Temperature Gauge Reporting Wrong Temp?

[spanky1205]

I installed a two speed electric fan using relays and a BMW temperature sensor following directions I found on multiple websites. I bought most everything from the local pull apart except for the BMW Temp Sensor. I bought a new 91c/99c (195f/210f) temp sensor from RockAuto (link). The fan came from a 2000 Mercury Grand Marquis (similar to this) and I used an under hood fuse box, fuses and relays out of a Jeep Cherokee (similar to this picture). The fan turns on and turns off like it is supposed to, except my temperature gauge about 230 when the fan comes on and 210 when the fan turns off. I have attached a picture of my temp gauge, I am calling the line half way between 210 and the red 230. I have lost count of the number of times I rechecked the wiring. I verified the 180* t-stat is opening, replaced the radiator cap, verified the fluids are full, and made sure there are no coolant leaks. The temperature sensor is installed in the water neck just above the t-stat. Prior to installing the E-fan the temp gauge always read 190ish and my heat worked great. Unfortunately It will be mid summer before I can work on the truck again I am trying to learn what I can in the mean time. So my questions are:

1: Is it normal for our old gauges to be that far off? Seems like the brand new sensor would be more accurate than my 30+ year old gauges.

2: If my gauges are reading right, what would throw off the new sensor so much?

3: BMW makes another temp sensor that kicks on at 195f and off at 180f. Should I swap for that? Based on reading those temps are too low.

4: If my gauges are wrong and the new sensor is more accurate, does that mean I have been running the truck too cool? If so what effect does that have? Would it have an effect on fuel economy?

5: Is there a reliable way to verify coolant temperature without installing additional gauges? I would rather skip the AUX gauges for now and upgrade all of my gauges to something I like when money allows.

6: Any recommendations on a set of AUX gauges I could install (if there is no other reliable way), that will not break the bank? I was thinking about something like similar to this.

7: This probably should have been question #1. Am I reading the temp gauge wrong? Seems like a no brainer, but I have done dumber things.

Thanks for hanging in there and reading everything, the more I wrote the more questions I had. I did not expect this to be so long.

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[spanky1205]

In an effort to help clarify what I was trying to explain in the first post. Here are a couple photos showing the temp when the fan turns on and when the fan turns off. Until I installed the electric fan the truck always ran below 210.


Fan turns on:

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Fan turns off:

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[1987 GMC Jimmy]

You really need an infrared temp sensor to verify your temp at the cylinder head where the GM sender threads in. I’m willing to bet, though, that your aftermarket stuff is right on the money, and your OE stuff is wrong. Besides the fuel gauge on a dual tank pickup, the temp sender is the most universally asinine of all of the gauges on these trucks. I’d say it’s normal and actually not bad looking at the cooler temperature. When it’s pegged hot or three quarters of the way, something’s off. I’d love to know what the secret is because mine does kind of what yours is doing but a little worse. I’ve tried like three different senders and got it close for a little bit, but it’s usually wrong. Not to mention the truck runs at a perfect 195*. I just haven’t had time to dig into it like I’d really like to... Are you still running TBI? If so, you really need to get the 195* t-stat so the ECM can nestle into the correct loop status and not run rich. I hope that touched on the majority of your concerns. I’ll attach a pic of my temp gauge. I got it at Home Depot for $30, I think it was. Harbor Freight sells a really cheap one, but don’t expect it to last for the long haul. It’s accurate, but it’s very cheaply made and mine just broke after a few uses. The one I have now is the real deal or has been so far.

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[spanky1205]

Thanks for the infrared temp gauge idea, that will be easier than anything I would have dreamed up. I no longer have all of the TBI stuff installed on my pickup, currently it is in a box hidden away just in case I decided I want to dive into tuning and rewiring everything. For now I am running an engine I picked up on the cheap since the original had a bad knock and something that looked like sand everywhere inside. Since I picked up the truck to learn from I plan to get everything fixed up and running well, then when I have ran out of projects to dream up I may look into something like megasquirt since the engine will be no where near a stock TBI setup by then. That is way down the road. Thanks again for the temp gauge idea.

 

[1987 GMC Jimmy]

Thanks for the infrared temp gauge idea, that will be easier than anything I would have dreamed up. I no longer have all of the TBI stuff installed on my pickup, currently it is in a box hidden away just in case I decided I want to dive into tuning and rewiring everything. For now I am running an engine I picked up on the cheap since the original had a bad knock and something that looked like sand everywhere inside. Since I picked up the truck to learn from I plan to get everything fixed up and running well, then when I have ran out of projects to dream up I may look into something like megasquirt since the engine will be no where near a stock TBI setup by then. That is way down the road. Thanks again for the temp gauge idea.

You’re welcome. Hope you can figure out what’s right and what’s not.

 

[spanky1205]

I started my truck tonight and let it run through a few cycles of the fan turning on and off. While it was running I recorded the temperatures of a few spots on the engine. The problem is I have no previous experience to go by. I am hoping someone with more experience that me can look at the temps below and tell me if they are good or too hot.

Top of water neck: 175-193
Above coolant sensor in driver side head: 227-235
Top of radiator at hose inlet: 217-230
Bottom of radiator at hose outlet: 90-120

My guess is I am running too hot, what does everyone else think?

Thanks again,

 

[Charlie]

I usually use the infrared tester on the thermostat housing on intake. If your test "top of water neck" is that, then the reading seems about normal range. I have tried 2 other factory gauges other than the original and they all do not show same readings and if I remember correctly, other people have said they weren't always accurate. If you do not have boiling over when stopped after driving, then I think you are ok.

 

[spanky1205]

I usually use the infrared tester on the thermostat housing on intake. If your test "top of water neck" is that, then the reading seems about normal range. I have tried 2 other factory gauges other than the original and they all do not show same readings and if I remember correctly, other people have said they weren't always accurate. If you do not have boiling over when stopped after driving, then I think you are ok.

Yes, what I am calling the water neck is what you are calling the thermostat housing. I could not think of the name when I started writing the post, so I looked up the name on rockauto.com and that is what I found. Regardless, thanks for the info. It is appreciated.

 

[Charlie]

Yes, what I am calling the water neck is what you are calling the thermostat housing. I could not think of the name when I started writing the post, so I looked up the name on rockauto.com and that is what I found. Regardless, thanks for the info. It is appreciated.

 

[1987 GMC Jimmy]

I started my truck tonight and let it run through a few cycles of the fan turning on and off. While it was running I recorded the temperatures of a few spots on the engine. The problem is I have no previous experience to go by. I am hoping someone with more experience that me can look at the temps below and tell me if they are good or too hot.

Top of water neck: 175-193
Above coolant sensor in driver side head: 227-235
Top of radiator at hose inlet: 217-230
Bottom of radiator at hose outlet: 90-120

My guess is I am running too hot, what does everyone else think?

Thanks again,

The second reading concerns me. With a 180* stat, that reading should be 180 plus or minus a few degrees.

 

[75gmck25]

Just for comparison - I just installed a new digital temp sensor and gauge on the passenger side head of my truck. After idling for about 10-15 minutes in 45 degree weather, the temp was running about 175-180 with a 195 degree high flow thermostat. It did get up to nearly 195 before the thermostat opened, but then dropped down when water started to flow. Even after driving for a while it will not run more than about 190-195 degrees in this cooler weather. I have a Stewart Stage 1 high flow water pump, stock shroud, stock steel fan, and HD thermostatic fan clutch.

I also have an infrared tester, and could check to see how close the head temp is to the temp reading on the new gauge. It may be that the exterior of the iron head gets hotter than the temp of the coolant, but I don't really know.

Bruce

 

[highdesertrange]

infrared thermometers work good but keep in mind they will show about 10° below what the actual coolant temp is. before you start buying more stuff check your sending unit and your gauge,

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with this chart you can check the sending unit and the gauge. highdesertranger

 

[hatzie]

If you compiled that chart based on readings from your sender it only applies to that year gauge and sender and other model years that used the exact same units.

The temp sensors across the model years of these trucks are a conundrum.

GM used at least four different temp senders and maybe as many as seven between 1973 & 1991.
They have kinda close but different enough temp to ohms that the needes will not rest at the appropriate place with the wrong sender...
To add to the fun the aftermarket "equivalent" senders are not spot on with the original GM parts... even more fun is the fact that the AC Delco replacement for the Nail-Head senders doesn't match the original GM part either...

I've tested the gauges in five gauge panels using a benchtop meter, calibrated decade resistance boxes, and a reasonably noise-free 0-30 VDC lab power supply set to 14.5V
My guinea pigs were 1976, 1979 tach, 1985, 1987, & 1990 clusters. The 85 & 87 temp gauges were very close to the same. 1976, 1979, & 1990 took noticeably different resistances to drive the temp needle to each mark.

They will work with the wrong senders but the temp gauge will be off a bit.

From 1977 on the fuel and oil gauges don't have any noticeable variance from year to year. Pre 1977 the fuel gauge is more linear. After 1977 GM weighted the fuel gauges to spend more time in the mid-range giving the appearance of better fuel mileage. There's a Chevrolet Service News article about this. Chevrolet Service News magazines eventually made their way into the wild and I've collected them.

 

[Snoots]

First thing, before you have started your truck for the day, what is the ambient temperature?
Get in your truck and turn your key to the 'on', not 'start' position. Where is your Temp Gauge pointer now?

What I'm getting at is, the 'baseline' or initial position of your gauge pointer should be well below the 100 level or first mark.

If not, I'd pull the gauge lens, carefully pull off the pointer and move it to a more appropriate reading.

Then base any further troubleshooting from there. I would not buy the 3-gauge setup you hinted at earlier.

 

[highdesertrange]

that chart I posted is for all years of GM with electronic gauges. they all read the same. highdesertranger