to open again. One way to eliminate this condition is to drI'll a 3/16 hole in the thermostat so some water always flows through it and that will eliminate the weird cycling that you saw at low load conditions, cool outside temperature conditions.

 

Almost sure that I drilled hole(s) in the thermostat when I installed it but that was at least five years ago. One of the reasons that I believe this is real is 5an once I got back on thethrottle, it took a while to cool back down, as in minutes. If it was an air pocket or voltage anomaly, I would expect it to return to baseline almost instantly.

 

Oh and the temp sender is in the intake but not the thermostat housing.

 

Change the stat. Cheap enough to try.

 

I’ll certainly do that but I now suspect that I have an issue that’s been hidden by the stat all along.

 

I've thought a bit more about your overheating. I've come up with some possibilities.

One is a tiny head gasket leak, blowing a little exhaust gas into your coolant. Your engine temp shouldn't change instantly "when you get on the throttle." Up hills, down hills, coasting or floored my coolant temp barely moves. That's the thermostat's job. Yours MIGHT be stuck. Drop in a new Stant,

There are test kits for exhaust in the coolant, you can also top the radiator to the neck, start the engine with the cap off, and once the engine warms up and thermostat opens coolant will start spurting out of the radiator every time that piston comes up if that's the problem.

The test kits are tricky to use; the test fluid goes stale, and the guys at NAPA will often sell you stale fluid. Your breath, which contains C02, should cause the test fluid to change color. Test the test fluid before testing your coolant.

Second is a worn down impeller on your water pump. Unless it's got sufficient RPMs it's maybe not moving enough coolant through a partially blocked radiator.

Third, and I think most likely is your radiator core is partially blocked with calcium, minerals.

Why? Because you and prior owners seem to have been dealing with overheating issues for a long time. Usually that means someone was topping it up with tap water, which gradually clogs the radiator. Drop by a radiator shop and have them take a peek. They may be able to "boil it out" with acid versus having to replace it.

I know you started this thread talking about coasting downhill but, what does it do if you simply start it up and let it idle in the driveway for 30 minutes? Does it overheat just like coasting?

 

The last item you suggest is particularly interesting. I won’t get out for a drive until Tuesday but at the conclusion of the run, I’ll leave it idling in the driveway and see what it does.