Is your Spal fan(s) on a thermoswitch or just a toggle switch on the dash.

Is there any chance you have them running the wrong direction? It's a simple mistake, and... you don't need to tell anyone, LOL!!!

 

Two thermostats, one for each fan. No, they run the right direction but I’ve certainly done far dumber mistakes

i have not not noticed the fans running this year though. As I said, it has been quite cool so didn’t think anything of it. When driving, I don’t notice the fans come on, only when idling at lights. Did I mention the sidepipes were loud?

 

Start with the simple stuff, and work your way to more complicated.

make sure the distributor is locked down. Check movement of the mechanical advance and springs.

I have side side pipes too, no popping ever, if your popping your lean.

Ensure vac advance diaphragm is functioning, if not then you would be significantly retarded off the throttle.

and as suggested is it getting hot idling?

 

I have found most aftermarket fans very under-engineered, in terms of the wiring and relays. Wires that aren't fat enough (like 14AWG) and relays that aren't rated for the current a motor pulls when it starts. A 10A relay won't cut it -- not for long.

Often you can double up on both wiring and relays and solve the problem. In the mean time I see a lot of aftermarket stuff that runs for 6 months then the relay contacts are fried.

I wouldn't wire a fan, fan relay, with anything less than 10AWG wire myself, and give it an equally fat ground to the nearest section of the frame. Use Bosch 30A relays.

Fan doesn't do squat if you're rolling more than 35mph though, this is all just an aside to your actual problem.

 

Why? The thermostat doesn't care if you are going uphill or downhill. O.P. overheating issue is downhill only. Temp is fine the rest of the time. This indicates that the cooling system is not the issue but rather something else that happens with high vacuum, downshifting, who knows?

 

I ttend to agree. I’m not seeing this as a radiator, fan, thermostat or blown head gasket problem. At least not as it’s being described to us.

 

We don't know if he's coasting or engine braking from his description.

I suspect the problem has to do with engine speed --- low water pump output --- as opposed to timing, rich/lean, etc. If it overheats sitting in the driveway idling we have our answer.

I wish we could place bets on diagnosis.

 

I cannot wrap my head around an engine creating less heat under load uphill, vs more heat generated coasting with no load.

 

It has to be creating more heat uphill, under load. That's thermodynamics, and it's the LAW.

It's simply not getting RID of heat as well coasting downhill, engine idling.

Which is why I think it's the water pump (impeller, slipping belt, whatever) or a clogged radiator that just ain't moving enough coolant.

But... fixing cooling systems ya gotta first rule out the stupid stuff like not holding pressure, gauge issues, low coolant level, etc. Low coolant level is more common than most expect and it's often due to a cracked overflow hose or tank. Spits a little out on every hot shutdown and sucks air back in instead of coolant as it cools down, overnight.

 

1)Radiator cap/change of altitude fits. Bad cap might let that happen. It's not holding pressure/whackey. You can buy a new cap for $8.00 and try it.

2)Car sat a lot. Going uphill stirred up the scale, and clogged something on the coast down.

 

How about the signal wire to the temp gauge being faulty. On level or uphill roads, the signal wire maintains contact. When headed downhill, the wire separates (gravity, I guess) and the gauge goes toward full scale HOT. Level ground puts the wire back in contact and it reads normal, again.

Sounds weird...but possible. Easy to test. Turn ignition ON (engine OFF); someone watches temp gauge in car and someone else tugs on temp sender wire to see what happens. Gauge should read actual engine coolant temperature. If you see it go HIGH, there is a break in that wire somewhere inside the insulation.

 

I had a car here that would stall doing a LEFT U turn. It was a bad wire in the tach drive dizzy. It would ground out on sharp left turns, not right.

It took forever to figure wtf?

When you have a car that Lord only knows how many paws have been on it, till you go over everything, ya just don't know. Then you have the defective crap out there, argh.

 

OP could very easily have a gauge wiring issue.

I'd diagnose a sender connection problem as you suggest by running DETOUR wiring in parallel with the original one, then go for a drive. 10 minutes!

 

120 - 140 tells me that the T-stat is not working. Do you have an auto tranny that goes into free wheel coasting down a hill in off throttle where your rpm drops to about idle while coasting?

I would put in a new t-stat and completely fill your system. Vettes lower than the motor radiator is hard to fully fill even doing the burping of the upper radiator hose. I lived in Vegas and like you needed all the cooling I could get. The hottest driving I ever experienced was on I-15 going towards Barstow and it was 124 F. I went to this taller T-stat housing as my filler and I now can completely fill my cooling system. Summer I run 100% distilled water with royal purple rust inhibitor.



https://www.summitracing.com/nv/part...3420/overview/

https://www.summitracing.com/nv/part...1600/overview/

 

I have been considering adding a taller thermostat housing to my car. Thanks for the link.
Eddy

 

I've never had any trouble burping MOST cars. Some Nissans required lifting the front end AND using a tall funnel that attached to the filler neck.

My MO for burping MOST cars is to fill the radiator, start the engine (cap off), let it FULLY warm up. At some point the thermostat opens and the level in the radiator drops visibly. Top it back up, let it continue to run. You'll see various bubbles coming out and often at some point, as the coolant heats up, coolant begins pouring out the neck of the radiator. Put the cap back on.

You'll want your heater turned on while you do this to circulate through the heater core as well as the block.

THEN I fill the overflow tank up to the brim, and shut the engine down. As the engine cools down, it will suck in enough coolant out of the overflow tank to fill any hoses above the radiator neck, either the first or 2nd night.

The only mistake you can really make is rushing the process, not waiting till the t-stat has fully opened. My current car cools so well it may take 20 minutes or more sitting idling.

I solve a LOT of overheating issues by checking the overflow tank & hose for cracks. They let fluid leak out, but only after the engine is shut down and heat soaks, and pushes some coolant out in whatever parking lot you're in. Then, when the engine is cool again, it sucks air back in instead of coolant. So over time the engine ends up low on coolant, with resultant problems.

I like that t-stat housing extender!

 

That's my thoughts. Lots of cool coolant in the rad but there is low volume getting into block due to low pump speed. The cooler coolant is not displacing the block coolant fast enough threby raising the temp. Running way too cool otherwise.

 

Ok folks, a bit of an update.

Went on a run yesterday and simply put, couldn’t recreate what happened on the previous run. Didn’t have time to check the timing, but did verify that the temperature reading was at least ballpark. My gauge reads 100, then an unmarked line, then 210. Throughout the run, it wandered back and forth around the unmarked line on the gauge.

Here’s what I know:

- Belts and distributor were tight

- Air temp was at least 25 degrees higher than my last run, even above the snow line.

- Car ran VERY well. Put me back in the seat on a steep uphill grade at over highway speed. Yes it did warm up some.

- Thermostat definately bad. At one point, the hose read 140 degrees well after warn up.

- At least one of the spaals was running. Probably didn’t get warm enough for both.

- I could NOT reproduce the heating up on downhill coasting.

- I did note wandering of temperature and gas gauges. Gradual, not rapid.

How would I verify the health/stability of my battery and charging system?

 

H-m-m-m-m-m-m. "wandering of gauges". Whole new ballgame now. What was the Oil Press doing? This might be a voltage issue and not a cooling issue.

 

Oil pressure was very stable as the gauge is currently disconnected Converting to electric.

Starting to suspect 5he same. Fluctuations weren’t wild by any means, but I thought I had a quarter to an eighth of a tank more than when I looked the second time. Ammeter never worked.