Engine temperature gauge , is this reading correct

[DoneonOT]

Recently purchased 72 Chevy with a big block 400 . is this what the temperature gauge is supposed to read ? I don't have a temperature / heat gun so I don't know the true temperature of the engine . Second , I noticed just today the needle jumps back and holds itself steady right in the middle of the hot and cold readings, as if it was showing a good reading of not too hot and not too cold. It says there for about 4 mins then goes back to a hot reading. Do I need a new temp sensor?

[demian5]

1. You need a temp gun to make sure you are not running too hot.
2. Over the years, as parts are changed or fail, the sensors get changed out and may not have the proper resistance.


I am sure there is someone with a chart that shows what the correct resistance is for the sending unit at the given temperature.

Post a picture of your sending unit. Mine has two (one in each head) for some reason and they are both plugged in (??). I am thinking one is for the ESC.

[BugzC10]

Does it go back to the middle again? Does it continually go back to the middle, Then back to hotter setting? Makes me wonder, what thermostat are you running? Could it be, when the termostat opens it cools ( guage goes to the middle), then the thermostat closes again, making the guage go to hot. Trouble is...it should stalbize. Just thinking out loud....

[AussieinNC]

A lot of variables in this situation:

Engine cold, remove the temp sensor wire and ground it to the engine...turn on the Ign but dont start, temp gauge should go to full hot...Yes?

Replace the sensor wire on the sensor and with engine cold, turn on ign but dont start...where does the needle show on the temp gauge ?

Does engine have a fan shroud? Big blocks MUST have a fan shroud...
Does engine have a fan clutch?
Does engine have a thermostat installed? If its missing, you should put one in place asap...thermostats slow down water flow to allow for heat to be drawn out by radiator...
How many rows in radiator? Big Blocks should have 4 row...
Does the truck have electric fans installed? If yes, is the rotation correct on the fans? The fans, if present, should push air thru radiator front to back, or pull air front to back...

Radiator cap working ok? Recovery system in place?
You running just water or coolant mix?

Then you get into ignition timing correct?
Carb jetting not too lean?

Then electrical issues with gauge itself, resistor systems, is coolant sensor matched to gauge?

Suggest you invest in a heat gun...try Harborfreight...

A big block running that hot will not like it for too long...

Stay safe mate...

[davepl]

You really need a heat sensing gun, they're cheap and indispensable!

That's about where my big block will run on a warm day.

[demian5]

1.

Originally Posted by demian5

1. You need a temp gun to make sure you are not running too hot.

[Joesjunk]

I had a truck that read hot like that, it was just the sending unit. I put an original one in it from a low mile motor and it was perfect

[DoneonOT]

Well the consensus seems to be , GET A HEAT GUN.

Yes there is a fan shroud

Yes, I will try the temp sensor trick and ground it to the block

Engine has a clutch fan .
Yes it does have a thermostat recently installed
Yes it has original radiator with four rows
No electric fans
As far as I know radiator cap is working ( had the whole radiator pressure tested assuming they tested the cap too)
There is no recovery system in this truck , did not come with one . ( factory optional )

As far as the ignition timing And carb jet , I won't explore that just yet ...

Took it on a 40 minute drive and saw the needle go to the middle one time and then back to hot . So I don't think it's a repetitive thing .

I'll take a picture of the sending unit and put it on the thread .

[Joesjunk]

I had a truck that read hot like that, it was just the sending unit. I put an original one in it from a low mile motor and it was perfect

[OregonNed]

I bought a nice sender from Lectric Limited. Works great. Also picked up the wire harness for it too

[DoneonOT]

I ran the temp gun on the thermostat housing on a hot day, at idle, in my garage (after driving it for a bit). It ran 205-210 and I shut it down at 212 at the hottest. On a cool day (Wisconsin weather 50-40's) it reads like 195-200. I think it just runs hot when its really hot. I have to keep the truck moving to keep the temp. down. Thanks for the info. I did get a new temp sending unit from Letric Limited and will install prob in the spring.. see if that changes anything. I saw on another post that a really concerning temp is 220. otherwise it should be fine.

Thanks for help

[clay68c10]

I had a 454 a few years ago, it ran on the upper side of the gauge like that. It had EFI so i could watch the temp from the computer, it was always 200-205. Had a 195 thermostat. Big blocks just run hot in my experience.

[davepl]

On my truck, on my gauge, that'd be about 195F I think.

[Big Kev-O]

Autozone gave me the temp sensor for a truck without a gauge. My gauge acted real funny, It would slowly creep towards hot then suddenly peg out. Turns out sensor was for an idiot light not a gauge. Once I got the new sensor it always read a bit cold(like all reproduction sensors). Yep time to get a temp gun.

[Jcbsat]

X2 on the bad sensor. Years ago I had a BMW 2002 that had a creeping temp gauge. After driving for a while the temp needle would start to edge higher to uncomfortable levels due to increasing resistance. If you flicked the bright lights or honked the horn the needle would jump and then drop to normal, only to do it over again. These cars had a bad design with a remote thermostat that involved 4 or 6 (!) hoses between the radiator, thermostat, and block!

[70STOVEBOLT]

My '70 has a mostly stock 402 with a 7-blade clutch fan, stock shroud, 3-row radiator, and 160 degree thermostat. This summer when it was 80-85 degrees, warmed up the needle would stay almost in the middle of the range, maybe slightly to the left of center. I have only used an IR temp gun on the exhaust manifolds to try and find a misfire, so I don't know what the actual coolant temp is for that reading. I am thinking about getting an IR gun from Harbor Freight on Black Friday. If I do, I will report back what the reading is.

[Andy4639]

This is the rule of thumb if everything is ok in the system.

Remember the cooling rule of thumb. runs hot at low speed - not enough fan. Runs hot at highway speed - not enough radiator. At 70 MPH your getting a huge volume of air through your radiator.

This also means the air flow is in fact going through the radiator and not around it.

[DoneonOT]

Stovebolt, I would live to hear what numbers you are getting... also include the outside temp please.. thanks

[70STOVEBOLT]

I will let you know if I pick up an IR thermometer.