Temp Gauge Fluctuates

narib2008 · Oct 10, 2009

Hey guys,
I noticed lately my temp gauge fluctuates while driving.Most of the time the needle stays at the N on the guage (NORMAL).However while sitting for a while the gauge seems to rise up to the R area.During driving it never seems to get hotter then O....Anyone have any ideas?I already changed the thermostat.No luck after that.

40oz · Oct 11, 2009

Is this a joke?

um, it's still in the "normal" range, right? I would think that it therefore is within the range of normal temps for an engine. But that's just me

wmburns · Oct 11, 2009

Normal temperature swings.

+1 on above.

Is the AC off? If so, the fan cycles on/off depending upon the engine temperature. Seams normal operation to me. As the temperature goes up, the fan turns on. Temperature comes down, fans turns off.

With the AC on and slower speed driving, the Fan runs all the time. This will tend to even out the normal temperature swings.

Further, what does the gauge do when driving steady on the highway? I bet is stays stable. That's because the fan does not run when the car's speed is above 45 MPH. So under steady driving on the highway, the thermastat is mainly controlling engine temperature.

Relax. Have a beer. Your ride is fine.

narib2008 · Oct 12, 2009

Sorry guys im just use to more precise gauges i guess.The temp gauge should honestly never move unless the gauge uses a very old rheostat technology.Most of the time my temp gauge only goes to "N" and if i let it sit it will rise to about the middle of "normal".I do not use the AC at all...Im honestly thinking it could be the coolant temp sensor/switch.I know there are two on the intake manifold.Can anyone tell me which one does what?Thank You

GDawg · Oct 12, 2009

I assume you are talking about a 2008 GT. If so, I don't know. But you really need to add in more info. Like location and car details. We can't read your mind.

Maybe I can pull up some engine diagrams for you but it would be better if you can take a pic of your coolant sensors and then we can tell you which one is for the gauge.

twogts4us · Oct 12, 2009

40oz said:
Is this a joke?

um, it's still in the "normal" range, right? I would think that it therefore is within the range of normal temps for an engine. But that's just me

If you noticed, the NORMAL section of the gauge is nearly the entire sweep. Typically mine will rise to 12 o'clock after it warms (about 10 mins +-) but never goes elsewhere, north or south. It's like clockwork. I'd question whether the fans are running, is the thermostat operating, sensor gone south. My first check would be the thermostat.

It's a very legit question...

GDawg · Oct 12, 2009

And very briefly I looked at the engine of a 2008 3v. Looks nothing like my engine. I'd have to look deeper to see. (The '99+ to '04, I think, only had one location) Mine has two on the front.

wmburns · Oct 13, 2009

clean radiator. Data log coolant temperature

OK, here is a new set of recommendations. Since the thermostat is new, let's assume it is OK. Next let's look at those things that can affect cooling. Good maintenance practices say that the radiator should be clean and the coolant is the correct concentration. Flush the radiator and install fresh coolant. Recommend no more than 50% concentration. More water means better cooling. However, get the concentration too low corrosion can be a problem. It is best to stay above 40% unless it gets very cold in your location.

Next, get a garden hose and thoroughly clean the outside of the radiator. This will ensure adequate air flow.

Verify the fan. Let the car idle until at operating temperature. Unless it is very cold outside, you should observe the fan cycling on/off to maintain engine temperature. While you are at it, verify that the fan turns on when the AC is on.

Check to see that the radiator cap is holding pressure. Is the upper radiator hose rock hard at full engine operating temperature? If not, suspect that the cap is not holding pressure or there is a leak somewhere.

Now we know that the system is clean with the good coolant and in good working order we can get down to brass tacks.

Get your ODB-II scanner and data log the engine coolant temperature. This will give us actual numbers to work with. Also log the TPS and engine load parameters. We can then see if there is a correlation between engine load and engine temperature. This will also help determine if the cluster gauge is at least reasonable.

If your scanner also allows it, log the fan as well. We can then see if there is a correlation between engine temperature and fan operation.

As an additional cross check, an infrared temperature scanner is affordable. Use the scanner to double check the temperature numbers reported by the PCM.

Let us know the results. Your stang will love you for it. Good luck.

twogts4us · Oct 13, 2009

Sorry I missed the new stat comment in the original email...good post by wmburns.

narib2008 · Oct 13, 2009

Thanks man...When doing the t-stat.I did clean out the radiator fully.It has proper coolant.Also i am not loosing any coolant at all.I will follow it on my monitor tommarow and see if there is any correlation between the sensors you listed.I will let you know.I believe it is a coolant sensor issue...

wmburns said:
OK, here is a new set of recommendations. Since the thermostat is new, let's assume it is OK. Next let's look at those things that can affect cooling. Good maintenance practices say that the radiator should be clean and the coolant is the correct concentration. Flush the radiator and install fresh coolant. Recommend no more than 50% concentration. More water means better cooling. However, get the concentration too low corrosion can be a problem. It is best to stay above 40% unless it gets very cold in your location.

Next, get a garden hose and thoroughly clean the outside of the radiator. This will ensure adequate air flow.

Verify the fan. Let the car idle until at operating temperature. Unless it is very cold outside, you should observe the fan cycling on/off to maintain engine temperature. While you are at it, verify that the fan turns on when the AC is on.

Check to see that the radiator cap is holding pressure. Is the upper radiator hose rock hard at full engine operating temperature? If not, suspect that the cap is not holding pressure or there is a leak somewhere.

Now we know that the system is clean with the good coolant and in good working order we can get down to brass tacks.

Get your ODB-II scanner and data log the engine coolant temperature. This will give us actual numbers to work with. Also log the TPS and engine load parameters. We can then see if there is a correlation between engine load and engine temperature. This will also help determine if the cluster gauge is at least reasonable.

If your scanner also allows it, log the fan as well. We can then see if there is a correlation between engine temperature and fan operation.

As an additional cross check, an infrared temperature scanner is affordable. Use the scanner to double check the temperature numbers reported by the PCM.

Let us know the results. Your stang will love you for it. Good luck.

40oz · Oct 14, 2009

twogts4us said:
If you noticed, the NORMAL section of the gauge is nearly the entire sweep. Typically mine will rise to 12 o'clock after it warms (about 10 mins +-) but never goes elsewhere, north or south. It's like clockwork. I'd question whether the fans are running, is the thermostat operating, sensor gone south. My first check would be the thermostat.

It's a very legit question...

Mine doesn't even say "normal." Should I be worried and how can I fix it?

The gauge stays in the range of normal. There is a reason it says "NORMAL" through that range. The lack of any issues besides the normal fluctuations of temperature would lead me to believe it's normal for the car's coolant to be in the normal temperature range. But that's just me.

If the thermostat was missing or sticking open, the temps would fluctuate all over the place, and it would run like crap sometimes and really good others. If the thermostat wasn't opening the needle would climb to the top of the gauge and stay there until it forced steam out somewhere. If the fan wasn't working the temps would climb higher than "R" on the gauge when the car idled for any length of time.

wmburns · Oct 14, 2009

Is this normal operation?

OK, I have to admit having mixed emotions about this post.

The original post did not include information such as year, model, engine, or mileage. This left us to guess if it is a brand new car or an older high mileage car. Further, the temperature ranges were not given in degrees to make a better determination.

My initial thought was that as long as the temperature gauge is around the “normal” range, it OK. Besides, it is intuitively obvious to me that if the fan cycles on at certain temperature and turns off at a certain lower temperature, then the engine temperature must vary between the two set points.

However, the OP didn’t seam to like the answer.

The next advice I could offer is basically a general shot gun approach towards cooling system issues.

Next advice was to monitor certain operating parameters. My hope was that by monitoring the engine temperature and the fan state a graph could easily be produced. It would then be easy to see the relationship between engine temperature, load, and fan state.

I personally have owned several cars that the temperature gauge would have minor variations within normal depending upon fan operation.

Good practices say to test first, isolate problems, and then perform repairs as necessary. This avoids replacing good parts. Will be interesting to see how this issue pans out. Bets anyone?

40oz · Oct 14, 2009

I don't know where the OP got the idea that "more precise gauges" don't actually measure things. Or that engines maintain a constant temperature regardless of load or conditions. But whatever

I think what the OP wants is an idiot light.

GDawg · Oct 14, 2009

40oz said:
I don't know where the OP got the idea that "more precise gauges" don't actually measure things. Or that engines maintain a constant temperature regardless of load or conditions. But whatever

I think what the OP wants is an idiot light.

True dat...

There are simple tests to test the coolant gauge sensor. OEM gauges are not all that and the idiot lights are to tell you good or bad, not specifics.

People need to give more info/history/details about their car and geographical location for us to better help them...

narib2008 · Oct 16, 2009

97 mustang GT 67k miles...Temp gauge when fully warm fluctuates from the N too the R on the gauge.All my car before this would stay exactly in the middle.The rheostat device being used is not technology that is from the 60's that will fluctuate depending upon minor temp changes 190f-210f.Most late model vehicles run in this range and do not fluctuate because the coolant temp sensor,ecm,gauge,etc compensate for this.So by having a gauge that fluctuates about 25% is a big issue.I honestly believe it is a coolant temp sensor.However i have not had the time to look into it.I will let you know when i do so everyone can sleep a little better

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