Overheating Only Under Load

[DadAndLadZ]

Hey all: 

We have our 260Z idling reasonably well now, and the temp gauge shows it staying right at normal operating temperature as long as we let it idle — but the second I shift into first gear and let the clutch out, the temp immediately climbs to 250°+. Here is a little video demonstrating the issue:

We have some coolant system flush stuff in there right now and plan to drain it soon, as it’s been in there about three days now and we have run the engine a little each day. But before I put any more coolant in there I want to hear some ideas about what might be causing this. The car never acts like it is actively getting too hot, but the gauge says otherwise. Your two cents is much appreciated.

Thanks!

MC and the Boys

[madkaw]

Bizarre for sure . Laser temp gun to verify readings .  Not sure what to suggest other than replacing the sender sensor to start 

[DadAndLadZ]

4 minutes ago, madkaw said:

Bizarre for sure . Laser temp gun to verify readings .  Not sure what to suggest other than replacing the sender sensor to start 

Yeah I am gonna swap the sender unit out, I thought maybe there was air in there but not sure that would cause this particular behavior?  

[SteveJ]

An air pocket would reduce heat transfer, and the reading would go down.

The sender works by decreasing resistance in proportion to the increase in temperature.

Do what Steve @madkaw said.

[Dave WM]

wrong thread.

Edited August 13, 2022 by Dave WM

[Zed Head]

What happens if you just raise the RPM while sitting in neutral?  Can you get the needle to go up and down without a load?

It kind of looks like what you might expect if the radiator was not actually removing much heat.  Is your heater core intact and connected or has somebody "bypassed" it?  The bypass allows coolant to pass through the block and back to the pump without passing through the radiator.  Effectively reduces pump capacity.

Another thought is that the water pump impeller blades are shot and not moving much coolant.

[DadAndLadZ]

7 hours ago, Zed Head said:

What happens if you just raise the RPM while sitting in neutral?  Can you get the needle to go up and down without a load?

In neutral if I rev it up, the temp goes back down to normal, even after the gauge shows it going into the red. You can see this toward the end of the video.

[Captain Obvious]

I've seen plenty of intermittent temp gauge senders, but they're usually random and not repeatable like yours seems to be.

First thing I would do is be double dog sure that the coolant system is full. And next is that I would replace the sender unit. It's easy and cheap, so I would just put a new one in there and see what happens.

[DadAndLadZ]

1 minute ago, Captain Obvious said:

First thing I would do is be double dog sure that the coolant system is full. And next is that I would replace the sender unit. It's easy and cheap, so I would just put a new one in there and see what happens.

Yeah planning to replace the sender this week. Radiator is full but again I am unsure if there might be pockets of air lurking somewhere in there… Trying to avoid buying specialty tools like the burping funnel kit until we absolutely have to because our budget is tight but will if it looks like that’s the problem. 

[Captain Obvious]

I don't think you need any fancy burping tools for these cars. Just run it, let it cool, and top it off a couple times. That's all you should need to do.

The 280's used a recovery tank, but I don't think the 260's did. They were a dump on the ground system. So there would normally always be some air in the system. Hopefully just one pocket corralled at the top of the radiator, but some air.

[Zed Head]

13 hours ago, DadAndLadZ said:

In neutral if I rev it up, the temp goes back down to normal, even after the gauge shows it going into the red. You can see this toward the end of the video.

That was after the driving.  I'm wondering about repeatability under no-load conditions.  Just another clue.  You don't have many right now.

[DadAndLadZ]

Just now, Zed Head said:

That was after the driving.  I'm wondering about repeatability under no-load conditions.  Just another clue.  You don't have many right now.

It doesn't overheat until you drive it.