Fluctuating coolant temperature

[burlsube]

I am trying to track down the cause of my fluctuating temperature. Over the past few months, I have been dealing with a lot of heating issues. The PHH went, the radiator split and a few other misc hoses were replaced.
When I first start the 80, the truck warms up to about 140/150 according to the Scanguage, it will then go 160 and then back down. It will continue go up and down every few minutes. It never goes above 186. If I park and start it up again, it will not have that behavior and will just heat up to 184/186 and stay there.
I'm not burning any coolant and the coolant is at the correct level.

I'm not sure if Im dealing with an air bubble, a sensor, or a combination of both.

Any advise on where I should start looking?

 

[mct75]

Any advise on where I should start looking?

If it never goes above 186 just look at the road and keep driving.

 

[Irish Reiver]

186f is pretty much where it should be. Maybe you are just seeing the thermostat opening and closing.

 

[Marvelicious]

My Torque readout shows mine eventually evens out at 186, but it takes some time to get there. You really don't want a modern gas engine to run much colder than that. Does it continue to do the yo-yo thing if you keep driving for 10-15 minutes at highway speed?

 

[burlsube]

Ill have to try on the highway tomorrow. This is usually around town so 15 mins is usually the longest trip.

 

[Marvelicious]

Yeah, 15 minutes of town driving might not be up to temp.

My other thought is that you might have some air somewhere in the system. Whenever I do cooling system work on anything, I make sure to get it up to temp, then drive up the steepest hill I can find: let all the bubbles find their way to the radiator where they can purge their way into the overflow. Some vehicles are worse about this than others, and I haven't been into the cooling system on my 80 yet, but it's a thought. I used to have a pickup with a body lift which put the heater core higher than the radiator and that made this step absolutely necessary, so it became part of my process.

 

[robnicko]

How is the radiator cap condition / age? Does sound like an air pocket in there
Do you see a vast fluctuation in the reservoir level?
WIth mine if the reservoir level is at the full mark cold will rise about 1inch once warm and then go back to full when cold

 

[Kernal]

Thermostats can go bad, what's the age of the Tsat?
Do you hear the fan clutch engage when the engine is first started and then quiet down after awhile??

 

[jonheld]

Any advise on where I should start looking?

There are 100s of threads regarding the cooling system on this forum. I would start there.
It is normal for coolant temperature to fluctuate depending on ambient temps and driving conditions. On my 97 LX450, 186F is pretty standard at this time of year with a modified blue hub fan clutch and a cooling system that gets looked at and maintained periodically. In the summer months, 190F/192F is the norm for me.
The temps you quoted 140/150/160 do not indicate a fully warmed up engine.

Some things to consider:
The cooling system self bleeds. Any air trapped in the system will work its way to the top of the radiator after a few running cycles.

With a cold non-running engine, the radiator should be filled up to the small hole in the filler neck for the overflow bottle. The overflow bottle should be filled to the "FULL" line.
The radiator cap is designed to bleed off excess coolant into the overflow bottle as the system warms and pressurizes above 13 PSI (approx). As the system cools and the pressure goes negative, the radiator cap allows coolant to be drawn back in from the overflow bottle to keep the radiator full.
The level in the overflow bottle should rise and fall along with coolant temperature.
This will ONLY work properly if the system starts at full capacity with no air to compress, the radiator cap (only OEM please) is functional, and there are no leaks or faulty hoses in the system.

The OEM thermostat begins to open at 180F and is fully open at 203F (82C-95C) according to the FSM. Personally, I think if you're anywhere within this range, you'll be just fine.