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Discussion Starter #1 (Edited)
My 69 302 has all Edelbrock Performer RPM parts and a big horizontal flow aluminium radiator with twin electric fans. Overheating is not a problem and it all behaves itself just fine when cruising around the town showing 1/2 on the temperature gauge and when stuck in a lot of traffic it can creep to 2/3 on the gauge.

But, I have recently done a few long drives on faster roads of about 60 - 70 MPH and have noticed the gauge can drop down to about 1/4 on the scale. I assume this is from the airflow through the radiator at this faster speed. I have a 5 speed manual gearbox and in top gear, I am barely touching the throttle to achieve 60 - 70 MPH. Is this lower temperature a problem or quite normal? I cannot remember what the gauge did before all my modifications and my old 3 speed automatic gearbox. The car drives just fine and this is just a query. As soon as I get back to town driving speed or traffic, then the gauge increases to the more normal temperatures above.

Jeremy.
 

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First off...get an actual temperature gauge(even if you only install it temporarily) to see what is actually going on. You need real temperature numbers....not 1/4, 1/2, or 3/4...that doesn't tell you anything. If you assume that those gauge positions correspond with say 150 degrees, 180 degrees and 220 degrees then yes, 150 degrees is too cool and the only way that could possibly happen is if you dont have a thermostat installed(or you have one that opens at a lower temp than I have ever heard of one opening) but all that is speculation without true temperatures to go by.
 

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I just checked my picture archive. I used a 180 thermostat.
What type is it? And when did you install it...a Stant t-stat for instance will fail in the open position, allowing an engine to overcool(or not warm up to correct operating temp, whatever, which for the record is far better than failing in the closed position). You can always test t-stat operation by dropping it in a pot of boiling water and watching to make sure it opens when you do so, and making sure its closes in a pot of room temp water
 

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Without actual temperature numbers it still sounds like you have a thermostat that may be not closing all the way. I've had it happen. Time for a new a new thermostat I'd say. And while you're at it, using a correct 195 degree thermostat would only do your engine good.
 
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Discussion Starter #8 (Edited)
What type is it? And when did you install it...a Stant t-stat for instance will fail in the open position, allowing an engine to overcool(or not warm up to correct operating temp, whatever, which for the record is far better than failing in the closed position). You can always test t-stat operation by dropping it in a pot of boiling water and watching to make sure it opens when you do so, and making sure its closes in a pot of room temp water
I do not know the make as the box it came in was long gone when I installed it. The pictures of either side of it show 54mm and USA on one side and 11 02 A 180 on the other side. As it has USA on it hopefully it is not a cheap quality one. I do not think the thermostat is stuck open as the engine warms up to its normal temperature nice and quickly as you would expect. And I am pretty sure I tested it anyway before fitting it. Also I drilled 2 tiny holes in the outer metal part to help with burping air after filling it. And the thermostat was installed with the 2 holes at the top.

Really all I am asking is, is it normal for the temperature gauge to drop a bit when cruising at higher speeds ( 60 - 70 MPH ) than just driving around local slow ( 30 MPH ) roads.

My modern daily driver is from 2001 and that sticks in the middle of the gauge if driving over 100 MPH or stuck in heavy traffic. But I assume that is modern cars for you.
 

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I do not know the make as the box it came in was long gone when I installed it. The pictures of either side of it show 54mm and USA on one side and 11 02 A 180 on the other side. As it has USA on it hopefully it is not a cheap quality one. I do not think the thermostat is stuck open as the engine warms up to its normal temperature nice and quickly as you would expect. And I am pretty sure I tested it anyway before fitting it. Also I drilled 2 tiny holes in the outer metal part to help with burping air after filling it. And the thermostat was installed with the 2 holes at the top.

Really all I am asking is, is it normal for the temperature gauge to drop a bit when cruising at higher speeds ( 60 - 70 MPH ) than just driving around local slow ( 30 MPH ) roads.

My modern daily driver is from 2001 and that sticks in the middle of the gauge if driving over 100 MPH or stuck in heavy traffic. But I assume that is modern cars for you.
Well, you do have more airflow at high speeds...but a properly regulated engine regardless of age isnt going to move much from the 1/2(normal operating temp) range. The t-stat dictates the minimum engine temperature...and the fans(when the flow enough air) dictate the maximum engine temperature. The fact that your temp is swinging so wildly(assuming it really is and that the gauge isnt just bad) indicates an improperly functioning cooling system...whether thats a t-stat stuck open, or radiator fans not flowing enough air, too small of radiator etc. Is the range that it is fluctuating in going to hurt your engine? Most likely no...but its certainly worth a look at the cooling system in general and doing whatever needs to be done to lock it down...but the first step is STILL to buy a real temperature gauge so you can determine what is really happening...you can just hook it up and leave it loose in the cabin...in this case its a diagnostic tool. I am sure they sell actual diagnostic gauges instead of in-dash ones too but either can be used as long as they have actual numbers and are accurate.
 

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Discussion Starter #10
I will look into fitting a temporary temperature sender that is more accurate than the stock one and shows proper readings.

I have one of those handheld digital temperature devices. Could I not just measure the temperature at the sender on the engine starting from cold when idling and note the readings for 1/4, 1/2 and 3/4 on the gauge?
 

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I will look into fitting a temporary temperature sender that is more accurate than the stock one and shows proper readings.

I have one of those handheld digital temperature devices. Could I not just measure the temperature at the sender on the engine starting from cold when idling and note the readings for 1/4, 1/2 and 3/4 on the gauge?
yes indeed. The only problem is that it will be hard to get a reading while at speed...though I guess you dont need to if you just nab those temps when the needle is pointing at reference points while its warming up. You might well find that it is operating in the 180-220 degree range the entire time which is fine
 

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Discussion Starter #12
yes indeed. The only problem is that it will be hard to get a reading while at speed...though I guess you dont need to if you just nab those temps when the needle is pointing at reference points while its warming up. You might well find that it is operating in the 180-220 degree range the entire time which is fine
Excellent, I will give it a go in the morning.
 

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It has been noted on this forum several times that running an engine too cold is detrimental to it's health. The oil needs to get hot enough to "cook off" any moisture that gets in it.
 

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Discussion Starter #14 (Edited)
It has been noted on this forum several times that running an engine too cold is detrimental to it's health. The oil needs to get hot enough to "cook off" any moisture that gets in it.
Luckily this is not happening for all driving and only when cruising at speed. With local driving and on slower roads, the engine gets up to temperature just fine. And as soon as the faster driving is over and I am back onto slower roads, the temperature gets back to normal within a minute or so.

But I am going to look into why it cools down when driving at speed.
 

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What awhtx is describing is likely not happening with your installed incorrect 180 degree thermostat. Not optimally anyway, regardless of your driving speed. While the thermostat possibly may not be to blame for the actual problem you are chasing (but it's very likely) it is still wrong for the car and needs to be changed. Other discussion and troubleshooting attempts will be pointless until it is done.
 

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I found that anything more than one tiny hole in the stat will let the water flow more than it needs to.
 

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I dont drill holes in the t-stat at all...it defeats the purpose. There are other ways to get air out of the system....and if you dont have that problem to start with there isn't a reason to drill holes in the first place.
 
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