Symptoms Of Bad Coolant Temperature Sensors - AAMCO Colorado

What Does the Coolant Temperature Sensor Do?

Basically, your engine coolant temperature sensor (CTS or ECTS) monitors the temperature of the coolant circulating through your car’s engine. In turn, this gives an idea of how hot your engine is. If your engine is overheating, it’s up to the coolant temperature sensor to pick up on that information and trigger the dashboard light, as well as any other responses your car is set to carry out.

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Many modern cars will automatically shut off the engine once it reaches a certain temperature to protect it from excessive damage. If your coolant temperature sensor isn’t doing its job properly, the engine may not receive the signal to shut off  and you might find yourself facing serious engine damage.

Poor Fuel Economy

Part of the coolant temperature sensor’s job of accurately reporting your car’s coolant temperature is to ensure the correct mixture of fuel is used. A higher ratio of fuel is used the cooler the engine’s temperature, so if your ECT is reporting a cool temperature when your engine is actually warm or hot, more fuel will be used than necessary. A failing coolant temperature sensor will commonly report a cooler temperature than is accurate.

Irregular Temperature Readings

If you notice more fluctuation in your engine temperature or it looks higher or lower than usual while you’re driving, it could be the engine coolant temperature sensor. In particular, if your engine doesn’t seem to be warming up as it typically does, you should have a mechanic take a look at your cooling system. While it could be cooler outside temperatures, it also is likely that the temperature being reported is inaccurate and lower than it actually is.

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Black Smoke from Your Exhaust

Along with the fuel ratio being controlled by the coolant temperature sensor, if there’s too much excess fuel being used, it may not be able to burn off in the combustion chamber. When this happens, you’re likely to notice black smoke coming out of your exhaust because the remaining fuel is being burned off there instead. While a little bit of black smoke won’t render your car undrivable, it is an indication that you should avoid driving your car and instead get it to a professional mechanic for a cooling system repair service.

Your Engine is Overheating

While a failing engine coolant temperature sensor may give a permanent cold signal, it can also malfunction in a way that sends a hot signal when your engine temperature is safe. In this case, your car will tell you that your engine is overheating when it actually is not, meaning you’ll likely have a dashboard warning light on and your car may not operate properly.

Your Check Engine Light is On

Alongside the signal that your engine is overheating, you may see your check engine light come on if your coolant temperature sensor is failing or has failed. If your car’s computer senses a problem with the signal your sensor is sending, it may trigger the check engine light. Regardless, your check engine light coming on is a sure sign you should have your car checked by a trusted mechanic.

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First some background on the car;

• with 123,xxx miles
• SOS Stage One build for Aerocharger twin turbos. 800 miles since rebuild
• SOS WTA intercooler mounted in standard position in front of the lower radiator
• OEM radiator replaced last year with new Koyorad radiator and running Honda Blue Coolant
• Current horsepower is 450 on pump and 525 on E98
• Can bus GaugeArt display

Currently in Arizona it's 100F ambient. I have seen coolant temps on my display at over 210F and thought that maybe the thermostat was failing so I ordered a Stant (part number ) which didn't fit. So I put the original back in. Filled and bled the system (twice) and still have high temps, or at least they seem that way to me. Maybe because I now have a digital readout where before the OEM gauge stayed pretty rock solid. Took a drive and the coolant temp ranged from 192F to 200F, depending upon moving or stopped at a light. Oil temp was between 205F and 220F. Ambient temp was 100F. So some questions and possible fixes;

1. Maybe now that i can see actual readings, don't sweat it. This is normal? What temps do you normally see on a warm day?
2. Could the t-stat be stuck partially open? Takes around 8 minutes at idle to warm up to 185F and then settles between 192F and 199F.
3. Radiator fan kicks on at 195F. And if the AC is on, the radiator fan remains on, even if the coolant temp drops below 188F. Is this normal?

I'd like to get the car back on the track but I don't trust coolant temps yet. The intake shroud is tight against the radiator and the OEM fan and shroud are in good shape and working. Just wondering if there is something simple and not too expensive to correct this problem. That is, if there even is a problem. I guess i could;

1. Move the intercooler farther away from the radiator with spacers to give more room and possible air flow to the radiator
2. Replace the thermostat regardless
3. Replace the OEM fan and shroud with a twin SPAL fan and shroud with a new 40A fuse
4. Replace the relatively new Koyorad with the latest MASIV
5. Install an oil cooler

I'd like to get this done sooner than later and with as little money as possible. Looking for Hrant, RYU, Coz and DocJon for some assistance. Thanks in advance. I can't advise on specifics for your car; but, I can provide some observations that may be relevant based upon my fiddling around with the installation of digital fuel injection on a truly vintage engine (think cast iron lump and push rods).

First off, under most circumstances the the coolant temperature is never steady. I suspect that car thermostats tend to be like the thermostat in your home, they are bang - bang controllers. Pretty much either fully closed or fully open (on or off in the case of your home thermostat). I have stuck a thermocouple under the hose that exits from the thermostat on my vintage lump and at idle and low output operation you can definitely see the coolant temperature swinging as the thermostat is going open and then closed. The temperature swing is around 3 - 5 C on that car. The frequency of the swing changes as engine load increases becoming slightly longer at higher loads as the thermostat stays open longer at higher loads. That frequency and the magnitude of the temperature swing will be a function of the temperature setting(s) on the radiator fan, the cooling capacity of the radiator (which is fan and vehicle speed dependent) and the hysteresis in the thermostat. An observation here is that if the thermostat goes fully open and stays fully open you no longer have any active temperature control. If engine load increases further beyond that point the coolant temperature can only go up and the only limit to going up further is an increase in radiator effect (more air flow through the radiator or a bigger radiator).

The short answer here is that if you can see those temperature fluctuations in the coolant, that is a probably a good thing. First, it means that the thermostat is going open and closed (its working) and second the fact that the thermostat goes closed or approaches going closed means that the cooling capacity of the radiator is in excess of the waste heat energy being produced by the engine. According to the FSM, the thermostat starts to open at 76 - 80C and is fully open at 90 C. The low speed rad fan switches on at 84 C and off at 78 C so its dead band overlaps the thermostat dead band so will tend to modulate the swings caused by opening and closing of the thermostat. The fan switches on before the thermostat goes fully open so under some conditions the thermostat may not get fully open reducing the potential temperature swings. Again, short answer is that as long as the temperature is swinging and the upper limit is at or a nudge above the thermostats full open temperature your cooling capacity is exceeding the engines cooling requirement (at that particular engine load). If the coolant temperature goes above the thermostats full open temperature then you have exceeded the control range for the cooling system. That might be OK for short periods of time or as long as you do not exceed the temperature / coolant temperature that results in persistent venting.

Your temperature swings of 192 - 200 F (and up to 210F ) are higher (both the bottom and the top) than what the FSM says for the thermostat operating range. However, the problem may not be the thermostat; but, your temperature measurement system. Temperature sensors, typically RTDs have tolerances that can lead to errors. The other big problem is the formula that your logger or your ECU uses to determine temperature from the sensor resistance measurement. ECUs, loggers etc typically use a 3 point calibration curve to establish the coefficients in a typical 2nd order equation that the ECU uses to calculate the temperature from the resistance. The standard method has a formula named after somebody and there is a Wikipedia page describing the method, I just can't remember the name right now. The problem is that the curve generated by the equation typically does not match the RTD perfectly. As an example, I have attached the resistance curve for my 'vintage' coolant sensor that I measured using a water bath (circle data points). On the same sheet I plotted the resistance calculated from the formula with a three point calibration at 10, 30 and 50C (triangle data points). You can see that at high temperatures the formula generated temperature curve is well below the measured curve. In this case causing a temperature display measurement that is about 10C below the true temperature at 94 C. However, by changing the calibration points (spreading them out or shifting them up or down the measured curve) you can change the shape of the formula generated curve and can generate errors that swing the other way.

View attachment

In the my case, I use the ECU to control the radiator fan. In order to get more accurate fan control I had to move the calibration points to get the calculation curve to more accurate at high temperatures for fan control. The down side to doing that is that it messed up the accuracy of the low temperature measurements which screwed up my cold start and warm up routines. The NSX doesn't have that problem because fan control is provided be a separate dedicated RTD. The temperature sensor for the ECU is used for cold start and warm up control and you check coolant temperature with a separate sensor for the dash gauge.

So, my suggestion is that your first step would be to confirm that you actually have a cooling problem versus a measurement problem. Since you are using an aftermarket ECU, you must have entered a calibration curve for the sensor into the ECU. If it is a three point calibration curve (super common) get the calibration points and then get the formula for the three point curve to figure out whether your calibration curve is generating skewed readings at high temperatures. If it is, you may be able to alter the calibration points to improve the accuracy at higher temperatures. You should also pull your thermostat and do a water bath test to confirm that it is starting to open and fully open at the values in the FSM. It would also be advisable to pull the ECU temperature sensor (if that is where you are getting your gauge art measurement) and do a water bath calibration measurement on it to get a good calibration. If the tiny little curve in the FSM was used to get the calibration points the thickness of the line on the curve is probably > 5 C leading to the potential for significant measurement error at higher temperatures. There are also production variations between sensors that can lead to significant errors in high temperature measurement.

If it turns out that your measurement system is accurate and you truly are running hot somebody else will have to help you with that because I have no experience with that on an NSX. The one solution related observation that I will make is that unless an overheating problem is occurring at low vehicle speeds, different / bigger radiator fans are not going to fix the problem. At 100 km/hr the air flow through the radiator generated by the fans is irrelevant. The fans probably create a slight impediment to air flow at high speeds. I gather you are using the AEM to control the radiator fan rather than the OEM fan control unit? If that is the case I am guessing that you only operate the radiator fan on high speed? Your 'on' at 184 F and 'off' at 178 is perhaps a rather small dead band. 184 F is good for the on temperature as it is effectively the Honda value for on temperature for the low speed fan. Honda shuts the fan off at 172 F so you are shutting yours off a little earlier. The only potential down side to that is that the relay and fan is going to cycle on - off a little more frequently.

The 194 F 'on' setting is definitely suboptimal; causing the engine to run a little hotter than normal; but, should not be responsible for running really hot. I am not sure what would happen with the off setting at 205 F. I am not familiar with AEM; but, normally the programable outputs would be conditioned with a > or <, with the on temperature being conditioned as > 184 F. If the off setting is conditioned with < (which would be logical), above 194 F the on setting would be telling the fan to run; but, the off setting would be telling the fan to shut off until the temp reached 205F. If the off setting was conditioned with > then the fan would presumably shut off above 205 F; but, the fact that the on condition ( > 184 F) is still met it should be switching the fan on, so the AEM could be trying to turn the fan on and off at the same time. Depending on how the AEM managed the screwy fan settings, you could have had high coolant temperatures during low speed driving if the fan was switched off. However, at highway speeds the radiator fan is pretty much irrelevant. Regardless of what the AEM is doing with the rad fan, if you are still reading really high coolant temperatures at highway speeds you either have a cooling problem or your temperature measurements are incorrect.

You might want to go back to your tuner and ask them what the plan was when they configured the radiator fan outputs, Turning the fan off if the temperature goes above 205 F is counterintuitive regardless of how fast the car is or isn't moving.