R16 cooling issues: help me with a gut check

[GeoffM]

I've always fought overheating issues over the years, and Ive found that if I'm festidius with purging air from the system (jacking up front end, using vacuum on the overflow fill) overheating, reservoir overflows at shutdown etc seem to go away. I've replaced both caps, thermostat and also did a flush a couple of years ago when I replaced the hoses and installed a Champion aluminium rad.

I've recently found that if I'm driving at low load, low RPM's...like say cruising at 30 mph on a flat road in 4th, my temp gauge creeps up to "uncomfortable" in 5 minutes or so. BUT, if I'm stop and go, cycling through gears and generally running the engine at higher RPM range in a more spirited manner, it doesn't overheat.

So without touching a bolt yet, I'm reckoning that my first step should be pulling the water pump. My logic is that maybe the impeller is corroded to the point that it's not as effective.

Am I just in denial that I need to swap out a head gasket?

[Habitat.pat]

Several questions. What year? Do you have a fan shroud installed?

I would look @ the water pump or a collapsed radiator hose. Verify that your radiator fan blade is positioned correctly, about half way out of the shroud. If you have an IR temperature gun, check temperatures around the system.

I had heating problems with overheating & low speeds w/factory radiator. Turning on the heater dropped the temperature. I did a restoration & installed new pump, thermostat & Champion radiator. Temperature is much better but it still kicks up.

[GeoffM]

Thank for your reply Pat. It's a '69 but all signs point to an early R16. I do have a factory shroud installed. 4-blade metal fan.

The hoses are new within the last 5 years or so.

[Nissanman]

As mentioned you need to verify that the engine is indeed getting "uncomfortably" hot by using an independent digital thermometer.
Electrical gauges are a wee bit fuzzy and until you have an accurate reading you are not armed with a reliable number.
From your post, it appears that you have done some well worthwhile upgrades to the cooling system
The factory shroud on my '65 was little more than a finger guard around the fan blades.
I designed a full shroud from sheet metal and it has been the single best improvement to the cooling system so far.
Our Summers are very hot and dry, 37degC atm, but with the new shroud my backup electric fan rarely kicks in during driving, usually comes on only when the engine is shut down due to heat soak.

20231230_155049.jpg

Inside view of sheet metal shroud.

20240905_121907.jpg

Fitted shroud.

20240905_121852.jpg

Now the fan can pull air through the entire radiator core area and it really has made a big difference

1500_rad_shroud.jpg

Standard shroud: pretty useless for cooling performance, great to protect the digits!

Unless you are experiencing a misfire or a smell of coolant at the exhaust, probably not a head gasket
You could perform a compression test and/or a leak down test to verify the integrity of the system, both cheap and easy to perform.

[itsa68]

The lower radiator air scoop should be in place to help air flow at low speeds.
Forth gear at 30 mph must be a low rpm (1800-2000?) so your water pump might not be circulating coolant enough?

[Bwk2000]

Can’t tell you how many times I’ve installed new thermostats in various vehicles that did not work correctly right out of the box or malfunctioned shortly thereafter. I would try replacing yours with a low-temperature thermostat and test it first using a heated pot of water and meat thermometer. You’d be surprised how many don’t open at the stated temperature or don’t open fully - A low cost check that’s worthwhile.

Unlikely it’s the head gasket if you haven’t already detected signs of water/oil mixing.
Also unlikely to be the water pump if it’s not leaking out of the weep hole or creating rattling/grinding noises or obvious signs of vibration or excessive play in the pulley.

*Also worth checking that your timing didn’t advance.

[Gregs672000]

Agreed on trying a different/new thermostat and checking that your gauge is reading correctly vs laser. Also, an overly rich fuel condition can increase temps sometimes, perhaps it's running richer at low rpms? I kinda doubt it has to do with a slower turning or inefficient water pump...?

[redroadster]

Pull the thermostat out / put back no thermostat ,does the water race thru?
I've found it best to have the car frt end quite higher than the back to purge air bubbles which kill the pumps ability to move the water thru could need a rod out too

[nismou20]

I highly doubt the Water pump impeller is at fault. They are metal and not like modern cars with plastic impellers. Are you sure the cooling system has been completely burped? Retarded timing can cause overheating. Good call on the thermostat from previous forum members, easy to test!

[GeoffM]

Thanks for all your replies! I mean, this is simple stuff but it's nice to get some hive mind sanity check once in a while.

I'll pull the thermostat and run it unfettered for a while to see if it does anything different....but logic dictates that, if the thermostat was malfunctioning, you would see a continual rise to overheating town, where as, what I'm seeing is more of the timing related overheating (low rpm under load)...even though timing is spot-on and advance seems to be functioning.

I guess I should double check the vacuum line to make sure it's not leaking during sustained higher vacuum as well.

[Bwk2000]

One other thing you might consider checking is your oil pressure at low rpms.

[Cahhh68]

The sender for the temp gauge on my 68 1600 is mounted less than 1” from the front of the exhaust manifold. It is my experience sometimes in a short idle period that the temp on the gauge increases but as soon as RPMs go up the temp goes to normal. And after a short shut down of the engine after running at normal temp, the temp gauge shows a higher temp than when I shut it down. Then when starting and driving or revving a little, the gauge quickly returns to normal temp. This leads me to suspect the closeness of the sender to the exhaust manifold is heating the head between the two points and the coolant adjacent to the sender, and resulting in a temp indication that does not represent the temp of coolant in the engine in general. Considering heat differences between rich or lean fuel settings, I have been told lean mixtures run hotter than rich mixtures. I replaced the water pump about 20 years ago, it may have never been replaced before then, and found sludge had built up in cavitation areas inside the old pump. Intentions are to flush the system multiple times on a 2 or 3 year schedule but these recent years seem like they are somehow related to inflation.

[Gregs672000]

Good info, that ^. I'm only familiar with U20s mostly...

[GeoffM]

The sender for the temp gauge on my 68 1600 is mounted less than 1” from the front of the exhaust manifold. It is my experience sometimes in a short idle period that the temp on the gauge increases but as soon as RPMs go up the temp goes to normal. And after a short shut down of the engine after running at normal temp, the temp gauge shows a higher temp than when I shut it down. Then when starting and driving or revving a little, the gauge quickly returns to normal temp. This leads me to suspect the closeness of the sender to the exhaust manifold is heating the head between the two points and the coolant adjacent to the sender, and resulting in a temp indication that does not represent the temp of coolant in the engine in general. Considering heat differences between rich or lean fuel settings, I have been told lean mixtures run hotter than rich mixtures. I replaced the water pump about 20 years ago, it may have never been replaced before then, and found sludge had built up in cavitation areas inside the old pump. Intentions are to flush the system multiple times on a 2 or 3 year schedule but these recent years seem like they are somehow related to inflation.

That's pretty much what I'm experiencing. If I recall, years ago I shot the engine with a thermal gun when the temp gauge was showing hot, and all temps seemed to be normal,not hot. I'll try that again and see if there is a difference between the area where the temp sender is and other spots.