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Fluctuating Temp Gauge


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#1 Frank S

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Posted 26 February 2001 - 05:20 PM

Have you tried changing the sending unit?

#2 76BBSub

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Posted 15 April 2001 - 10:24 PM

My stupid '96 GMC Sierra did this all the time. Driving down the freeway, constant speed, the temp would constantly fluctuate about 50-60 degrees.  After a few days I figured it was normal, after a year and half I never had a problem with that.  BUT, the truck had so many other electrical bugs I traded it in on my '99 GMC Sierra.  The '96 got to where occasionally when you put it in reverse it would lose ALL electrical power for about 10 seconds.  NOT FUN!  Had some other bugs, too...I just hope it $hit the bed at the GMC dealership where I traded it in before they dumped it on someone else.
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#3 Hobie

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Posted 26 February 2001 - 08:10 AM

Hello all!
I have a 1995 GMC K1500 Z71 350 TBI. The temp. gauge fluctuates constantly between normal and 210^ in cool weather(40^ & below). Also during accleration it will rise to 210^ and drop back down, never staying at a constant temp. It constantly moves between normal and 210^, exempt during summer time it stays steady. I replaced the t-stat 3 times, having no affect. Also this past summer I drained and flushed the radiator due to the age and milage. I talked with a mech. from the dealer about it and he had no idea. He said since it does this only in cool weather and it never overheats he couldn't see any problem.

Does anyone have any ideas, or do I cope with it?



#4 Guest_Friz_*

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Posted 26 February 2001 - 07:20 PM

Did you post this over at PUTC?
I saw a similar problem there.
My 1980 CC did the same thing until I blocked off part of the radiator in the winter and then it was fine.
If you doubt the accuracy of you gauge, try to install a mechanical gauge at the same place. You should be able to "Tee" the existing fitting where the electric sending unit is and compare the two gauges side by side.

#5 Kansas Kid

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Posted 26 February 2001 - 02:41 PM

I wouldn't worry about it too much since it never overheats.  The stock gauges aren't very accurate.

Have you tried installing a good mechanical gauge just to see how it reacts?


#6 Hobie

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Posted 27 February 2001 - 08:05 AM

Thanks guys for your input. Seems like there is no major  problem. And yes Friz, I put this on th ePUTC board.

#7 GH6702

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Posted 26 February 2001 - 04:41 PM

My truck did that too.   My temp gauge was well under 210 driving down the highway, then all of a sudden it jumps up past 210, between 210 and 260.  I pulled over and felt around the engine, and it wasnt even hot.  Then I got back into my truck and a few seconds later, the temp gauge jumped back behind 210 and stayed there.  I think it is either the wire on the side of the block thats hooked up to the heat gauge or some electrical problem.  

#8 gmcblu

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Posted 15 April 2001 - 08:58 PM

The '92 K1500 w/ 350cid that I bought last month also has a temp gauge that fluctuates pretty badly.  The last owner had a 160 deg. thermostat in it b/c he towed in the mountains, and since I put the 195 deg. in it moves around between roughly 160 and 215.  At least that's what the gauge says...I don't believe the factory gauges either.

#9 RJRanch

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Posted 23 August 2005 - 10:38 PM

My truck did that too.   My temp gauge was well under 210 driving down the highway, then all of a sudden it jumps up past 210, between 210 and 260.  I pulled over and felt around the engine, and it wasnt even hot.  Then I got back into my truck and a few seconds later, the temp gauge jumped back behind 210 and stayed there.  I think it is either the wire on the side of the block thats hooked up to the heat gauge or some electrical problem.  

<{POST_SNAPBACK}>

I've got a "99" model Silverado with 5.3, 3.73 gears, pushbutton 4WD in it and just over 200K miles. Up until last 6 months it never got above 210 deg. I had water pump start leaking a bit at weep hole and replaced it along with thermostat and flush for radiator, heater core and block. Then towed a small load on trailer and it did good until i came to some hills... temp climbed again. Next i thought maybe transmission was running hot, new fluid and filter for it, then i had to pull pan back off tranny because it acted like clutch slipping in fwd gear..reverse was fine. Turned out it was a solenoid inside pan that had gotten broke, replaced that and tranny was fine, I crossed fingers and drove it for a day.. never heated up but i didnt need air and ran mostly flat roads. Next hot day, temp climbed up to 3/4 mark when i had air on and reached cruising speed, so ended up turning htr on and melting with windows open till it cooled back down. Scratching my head a bit, it is about 200 K miles on truck after all, maybe cat converters doing it?.. went and got replacement pipe and converters.."YIKES" put them on...and darn thing still overheats. Maybe someone in here can help with it. The sending unit reads right going by ohmmeter and gauge (when it works right) 113 ohms, i figured around 250deg. or so..about right for where temp gauge was reading at the moment, and 176 ohms is 212 deg. Voltage from plug to guage reads 5.03 volts ..right in ballpark. but if i turn on ignition with sending unit disconnected gauge still reads about 170 and floats around a bit. If i hook it up and turn on ign it may read right or might stay at bottom with engine near operating temp. Also got dancing fuel gauge thing by the way and replaced near all the front end parts other than housing and ring and pinion gears. (right wheel bearing/steering knuckle assy, right upper ball joint, left cv shaft, right inner axle shaft and bearings both ends, left inner axle shaft and bearings both ends, both carrier bearings for gear cluster, pinion bearing of front diff, and currently waiting on front half of transfer case to come in so i can replace output shaft bearing which happens to be part of housing..hmm.
Sorry such a long post, but i'm near desperate at this point.
Thanks in advance for my sanity and truck back.

#10 Bill55AZ

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Posted 24 August 2005 - 11:46 PM

It used to be that there is an additional voltage regulator (less than 12v but constant) for instruments and guage power, so that fluctuating battery charging voltage won't cause readings to vary. I don't know if they are still doing it that way. If all instruments are acting up, that would be the culprit. If only one is acting up, it is most likely the sensor or the wiring for that one circuit.
When it is coolant temperature, I think air bubbles can cause some fluctuating readings.
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#11 Wingnut

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Posted 24 August 2005 - 11:54 PM

And the winner for bringing up the oldest post for this week goes to....

I've got a "99" model Silverado with 5.3, 3.73 gears, pushbutton 4WD in it and just over 200K miles. Up until last 6 months it never got above 210 deg.  I had water pump start leaking a bit at weep hole and replaced it along with thermostat and flush for radiator, heater core and block. Then towed a small load on trailer and it did good until i came to some hills... temp climbed again. Next i thought maybe transmission was running hot, new fluid and filter for it, then i had to pull pan back off tranny because it acted like clutch slipping in fwd gear..reverse was fine. Turned out it was a solenoid inside pan that had gotten broke, replaced that and tranny was fine, I crossed fingers and drove it for a day.. never heated up but i didnt need air and ran mostly flat roads. Next hot day, temp climbed up to 3/4 mark when i had air on and reached cruising speed, so ended up turning htr on and melting with windows open till it cooled back down. Scratching my head a bit, it is about 200 K miles on truck after all, maybe cat converters doing it?.. went and got replacement pipe and converters.."YIKES"  put them on...and darn thing still overheats. Maybe someone in here can help with it. The sending unit reads right going by ohmmeter and gauge (when it works right) 113 ohms, i figured around 250deg. or so..about right for where temp gauge was reading at the moment, and 176 ohms is 212 deg. Voltage from plug to guage reads 5.03 volts ..right in ballpark. but if i turn on ignition with sending  unit disconnected gauge still reads about 170 and floats around a bit. If i hook it up and turn on ign it may read right or might stay at bottom with engine near operating temp.  Also got dancing fuel gauge thing by the way and replaced near all the front end parts other than housing and ring and pinion gears. (right wheel bearing/steering knuckle assy, right upper ball joint, left cv shaft, right inner axle shaft and bearings both ends, left inner axle shaft and bearings both ends, both carrier bearings for gear cluster, pinion bearing  of front diff, and currently waiting on front half of transfer case to come in so i can replace output shaft bearing which happens to be part of housing..hmm.
Sorry such a long post, but i'm near desperate at this point.
Thanks in advance for my sanity and truck back.

<{POST_SNAPBACK}>


I think you also get an award for "longest paragraph without a break" this week.
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#12 RJRanch

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Posted 26 August 2005 - 07:05 PM

Sorry about running on, but once i started to let it out..about truck, well you understand. I'm new to posting as well but i'll try to keep them short and to the point from now on. :thumbs:

Edited by RJRanch, 26 August 2005 - 07:18 PM.


#13 99silveradoz71

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Posted 26 August 2005 - 08:17 PM

I can't fault you for enthusiasm. Welcome
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#14 kewlgye

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Posted 26 August 2005 - 08:53 PM

Hello all!
I have a 1995 GMC K1500 Z71 350 TBI. The temp. gauge fluctuates constantly between normal and 210^ in cool weather(40^ & below). Also during accleration it will rise to 210^ and drop back down, never staying at a constant temp. It constantly moves between normal and 210^, exempt during summer time it stays steady. I replaced the t-stat 3 times, having no affect. Also this past summer I drained and flushed the radiator due to the age and milage. I talked with a mech. from the dealer about it and he had no idea. He said since it does this only in cool weather and it never overheats he couldn't see any problem.<p>Does anyone have any ideas, or do I cope with it?<p>

<{POST_SNAPBACK}>


My 1998 K1500 Z71 Vortec 350 was doing the exact same thing. I replaced the thermostat. Turns out that the old one was sticking, so the problem was solved. If you want to see if it's your temperature sending unit, run your truck for a week without a thermostat, and see if the temp still fluctuates. If it does, it's more than likely that sending unit or the gauge itself.
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#15 oldwrench

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Posted 27 August 2005 - 11:07 AM

Actually not a good idea to run without a thermostat. The cooling system was designed to only operate properly with the stat in place.

Climbing a hill with AC on your temperature will rise significantly (& cool as you go down the other side). The gauge should still move significantly.

I do agree though to replace the stat first.