[scottydl]

My '69 Executive (stock 400ci 2bbl) only has the original TEMP dummy light to indicate engine temperature, and last summer it started popping on when I was cruising through town at slow speeds. The thermostat was replaced and housing re-sealed, which didn't seem to change anything before she got put up for the winter.

Here are the current issues:
- Engine warm, after a drive and at idle: temp measures 220-224 (using an infrared temp gun aimed at the thermostat housing).
- Letting the engine idle for 10 mins or so, TEMP light pops on and temp measures at 238.
- Heat does not work... clogged heater core? Related to high engine temp?
- Vent selection doesn't work... I'm getting weak airflow from all vents, regardless of HVAC mode or fan speed. Blend door issue? Related to high engine temp?

Coolant level in the radiator looks okay... spits a little out the overflow tube at times when the engine is hot (makes sense) but there doesn't seem to be a major leak otherwise. These problems seem associated but I'm not sure which to tackle first.

[steve25]

Coolant flow thru the heater core should not be considered part of having sufficient engine cooling.

It sounds to me like you need to run DuPont brand two part flush thru the cooling system and I would do this with the heater core bypassed and deal with that issue seperate.

[ZeGermanHam]

Yes, your engine temp is too high. There's probably a 1% chance that any sort of parts-store flush product will help in any meaningful way. What is the state of the rest of your cooling system?

After I overhauled the cooling system in my '66, it went from constantly overheating in slow moving traffic to being able to idle for as long as you want and never getting above 190F. I replaced my old clogged radiator, installed a fan shroud and HD fan clutch, high-flow water pump w/clearanced divider plate, and a smaller A/C water pump pulley. No more overheating.

[Stuart]

I used to have a 1965 Grand Prix. I bought it from the proverbial little old lady who only drove it to church on Sundays. Initially it was having overheating problems; I took the radiator to a radiator shop to get it rodded out, and from then on it was fine. I assume it had just gotten sludged up from lack of use by the previous owner. The trick of course will be finding a radiator shop that can do the work these days.

[scottydl]

It has an aftermarket radiator that I installed myself probably 10-15 years ago (don’t remember the brand) and the car is driven well under 1000 miles per year on average. Could some sludge be formed & trapped in there as a result? There’s a decent radiator shop in town I could have check it out if that’s what I need to do.

Water pump I believe is original (but was tested a couple years ago) as well as the fan… not sure about the clutch. Factory fan shroud securely in place. DuPont flush kit was mentioned above… did you mean Prestone? I used one of those kits (bypass inlet still installed) many years ago, but it sounds like that may not accomplish a whole lot. The car has A/C technically, but compressor is seized and there’s no belt anymore, so that’s currently a non-issue.

[694.1]

The lack of heat from the core could indicate a bad head gasket.

[Sirrotica]

If you're getting water constantly out of the overflow, there is a good possibility that you're getting compression gas into the cooling system due to a leaking head gasket, cracked head, or block. Especially if the engine was assembled back in the 60s and has never been taken apart for any reason. Head gaskets do deteriorate over time, and will start getting a compression gases into the cooling system via the leak.

If you remove the radiator cap and look down into the radiator with the engine idling you may see tell tale bubbles that continually come out as the engine is running. Another way to test for compression gasses is put the end of the overflow hose in a cup of water and look for bubbles in the cup as the engine idles.

If the leak is minuscule you may need a tester to determine if there is a leak. Link to using a combustion leak tester:

https://www.youtube.com/watch?v=vYz4Kl1IcSw

I would rule out that possibility first, then start looking at the radiator, fan clutch, and other parts that also could have deteriorated from old age. As has been mentioned, old radiators do scale internally over decades effectively insulating the cooling system from transferring heat from the radiator to the airstream.

Typically a radiator will lose 30 degrees from the top to the bottom if it is able to transfer heat properly. With your heat gun you should be able to check that for a problem. That's how I would troubleshoot the problem, hopefully this will help you find the cause of your overheating problem.

[bluebandit]

Check to make sure your vacuum advance is working properly, and the advance weights are not seized. Not enough timing can cause it to run hot.

[scottydl]

Great tips! I’ll try to do some bubble checking tomorrow from the radiator. That head gasket talk did remind me to mention, the engine has been apart a couple times from the top down. Once for intake manifold, another time for actual head gaskets (I think?) if those are thin metal in these engines. I remember the mechanic (awesome guy who had his own shop and loved Pontiacs) explaining that. This was all probably mid-late 2000s, so not exactly recent but not THAT long ago considering the vehicle age.

[59safaricat]

No matter how many miles you put on your vehicle, the coolant still needs to be changed every 3 years. That includes 3 years of sitting.

[pfilean]

Sounds like your hot light is good. They are usually set to trip at 240 degrees.

If you have trouble getting het out of any setting, then I would start with replacing all vacuum lines for heater. Also, there is often a vacuum switch at the control levers which may leak and is virtually impossible to repair successfully and made from unobtanium.

The suggestions for flash the system is always a good start but also I would check to see if someone replaced the pump with one of those stamped impeller replacements. If so, throw it as far as you can and get a cast impeller model.

[PunchT37]

That fan should be roaring at 238 degrees with a little off idle speed. Check that 1`st.

[70GS455]

If you have a 210 deg thermostat then everything is fine. If you have a t stat rated at 160 then there is a cooling capacity issue

Sent from my SM-T817V using Tapatalk

[Formulabruce]

Nobody needs a 210 t stat these days.
I have seen " DEXCOOL" do this to cars many times..
Get a serious flush. May pull the block plugs too, then refuse.

[78w72]

Originally Posted by 59safaricat

No matter how many miles you put on your vehicle, the coolant still needs to be changed every 3 years. That includes 3 years of sitting.

curious where you got 3 years from... especially when sitting? is that what some mechanic thats trying to sell you a service/product said? or like the outdated 3000 miles oil change?

way too many variables to claim all coolant needs to be changed at 3 years regardless of miles, certain brands of coolants state 5 year change intervals, some are even as high as 10. most modern vehicles suggest 10 year or 100k miles for a coolant change, in classics 5 years in the norm.

https://www.walmart.com/ip/Super-Tec...e&athbdg=L1600

https://www.walmart.com/ip/Prestone-...E&gclsrc=aw.ds

[steve25]

The problem that still results even today is that many people buy un-cut coolant ( non 50/ 50 ) and then they cut it with tap water with all its minerals in it and that's the start even with a 100 % new parts in your cooling system of things getting clogged off.

[freebird69]

I had cooling issues with my 69 400 and it was several things that added up to make a difference. I replaced the radiator, replaced the water pump (stamped impeller to cast), added lower valance panel to block hot air, and added a foam gasket to fill the gap around the radiator to block hot air. I also tested a bunch of fans to find the one that worked best (ended up being a flex fan). Each one of these things decreased the temp by a few degrees. If it were me I would add a temp gauge so that you can see the temp while you drive.

[Schurkey]

Originally Posted by scottydl

- Heat does not work... clogged heater core? Related to high engine temp?

Is there a heater-control valve that doesn't open? Are the heater hoses BOTH hot?

Originally Posted by scottydl

- Vent selection doesn't work... I'm getting weak airflow from all vents, regardless of HVAC mode or fan speed. Blend door issue? Related to high engine temp?

WILD GUESS with no evidence: Vacuum problem to the air door motors, also prevents the heater control valve from opening to allow coolant flow though the heater core.

But I don't know the vacuum system of your vehicle. I could be really wrong.

Generally, the air doors are set up so that if there's no vacuum, you have strong air flow out the defrosters.

One wonders if you have a plugged A/C evaporator, that's blocking air flow from the fan. Common for debris to get thrown off the fan blades into the evaporator--pine needles, leaves, etc.

Originally Posted by scottydl

Coolant level in the radiator looks okay... spits a little out the overflow tube at times

Consider installing an overflow bottle and a rad cap suitable for the closed cooling system...

...AFTER you figure out why it's overheating.

[wh33lman]

The valve at the center of the fan clutch, which gets turned by the coil expanding as the temp rises, are prone to seizing.
Spray a little WD40 on it and use a slot screwdriver to manually loosen it up.

I even had a brand new one that was stuck right out of the box.

Once I figured that out...after flushing rad, back flushing engine, replacing clutch, checking timing, swapping distributer etc... I replaced the original and dropped 50 degrees immediately.
That valve controls the flow of a viscous fluid that engages the fan more as the temp rises...sort of similar to a torque converter.