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Monday, September 07, 2020

OEM Temperature Gauges Lie : Why You Should Always Question Gauge Readings

Two bluecars : Calsonic Livery and #bluecar tv2 R32 GT-R
Two bluecars : Calsonic Livery and #bluecar tv2 R32 GT-R

Las Vegas can be very hot. Days and days above 100F are fairly common in the summer. Days above 110F have been the normal recently.  Since RentJDM is dealing with 25 to 40 year old JDM cars that were never designed for that kind of heat, I've been looking into some cooling system modifications. No one wants to rent a car and not be able to use the air conditioning. 

This is what I think of, when I look at most gauges. The top tape measure is off 3/16th of an inch

The above picture illustrates why you should always take unknown, uncalibrated, unchecked readings with a grain of salt.  Now this doesn't mean you don't believe everything, but you have to bounce those numbers against something else.  If you use that top tape measure all your measurements, all your cuts are going to be 3/16 of an inch off. Until you check that measurement against another tape measure, your whole life will be 3/16ths off.  So remember this when looking at temperatures, air inlet temps, air/fuel ratio monitors, and boost gauges.  Compression and Leak down testing

Some people live and die by a single point of AFR, boost, or a temperature. So while the reading on your car may be a certain number, someone elses may be off a little. Your car could read high, theirs low.  When I was in the Navy I did Gauge Calibration as part of my collateral duties.  For a gauge, generally speaking you are allowed a range or a percentage of the full scale to be off, and still be a "functional" or a gauge you will not replace. 


I am a skeptic when it comes to most gauges. Four years calibrating them on ships made me skeptical.  That doesn't mean that I don't believe anything they say, just not everything they say. 

Showing 183F coolant temp at the engine outlet. 
The ECUTalk showing coolant temperature that the ECU sees. This is coolant outlet temperature from the engine. 
This time the coolant temperature is 13 degrees F warmer, still sort of ambiguously in the middle of its range. 


I joke around with people that the factory coolant temperature gauge might as well be hooked to a slip and slide, aka it doesn't really tell you much, and to prove that, the above readings are from a car in a 100F day here in SoCal. AC on, and sitting parked, driving on the street. The high temp is after shutting the car off, and then starting it back up. Just showing what some temps can easily get to at the sensor. This is significant, because an air bubble or pocket here, can see "water" temps in the 250F range. There is a point in an ECU, that if the water temp is too hot, it will NOT put fuel in the engine during cranking. Most cars will trail fuel off as temperatures increase. Many ECU's go down to nearly nothing as they max out.  I've seen it on many track cars, and helped many people fix this issue. 


This is before starting the car. Just showing how hot the sensor can be after sitting on a hot day


In the above reading at least the OEM sensor is showing fairly hot in its range. Still not to the "H" mark.  242 F  116 C at the engine.  If you make it to the H mark, you have probably fairly cooked it on the water side. 


This R32 GT-R is over 25 years old, and has a factory radiator in it. It also has the factory oil/water heat exchanger. Nismo lip on the hood. No Nismo ducts in the bumper. It has an undertray. I added some foam to the top of the radiator to try and keep air from escaping around the top of the radiator and core support. It has the factory radiator shroud. It has the factory fan clutch. The factory air conditioning cooling fan works as designed.   Cruising on the freeway and driving normally in a 100F day, saw outlet temperatures ECU readings of about 201F to 208F.  Closer to the beach, with cooler ambients, I saw temperatures drop down to 192 F.  As the ambient temps increased, the temperatures rose again to the 201F to 208F range.  All this on the factory temperature gauge is more or less ambiguously in the middle.  192F and 208F look to be about the same pointish.

The ambients here in SoCal are probably not going to be enough to really push this system, but I have some things I want to try and change. Some of which are some more ducting/cooling panels. Smaller water pump pulley to overdrive the pump(this car is mostly low speed in hot temps).  Maybe a new radiator. Cooling system flush and coolant replacement. Higher pressure radiator cap. GK tech fan. New fan clutch. 



RB26dett Coolant flow diagram. Notice the inlet and outlet paths



Coolant Capactiy (liters)
RB20/RB25DE/RB25DETRB26DETT
Capacity~9~9.2


Nissan Skyline Radiator Cooling System Fill and Bleed Procedure
Nissan Skyline Radiator Cooling System Fill and Bleed Procedure


Coolant temperature sensor 2 wire and 1 wire

Two wire coolant temperature sensor for an RB engine. According to NZ EFI, this fits about 50 different Nissans. 


The 2 wire sensor, is the sensor that the ECU uses as an input. It is a very important sensor. It effects startup, and running of the car.  We often see the cooling system not bled right, and an airpocket forms at the sensor. When this happens, the sensor may show 250F-300F. When this happens, the ECU thinks the car is hot, and it may not fire the injectors, based on the tune. So if you have hot start problems, that go away once you crank for a bit, the CTS is a good thing to look at, then the bleeding.  This gets back to having a Consult datascanner, so you can see what the ECU is seeing. Makes troubleshooting much easier. 

The 1 wire sensor only drives the gauge in the dash.  A cool fact, R32 and R33 are different resistances. You find these things out later, when you start a car up, and have the wrong sensor in. Then about 2 minutes in, the gauge pegs, the car is fairly cool.  NZ EFI says that the R34 is also different, need to look that one up. 



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