Overheating “slightly” and infrequently

[79K15 HIghSierra]

Alright fellas, riddle me this! Got my freshly rebuilt 350 installed, just back from the shop where timing was dialed in and my few rookie mistakes were mended (little things, thankfully). She runs great! Starts right up and idles great even in the cold. Here’s where my issue is. When I get it up to temp, I’m cruising down the road and when I get to about 2-3k rpm in preparation of a shift (it’s manual trans) it will tick up to 3/4 on the temp gauge. If I ride around 2k rpm it will do the same. Pretty much any time I feed her the beans it ticks up, when I let off the gas it goes back to the middle of the gauge. I replaced the thermostat when I did the rebuild and used the old one to plug a whole on the intake right behind the thermostat housing. I put the temp gauge wire on that for giggles, noticed the temp dropped below half, then when running it up to 2.5k rpm and hold it there, it went to dead center on the gauge and pretty much stayed there. Ticked up a hair when I ran it up to 3k rpm. So! My question is, am I getting a whacky reading because the temp sensor is right next to the headers? Is the reading from the intake close to accurate? I want to replace the radiator anyways as it’s the original from ‘79. Also my heater core is no good so I have the hose just running a loop from the send/return on the water pump. Thanks guys!

 

[Shorty81]

My temp sensor is in my intake also. They tend to run a tick cooler there. But I've checked heads and block with a heat gun and it's pretty close.

 

[Shorty81]

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[Paladin]

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Damn, that's Purdy!

 

[Bextreme04]

The fact it moves that quickly when you increase RPM makes me suspect you have an electrical issue. Did you make sure all of the grounds are present and clean? Passenger head to body, alternator bracket to frame, battery to frame, etc...

Also make sure the grounds on the ground block integrated into the parking brake body are clean and present. That is where the gauge cluster gets its ground. You could also have a weak battery or an alternator going out that is causing low voltage at idle and therefore changing your reading. Although my voltage drops at idle on my truck and my temp reading is rock solid.

 

[wanderinthru]

Similar, temp gauge showed hot, engine wasn't. Changed kinda like yours. Ended up being an 82 gauge reading a 78 sender. Got an 82 sender and all is well. Sender is in the head.

 

[Bextreme04]

The showing higher on the gauge could definitely be the wrong resistance range. The bouncing around is either a serious engine issue or an electrical one. Since it changes with engine RPM, even when unloaded, my guess would be electrical.

 

[79K15 HIghSierra]

The showing higher on the gauge could definitely be the wrong resistance range. The bouncing around is either a serious engine issue or an electrical one. Since it changes with engine RPM, even when unloaded, my guess would be electrical.

that’s kind of what I’m thinking. It was just tuned and timing set, all adjusted last week on a fresh rebuild. It’s moving up quite fast when I give it throttle. I’ll start with grounds and a new gauge and see what she does!

 

[tadslc]

If it's not a grounding issue, I'd buy an aftermarket gauge and sender, loosely install and see if it replicates the readings.

I also had a vehicle one time that had so much silicone tape around the sender that the sender had a hard time grounding out to the head and read very poorly. Doesn't sound like your issue since yours is repeatable but just something to think about.

Good luck, let us know what you find.

 

[79K15 HIghSierra]

If it's not a grounding issue, I'd buy an aftermarket gauge and sender, loosely install and see if it replicates the readings.

I also had a vehicle one time that had so much silicone tape around the sender that the sender had a hard time grounding out to the head and read very poorly. Doesn't sound like your issue since yours is repeatable but just something to think about.

Good luck, let us know what you find.

I did as you recommended! Aftermarket sensor and gauge and it’s read steady, held at about 190° even when feeding her the beans, glad it was just old gauge and wiring! Now to mount the new one and hook up lights. Thanks guys!

 

[MikeB]

Some electrical gauges will display different temps depending on voltage. For example, with ignition on, engine not running, and battery at 12 volts, a gauge might read 180. But fire up the engine, voltage jumps to 14 volts, and the gauge might read 190 or more. If you have a voltmeter, you should check battery voltage at various RPMs to see how much it varies. Higher quality gauges use circuitry that minimize or eliminate gauge fluctuations.

 

[dvdswan]

Do you have the proper amount of coolant in the system? Even if it is a little low, as you accelerate you will get an air pocket at the sensor location on the manifold, thus allowing the temp to rise and fall with the changes in RPM.

 

[Scribbles]

I had an air bubble in my system after cleaning out the radiator. It took a few tries but it finally came out and I had no more overheating issues after that.
I'm glad it was just wiring!

 

[80BrownK10]

Alright fellas, riddle me this! Got my freshly rebuilt 350 installed, just back from the shop where timing was dialed in and my few rookie mistakes were mended (little things, thankfully). She runs great! Starts right up and idles great even in the cold. Here’s where my issue is. When I get it up to temp, I’m cruising down the road and when I get to about 2-3k rpm in preparation of a shift (it’s manual trans) it will tick up to 3/4 on the temp gauge. If I ride around 2k rpm it will do the same. Pretty much any time I feed her the beans it ticks up, when I let off the gas it goes back to the middle of the gauge. I replaced the thermostat when I did the rebuild and used the old one to plug a whole on the intake right behind the thermostat housing. I put the temp gauge wire on that for giggles, noticed the temp dropped below half, then when running it up to 2.5k rpm and hold it there, it went to dead center on the gauge and pretty much stayed there. Ticked up a hair when I ran it up to 3k rpm. So! My question is, am I getting a whacky reading because the temp sensor is right next to the headers? Is the reading from the intake close to accurate? I want to replace the radiator anyways as it’s the original from ‘79. Also my heater core is no good so I have the hose just running a loop from the send/return on the water pump. Thanks guys!

If that's an original radiator it's a good one. Have it reworked at a shop, unless it's just too far gone.

 

[79K15 HIghSierra]

If that's an original radiator it's a good one. Have it reworked at a shop, unless it's just too far gone.

I’m glad you said this as I’ve been weighing options for replacement. It is original, and there’s a shop by me that will clean and repair for about $120. I’ll stick with it and not spend more on a aftermarket fancy one, thank you!