Which temperature gauge to trust? Or, how hot is too hot?

[jangell]

First: 3.0L EFI here.

I replaced the water pump a couple weeks ago. I noticed the engine temp was a little high the other day (near 220, according to the dash gauge), so I bled the system at the radiator, which I forgot to do last time (I have a self-bleeder at the water pump).

Today it was running a bit hot about, about 7/8 of the way to 220 from the unlabeled tick. But this time I had my laptop — TunerStudio showed 201 degrees. This was at highway speed (around 70 MPH in this case). It was 82 out today.

So, which gauge should I trust? MeagSquirt is reading from a sender mounted on the water pump, while the gauge is still using the original sender location on the back of the cylinder head (I think). Or perhaps both gauges are correct, since they’re measuring different locations?

I bled the radiator again when I got home. There was a hiss, some steam, and then coolant after a second or so, so it looks like I still had air in the system, assuming I’m reading that right.

I consider 220 to be too hot. I know it’s not a red line, but I’m paranoid from overheating issues in the past with the old engine. I also have a 2.5 hour trip each way next weekend, so I’d like to rule out any high temperature issues before then.

Any advice is appreciated. Thanks!

— Joe

[jangell]

And here are some pics of the gauges that I took while on the road. Apologies for the quality.

— Joe

[Bitsyncmaster]

The correct sender is very critical to have the gauge read correctly. I did some sender testing and I did get three senders reading the same. The gauge like most old automotive gauges compensate very well for car voltage changes.

I will look for that thread I did for the sender results.

Post #37 has the sensor values for temps. Other posts in that thread also have gauge testing

http://dmctalk.org/showthread.php?15...tterstat/page4

[FABombjoy]

All of the factory gauges should be considered rough estimates. They're indications will change with battery voltage, ground condition, etc. Trust the Megasquirt gauges more!

[Spittybug]

Has either one been calibrated? Procedure to do so is in MS instructions....

[FABombjoy]

If you're feeling enterprising, there are also methods of implementing AC control through megasquirt which can shut down AC in an overheat condition.

BUT look at your gauge RPM vs MS RPM. The original gauges really are rough estimates.

[David T]

You should check your gauges against an infra red non-contact thermometer. Where the sending unit is makes a difference and they could be off a few degrees. The place to check is the pipe the "O" switch is in once the thermostat opens.

[jangell]

Sorry for the late reply; I missed the notifications:

I've bled the radiator five times now (I get home or to a parking lot and bleed it). Each time there's a hiss of air before the coolant comes out. Thankfully I have the DPNW valve thing that you can just turn to bleed, rather than having to remove the hose, so it's pretty easy to do. I don't appear to have any leaks and the coolant level looks fine in the bottle, but each time I bleed it the engine temperature seems to get lower at highway speeds according to the stock gauge. Today, it was reading not too much higher than the mark between 100 and 220, and TunerStudio was showing about 197. This was about 10 minutes of highway driving, and it's about 80 degrees out and sunny.

I'm not sure if either sensor has been calibrated. I don't know how I'd calibrate the one for the stock gauge. My notes say I'm using the stock 2.8L sender in the head (the 3.0L sender was DOA). I'm guessing that the coolant temperature should be about the same for most engines, and that it isn't just that the 3.0L runs hotter than the 2.8L.

My note also say nothing about having calibrated the water pump sensor, so it is quite possible (re: likely) that I didn't. I feel like it was on my todo list and I forgot to actually do it. Once the car cools down I'll check the MS manual and see what it says to do. The fans are turned on by this sensor (and they do work), but if the head sensor is the right one it would mean that I'm letting it get too hot before the fans turn on. It looks like they're off by about 15 degrees -- the gauge reads a little over 220 when MS reads about 205.

I do tend to trust the tachometer in MS over the stock gauge, although they look reasonably close to me. That said, I haven't tried having the car sit at 1000 RPM and comparing the two either, so they might be more off than I think they are. I should do that sometime. Eventually I'm going to do that digital dash I keep threatening to do and then MS will be my only gauge.

I plan on adding AC idle up, once I add the circuit to my MS II hardware (next weekend I'm taking a 2.5 hour drive each way, so I'll do it after that). I didn't think of shutting off the AC if the engine gets too hot. I'm working on touch-screen climate controls as well; maybe I should have it turn the heater on automatically in those situations as well (my software could turn off the AC too, I guess, but I can't increase the idle when the AC is on, so I'll have to do the MS mod no matter what).

I have an IR thermometer, so I'll warm it up and see what it reads at on the otterstat pipe.

Thanks everyone!

-- Joe

[jangell]

I just did the IR thermometer test. I had trouble getting a good reading off the shiny otterestat pipe. It would read around 165-175, but it would be over 200 if I moved the thermometer body too close to the engine bay so that the waste heat from the exhaust system would interfere with the sensor.

I got a more expected 190-195 on the hoses at the water pump. The MS gauge would read 195-205 during this time (the fans were coming on and off, since I have them set to around that range). The gauge in the dash was reading a few needle widths under 220.

Here's a picture of where I took my samples.

Napkin 3 08-04-19, 12.39.02 PM.jpg

Thanks!

-- Joe

[jangell]

I finally pulled the coolant sensor from the water pump and calibrated it. The boiling water test got 190 ohms at 200-210 degrees; I'm a little fuzzy on that because the meat thermometer I was using would change depending how far I put it into the water, and an IR thermometer was useless as expected in this context (198-208, aimed at the sensor through the water).

Ice water came out to about 9330 ohms, and room temperature (about 75 degrees) was around 3100 ohms. I plugged these into TunerStudio and it consistently read about 5 degrees cooler than the GM settings I was using. I decided to set it back to GM, because I'd rather have it read too hot than too cold, and I'm not totally confident that I did this right. It'd be nice if they printed parts numbers on these things and provided datasheets with the temperature curves...

I went for about 20 minutes of driving on the highway; it's 81 degrees out and sunny today. Engine temperature in TunerStudio ranged from 198 to 202 when in the 70 MPH range, during which the stock gauge was up around 3/4 of the way between the first unmarked tick and 220. It only got higher than that when I turned off the A/C on surface streets and go stuck behind a slow truck (around 20 MPH); TunerStudio went up to 207 and the stock gauge moved 7/8 of the way to 220. It cooled back down after the fans came on, and went back to around 198 when I was driving at 35-45 MPH.

One other thing is that I forgot to put more coolant in after bleeding the at the radiator, which was now sitting at around the level of the hose in the back instead of half full. Once I topped it off, subsequent bleeds almost immediately dumped coolant with very little air.

So I guess this is just how hot my car runs, or that the gauge just reads 10-15 degrees hotter than TunerStudio. Tomorrow is my 2.5 hour drive (primarily highway driving), with another 2.5 hours back on Sunday, so we'll see how that goes. The temperature was pretty stable on the highway today, so I don't think I'll have any problems.

Thanks!

-- Joe