97 Temp Gauge rising and falling.

[Jack's GTS]

I had no temp gauge issues until I replaced my cooling fan assembly (I believe it is unrelated) a month ago. The gauge will go to 250 and then go back down to 195 even at near warm up temps. I don't think the engine is overheating. Any suggestion?

Thanks

[thetalonguy]

Sounds like a bad temperature sending unit

[Jack's GTS]

My thoughts as well. I'll give it a try. Cooling fan would kick in at warm up sometimes.

Sounds like a bad temperature sending unit

[dave6666]

What is the coolant level in the surge tank?

What is the coolant level in the overflow tank? The overflow tanks typically get stained so what you think you see as a level is often not.

What is the condition of the fan connector contacts?

This does not sound like a sending unit problem.

[dave6666]

It sounds like you have a large pocket of air in the engine. Upon engine heating, thermostat closed, the air gets very hot before the stat opens. Hot air does not work the stat like hot coolant does, but does read on the gauge. Once the 250* air finally causes the stat to open, fluid flows and you appear to at that point have normal cooling.

If this is the case, it is caused by cooling system neglect. You need to fill the system.

Burping the system is a joke. It is never needed. You just have to fill and maintain it properly and it will purge itself of air. It is designed to work that way contrary to people that preach otherwise. I've done way too many cars without it to believe otherwise.

If you need instructions on cooling system maintenance let me know. There are a few tricks, and several myths.

[Jack's GTS]

Thanks Dave. I will follow your advise and check the levels before replacing the sender. Your diagnosis seems spot on. The gauge would go to 250 and then back down.

It sounds like you have a large pocket of air in the engine. Upon engine heating, thermostat closed, the air gets very hot before the stat opens. Hot air does not work the stat like hot coolant does, but does read on the gauge. Once the 250* air finally causes the stat to open, fluid flows and you appear to at that point have normal cooling.

If this is the case, it is caused by cooling system neglect. You need to fill the system.

Burping the system is a joke. It is never needed. You just have to fill and maintain it properly and it will purge itself of air. It is designed to work that way contrary to people that preach otherwise. I've done way too many cars without it to believe otherwise.

If you need instructions on cooling system maintenance let me know. There are a few tricks, and several myths.

[Jack's GTS]

The fan would kick on at warm up and then kick on when gage went above 200. Should I fill the coolant slightly until it pukes out?

E=dave6666;146763]What is the coolant level in the surge tank?

What is the coolant level in the overflow tank? The overflow tanks typically get stained so what you think you see as a level is often not.

What is the condition of the fan connector contacts?

This does not sound like a sending unit problem.[/QUOTE]

[dave6666]

With the engine cold fill the surge tank brim full. That is the tank under the hood with the radiator cap on it.

The rubber line right below that cap is the line that goes down to the overflow tank. Unless you are wanting to pull the bumper cover off to gain access to the overflow tank, that hose is how you fill the overflow tank. And that tank being empty is often the root cause of cooling system problems. OR, a leaky connection where that hose connects below the radiator cap on the surge tank.

The cold - hot - cold cycle of the cooling system is as follows:

* System heats up and pressure builds in the surge tank.

* The radiator cap allows coolant to escape to the overflow tank as the pressure rating of the surge tank cap is exceeded.

* Upon cool down the cooling system will begin to go under negative pressure - vacuum - and the surge tank cap will open to allow the coolant in the overflow tank to return to the system.

Here is what goes wrong with that. Let's say the the overflow tank is allowed to dry up. It is vented to atmosphere, and the water component in the coolant will evaporate. So the engine heats up and pukes coolant to the overflow tank. During driving and while sitting and cooling, the coolant level in the overflow bottle slowly goes down over time due to evaporation. When there is no longer enough coolant to suck back in the full amount that was displaced during heat up, it will suck in air during cool down. Multiply that times days / months / years and you get a huge air pocket in the system.

Your temps will also go up as the pressure in the system reduces the boiling point of the coolant. You are not running compressed coolant as much as you are running compressed air then.

Another issue is the connection of that little hose below the cap on the surge tank. The hose that goes to the overflow tank. If that connection is loose or if the hose is rotted, it will guide the puking coolant to the overflow tank like it's supposed to, but upon cool down will let air back in under vacuum. Check that connection and put a corbin clamp or cable tie on it to snug that connection.

Back to your car, after filling the surge tank you will still have air in the cooling system. Normal drive cycles of cold - hot - cold will work that air out. This is done by having everything in good working order with the overflow tank; the hose connection and the level of coolant in the tank.

Filling the overflow tank is as I mentioned, with that small hose. You check the level with a flashlight through the wheel well plug. I would try to determine if your overflow bottle has a deceiving stain in it by adding 16 ounces of coolant and see if you can see the level. If not, you will have to wing it on the level. If you add to much it will spill out the top no harm no foul. But while you are continuing to purge air from your system when you drive it, that level will progressively drop over time. You'll just have to figure out a way to blindly maintain it then, as I already mentioned how critical it is having coolant in the overflow bottle always.

Here is how you fill the overflow bottle:



Here is where you check the level in it:









Here is what the coolant bottle looks like behind the bumper cover:

[Jack's GTS]

Dave. Thanks! Great detail and explanation. I'll work on this tonight and keep you posted. When are you opening up a shop in NJ?

[RTTTTed]

Good work Dave.

[Jack's GTS]

Dave was spot on with his assessment. It took 3 cold to hot to cold sequences after adding coolant to the pressure tank and overflow bottle to stabilize the gage. Can't thank Dave enough.

[dave6666]

Just check that hidden bottle in the fascia at least once a season. And what evaporates from that vented open to air bottle would be the water component of the coolant but not the glycol component. Meaning, you can top it off with just water. Although adding premixed coolant won't hurt anything either.

[Edward 96GTS]

if fascia bottle is empty, should it be filled to the top? or half way?

[Bugman Jeff]

There are marks molded into it that can be seen through a slot in the wheel well, they just get hard to read over time. It should be about half full. If you over fill it, the only thing that will happen is you may dump some excess coolant out the overflow once it gets hot.