Temp gauge fluctuates a little

I have a 1991 ranger 2.9liter xlt 4x4with 77k original miles. I blew a headgasket and replaced it. Also flushed the radiator, replaced 2 temp sensors, thermostat, tps, and iac. The temp gauge was fluctuating up and down and getting too hot. I checked the wires on sending unit and they work. I replaced the thermostat again and its better, but still the gauge goes up and down a little. Is this normal?

It is if you still have air in the system.
Did you turn the heater on when you bled the system?
Did you test the radiator cap?

Now the less optimistic thoughts:
Did you run a leakdown test to ensure that the cylinders were sealed?
Did you check the head for flatness (warpage) when you did the head gasket?
id you ensure that the surface of the block was flat and free of eroded passages that might allow passage after the head gasket replacement? Did you clean it completely?

Heads were shaves. Leak down test was good. New radiator cap. Heat was on when burping the radiator. Block was good and cleaned properly. Maybe i need to burp it again.

There’s probably air trapped in the cooling system.

Try this.

Get the engine up to operating temperature.

While the engine is idling, loosen the upper radiator hose clamp and slip a small flat bladed screwdriver between the upper radiator hose and the hose neck on the radiator.

Allow the engine to idle until all the air is purged out and nothing but coolant comes out.

Remove the screwdriver and tighten the hose clamp.


Sounds like you did a thorough job. I tip my hat to you.
Perhaps if another burp doesn’t work, a good radiator testing/mapping/boiling is in order.

One other possibility comes to mind. Radiator and heater hoses are a sheath over a rubber lining. Sometimes when an engine is overheated (or even as a root cause) the inner liner can collapse when hot and block the coolant. Heater hose and radiator hose is cheap. It might be prudent to change it out.

Once the coolant reaches a stable temperature on all my vehicles it usually stays there pretty steady. But there are exceptions. Long uphills might cause the coolant temp to slowly rise, and if it is really cold outside it might drop , especially when going faster.

My guess, air remains in the cooling system. Do what Tester says above, but before you start drive the front wheels up on ramps. That will improve your odds.

If that doesn’t fix it, other ideas …

  • pressure test the cooling system including the radiator cap
  • water pump is faulty or debris is blocking the inlet or outlet
  • make sure the radiator fan is spinning when it should and at the correct rpm

I don’t know if it is relevant but I disconnected the heater in my '97 ranger (we live in Texas and really don’t need a heater) by simply plugging the hoses. This caused the gauge to go up and down, even when driving. I replaced the plugs with a loop which allowed the water to return and it fixed the problem.

It is relevant. It suggests that a plugged heater hose or heater core can cause engine temp to fluctuate.

Until the problem is solved, information like this could be the key to the solution.

I agree, it was the fluctuating temp that caught my eye. An obstruction in
the heater line might do the same thing.

I’m surprised that the lack of flow through the heater core could affect the engine’s cooling system. I suppose if the engine cooling system performance was marginal to begin with, the heater core could act as a helper radiator to keep the engine cooled. Maybe that’s the explanation. But that would only work in winter when the heat mode is selected.

How does the Ranger regulate the inside temperature then when you don’t want any heat? On all the vehicles I’ve owned it’s done by blocking off the coolant flow to the heater. Perhaps on the Ranger the heat selection done with the vent doors, and a full flow of coolant always flows through the heater core even when no heat is selected. I guess the cooling system currents could be affected throughout if the flow through the heater core was block off, as the system wasn’t designed or tested for that.

My 2000 Ranger has a heater shut off valve on the coolant hose but there is a 3/8" bypass hose that makes virtually the same connections.The bypass is very convenient for burping the engine.

It was very sensitive to outside temperature. When it was warm to hot
(80-100F) outside the gauge would register as you would expect but, when it
was cooler (60F or so) even driving down the street the gauge would go from
normal to cool and then back up. As I sat at a red light it would change
and then go back up. When I was at highway speed it would drop and go back
to normal. It did not cycle up-to-down-to-up with any regularity. It
seemed to come out of the blue but was fairly frequent on any trip. It was
always multiple cycles.

I asked a mechanic about this and he shook his head but thought it might
have something to do with the flow of the water to the heater coils. I put
in the loop which bypassed the heater coils (instead of just stopping it
going to the coils at all) and just returned the water to the radiator and
everything has worked well since then. To me, a couple feet of insulated
hose should have made no difference but it seems to have made all the
difference. The other interesting point is how sensitive it was to outside

When it all settled out, I decided not to look this gift horse too closely
in the mouth since I really never use the heater anyway.

It’s a great diagnostic test, but might not work as a solution for the OP as they may live in a colder area and need the heater to work.

It may be that they blocked the path to the heater core and diverted the flow back to the engine, bypassing the heater core. That would be the normal way. The poster that offered the thought blocked the lines without diverting the flow.

I believe it’s wise not to disallow any possible explanation until it’s been tested. Too many times over the years I’ve been running tests and had the system(s) under test react in ways I hadn’t expected. Until the solution is found and confirmed Id’ welcome all ideas. That’s how I learn.

Thanks for all the help! Im going to start with the cheapest and
easiest…i will burp the radiator again. Second step will be the hoses.
Bypassing the heater core is a no-go. I live in kentucky…it was 1 degree
outside the other day!

LOL, Must be nice to be in a temperate zone… I’m in New Hampshire! :grimacing:

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You can keep that cold weather. I like it around 80 degrees

What ever happened to global warming it was down to 7 here 2 night’s ago.

I wish mine had one, but sadly,…it doesn’t. I have a steep grade I can park it on, so hopefully I can burp it there and fix it.

I thought these wild temp swings were evidence of global warming.