Temp Gauge issue

[tdangle]

Last year, I had the same issue. The sending unit went bad, I replaced it and started acting the same way. Cold when you start and then would climb to hot peg and then drop to "normal" Temp gun verified engine was not overheating.
Tried 3 different sending units, verified the sending units with water heating on the stove and charting the resistance vs heat of the water. All of them were OK, verified the operating range of the gauge with a resistor box to ground. Totally stumped. Even went so far as to remove the head thinking I had a blockage in a water passage. Also disassembled the water pump looking for the cause. Then started worked all by itself Still don't know. Now have a new motor with vortec heads, got a turned down sending unit and all works perfect. Only thing different with my new motor is vortec heads require a coolant bypass hose setup.

[maynardogle]

Check to make sure all the ground steps between the engine block, frame and cab are intact. Bad grounds screw up gauges.

[custom10nut]

Well, I ordered a Delco unit for my year so I would hope that it would be the right one. I have to go through mo old parts bin and see if I still have the original one. Incidentally, I spoke with a guy with a 73 truck, and he’s having the same issue.
Would it be worth it to buy one on these resistors for the gauge, or are they only for electronic ignitions.
But now that I breached that thought. Could the fact that I put an Electronic Conversion Module in the distributor make it act like this? I don’t know, just a thought

[custom10nut]

Originally Posted by maynardogle

Check to make sure all the ground steps between the engine block, frame and cab are intact. Bad grounds screw up gauges.

Good point! I’ll check them

[HO455]

Where is your sender installed in the cooling system? It may be that you have steam pockets building up in the system. Steam will build up in pockets and then burp out into the coolant flow. When steam passes the sender it will cause the temperature at the sender to rise temporarily then drop after the steam has passed. Sometimes this can be cured by drilling a 1/8" hole in the thermostat . That is if your sender is in the thermostat housing.
This condition can start happening due to a failing head gasket. You can test your coolant for the presence of exhaust gasses to rule out this as a cause of your problem.
Just information in case the grounds prove to be fine.

[Beach-Burban]

Following along as just yesterday I posted the same question on the Suburban side...it reads:

I just got back with the Beach-Burban from a 3 1/2 hour drive (7 hour round-trip...longest distance since I've owned it) and I had a little anxiety because my temperature gauge would pin itself under certain conditions, causing me to pull over just to find that the engine was NOT overheating! The Burb drove beautifully in every other way: Plenty of power (454); speeds between 65 and 75; long uphill grades; TH400 never acted up; oil pressure held firm; no pre-ignition or detonation; no loss of coolant (not even in the overflow tank); no extreme heat radiating from the engine bay; and no crackling noises when shutting the engine down. This held true even when driving with air temperatures of 101 degrees! The gauge would float between the high mark to the pinned position depending on the grade I was on, but never affected drivability to the point that I had to say, screw it...there must be a communication problem between the sending unit and the gauge!?!? The radiator was flushed a couple years ago and fitted with a new pressure cap and T-stat, as well as having a new sending unit installed because the old (original?) was dead...but during that time period, I had only been driving an hour or so at a time (and mostly coastal areas) so the gauge maxed out a little above the middle line. Do sending units have different ranges of temperature levels? Is it possible I installed the wrong unit...or is it more likely that the original gauge needs calibration? Baffled, yes...but very happy with the overall performance of the Beach-Burban! Any help would be appreciated!

Woody

[custom10nut]

Originally Posted by HO455

Where is your sender installed in the cooling system? It may be that you have steam pockets building up in the system. Steam will build up in pockets and then burp out into the coolant flow. When steam passes the sender it will cause the temperature at the sender to rise temporarily then drop after the steam has passed. Sometimes this can be cured by drilling a 1/8" hole in the thermostat . That is if your sender is in the thermostat housing.
This condition can start happening due to a failing head gasket. You can test your coolant for the presence of exhaust gasses to rule out this as a cause of your problem.
Just information in case the grounds prove to be fine.

It’s not the steam pocket problem. The Sending unit is in the side of the head, between 1-3 plugs. There’s no oil/coolant mixture and there’s plenty of pressure In the coolant system. The Heat range theory is starting to sound like the culprit.
Is there a vendor that has a factory spec sending unit for a 70 350? Would putting a resistor in the line with the gauge fix the Issue?
As I said from the beginning, the engine runs beautifully and never overheats. So I know it’s circulating correctly. I just like to visually monitor conditions while I’m driving.
Tomorrow I’m picking up a temperature gun so I’ll be able to give a “Temp at the Head” in my next post.
Here’s another Fleeting thought.
Could a wire splice be causing the issue?

[BigBird05]

To everyone that is having temp gauge problems. Install a cheep manual gauge and run it along with your factory gauge. Then you can see what is going on and stop guessing.

[Steeveedee]

Originally Posted by BigBird05

To everyone that is having temp gauge problems. Install a cheep manual gauge and run it along with your factory gauge. Then you can see what is going on and stop guessing.

After monkeying around with mine, that's what I did. I'll eventually go back to a gauge in the dash, but this one is working, so...

I don't know about "cheapo", though, it's an Autometer "Phantom" temp gauge left over from when I was going to get rid of the eejit lights in my hot rod Skylark, but I sold it before getting to that.

[toolboxchev]

Heat would increase electrical resistance in my mind.

[HO455]

Originally Posted by custom10nut

It’s not the steam pocket problem. The Sending unit is in the side of the head, between 1-3 plugs. There’s no oil/coolant mixture and there’s plenty of pressure In the coolant system. The Heat range theory is starting to sound like the culprit.
Is there a vendor that has a factory spec sending unit for a 70 350? Would putting a resistor in the line with the gauge fix the Issue?
As I said from the beginning, the engine runs beautifully and never overheats. So I know it’s circulating correctly. I just like to visually monitor conditions while I’m driving.
Tomorrow I’m picking up a temperature gun so I’ll be able to give a “Temp at the Head” in my next post.
Here’s another Fleeting thought.
Could a wire splice be causing the issue?

A resistor will not help. It will make the guage read incorrect at all temperatures.
A wire splice could very well be the problem. Any connection can add resistance to the circuit causing the guage to read incorrect.
Here is a link to a Corvette forum with a good discussion on guage accuracy and senders. Scroll down to post 24 and there is mention of BarryK and his data in PDF format. It has a lot of sender data in it. Unfortunately it is too large of a file to post here, but definitely worth looking at.

https://www.corvetteforum.com/forums...-accuracy.html

[tdangle]

Originally Posted by HO455

A resistor will not help. It will make the guage read incorrect at all temperatures.
A wire splice could very well be the problem. Any connection can add resistance to the circuit causing the guage to read incorrect.
Here is a link to a Corvette forum with a good discussion on guage accuracy and senders. Scroll down to post 24 and there is mention of BarryK and his data in PDF format. It has a lot of sender data in it. Unfortunately it is too large of a file to post here, but definitely worth looking at.

https://www.corvetteforum.com/forums...-accuracy.html

Adding resistance will cause the gauge to read lower. As the sending unit resistance gets closer to ground it will read hotter. Ground the wire and it will peg hot, unhook the wire (from the sending unit) and it will read stone cold.

[54blackhornet]

Guys I have the opposite issue. I can drive all day and my temp gauge hardly gets to the middle mark. Takes forever in colder conditions. Should I worry about it ? Truck has a shroud and clutch fan.

[Beach-Burban]

While searching for additional threads on this topic, I read a number of posts that "might" explain the problem that some of us are experiencing. Apparently there is a consensus that GM temperature gauges for our trucks "changed" in 1979 and the new resistance values of the gauges required a temperature sending unit that had a different resistance curve. That could explain some issues if the wrong year sending unit was used. Additionally, a number of members reported that even when they ordered a replacement sending unit for the right year of their gauge, there was no certainty that the replacement would have an accurate resistance curve...and I guess that this issue was so widespread, that American Autowire started making a reproduction of the GM #1513321 sending unit with a precise resistance curve to match the 1978 and older gauges. I read enough success stories to situations that sounded so much like our issues, that I payed the $27 to have one sent to me. I'll let you all know if it was worth it!

Woody

[custom10nut]

Originally Posted by Beach-Burban

While searching for additional threads on this topic, I read a number of posts that "might" explain the problem that some of us are experiencing. Apparently there is a consensus that GM temperature gauges for our trucks "changed" in 1979 and the new resistance values of the gauges required a temperature sending unit that had a different resistance curve. That could explain some issues if the wrong year sending unit was used. Additionally, a number of members reported that even when they ordered a replacement sending unit for the right year of their gauge, there was no certainty that the replacement would have an accurate resistance curve...and I guess that this issue was so widespread, that American Autowire started making a reproduction of the GM #1513321 sending unit with a precise resistance curve to match the 1978 and older gauges. I read enough success stories to situations that sounded so much like our issues, that I payed the $27 to have one sent to me. I'll let you all know if it was worth it!

Woody

And I’m waiting, with baited breath. If this will fix it, I’m in.
I looked it up and see what you ordered. As soon as I see your response I’m going to order one, if it works

[BigBird05]

You are right. GM did change the senders. You will need to make sure you get the old one and not the new one. A new model sender will make the older gauge read colder.

[HO455]

Originally Posted by 54blackhornet

Guys I have the opposite issue. I can drive all day and my temp gauge hardly gets to the middle mark. Takes forever in colder conditions. Should I worry about it ? Truck has a shroud and clutch fan.

That sounds like my truck. It is normal operation. On the cold mornings does your heater start blowing warm air after about 5 minutes? For me I fire the truck up, let it idle it for about 30 seconds, then turn around ( I live on a one way street.) drive 5 blocks at 10 to 15 mph, wait at a stop light for a minute or 2. From there it is 3.more blocks to the freeway onramp. By the time I'm on the freeway the heater is blowing warm air. The guage at that point is still down in the bottom of the part of the guage.

[custom10nut]

Originally Posted by BigBird05

You are right. GM did change the senders. You will need to make sure you get the old one and not the new one. A new model sender will make the older gauge read colder.

But I actually sweeps across ad goes all the way to hot.
I’m now thinking that the heat range of the sender is the issue. I’m going to go ahead and order the one from Auto -Wire, and it should be fixed

[custom10nut]

Originally Posted by custom10nut

Posted via Mobile Device

So, on a whim, I pulled the T-stat out and ran it for a little while. It read normal operating temp on the gauge. I’m off to the auto parts store in the morning. I’m just glad I didn’t go and spend a bunch of money on parts I didn’t need.😬
I’ll update y’all when I get it in
Posted via Mobile Device

[custom10nut]

Well, I’ve determined that the coolant isn’t circulating. In the morning I’m changing the water pump out, and that SHOULD be the end of it.

[Beach-Burban]

Originally Posted by custom10nut

And I’m waiting, with baited breath. If this will fix it, I’m in.
I looked it up and see what you ordered. As soon as I see your response I’m going to order one, if it works

Well...I just got back from a 2 1/2 hour (one way) drive to monitor the readings with the new sending unit...and I would say that it was a "semi-successful" remedy!? If any members are familiar with "the 5 Freeway Grapevine" in So Cal, you'll know what a grueling test that is for a cooling system (more so for our older vehicles). This has been the site of countless "boil-overs" thoughout the years...to the point that there are radiator water spickets all along that section of the grade. Anyway...that's where I conducted my testing, and overall the answer is yes...the readings on the gauge more closely represent what's going on under the hood! The gauge never once pegged itself, even though it was indicating a very high temp reading...but then recovered quickly when the grade flattened out. I can live with that, however, I still want/need to know what the actual coolant temperature is when it gets that high...so I'm going to install an under dash gauge (and matching sending unit) that has gradient numbers in degrees. I'll report back after more testing!

Woody

[Beach-Burban]

***Update on Temperature Gauge***

So I ordered a couple of Stewart Warner gauges and a two hole under-dash panel bracket...an Ammeter, as my original hasn't worked since I bought the Burb, and a mechanical Temperature gauge to help analyze what my original gauge is telling me. I did a little research and found that these mechanical gauges will accurately measure the temperature of any liquid, so I plan on using this one to monitor my A/T fluid after I sort out things with my water temperature.

I utilized a plugged 1/2" NPT hole on the drivers side head of my 454 so that I could still keep the electric sending unit operating...which makes comparing the two gauges much easier. As this tapped hole is located a little above the exhaust manifold, I used a couple feet of 1/4" ID X 1/2" OD rubber hose to insulate and protect the capillary tube (all the way past the firewall and into the cab), as well as some thin metal tape (crumpled up for a better insulation factor) from the Sender Bulb out past the manifold a ways (as a heat shield).

After refilling and "burping" the coolant to the best of my ability, I went out for a 45 minutes drive in our local hills of Palos Verdes. Not much of a test compared to the "Grapevine", but I mainly wanted to get any air pockets filled in with coolant, check for leaks and compare "around town" readings. At the halfway point of the drive, I stopped and found that the radiator had sucked coolant in from the recovery tank (it was at the "Full Hot" line when I left, and was now at the "Add" line) so I topped off the recovery tank and headed for home. Once home, I checked the recovery tank again, and as it was still at the "Full Hot" line, I believe all air pockets should be gone.

Results:
Spoiler alert...the two gauges did not read anything close to what I had expected, but I didn't take any pictures until I got home! While driving, the S/W gauge read between 200 and 210 almost the entire drive (after initial warm-up of course), but when I pulled into my driveway and sat with the motor running, it stabilized at what looked like 212 degrees. While driving, the factory gauge had been holding steady just about on top of the first mark past "C" (well technically, it's the second mark past "C"), but in my driveway with the motor running, it rose to about halfway between that first mark and straight up. I then decided to shut the motor off (with the key in the "ON" position) and see how much the temperature would rise without any circulation/fan. The S/W topped out at just above 230, and the factory gauge made it to just above straight up.

Now I do realize that there are "control issues" with the testing methods in place here...brand new mechanical gauge verses a 48 year old electric gauge (with a new American Autowire sending unit)...and...mechanical sender bulb installed in the water passage of the head verses electric sending unit installed in the water passage of the intake (right next to the T-Stat)...but I was expecting/hoping for something that I could say "well, that makes sense"...but I'm still puzzled! I guess I need to return to the "Grapevine" so I can compare what the S/W gauge reads when the factory gauge reads past the "H". Once again...I'll report my new findings!

Woody

[HO455]

Science! Good stuff!

[Beach-Burban]

Originally Posted by HO455

Science! Good stuff!

HA...I don't know if this qualifies as "Science" HO455...maybe closer to "Uncontrolled Obsessive Behavior" at this point! Really...I just want to feel comfortable with the readings so I'm not always worried about doing damage to this "near-new" 1989 454. I'm glad you are enjoying the "read"...I always enjoy reading your success stories...and how you overcome obstacles getting to those successes. We learn things everyday!

Woody

[FirstOwner69]

Originally Posted by Beach-Burban

...If any members are familiar with "the 5 Freeway Grapevine" in So Cal, you'll know what a grueling test that is for a cooling system (more so for our older vehicles). This has been the site of countless "boil-overs" thoughout the years...to the point that there are radiator water spickets all along that section of the grade....

This won't help you at all, but...
Boy, does that bring back memories. We lived in Fullerton from 1968 through 1971. We had a '65 Impala SS with a 327 in 1968 and made a couple runs up to Yosemite. The Grapevine is the only place we ever boiled over.

Great place for a trial run!