rao

iTrader: (18)

I have seen lots of weird things playing around with these engines for the past several years, but this one has me stumped.

The drivetrain is a 402 LS2 using 2001 F-body electronics. This setup has been running for 13k miles and it runs very well I am very happy with the tune. Over the winter I moved the drivetrain and all electronics from one chassis to another and the new chassis is performing just like the old one.

Lately I started noticing that the fans were not cooling as they should so I started investigating. I replaced the thermostat (stock 186 temp) water pump, played around with the fan, etc. and nothing made any difference.

I then (way too late ) pulled out my IR thermometer and determined that the ECT sensor is reading 15-20 degrees too high , which would explain what I was seeing and also made me realize that I had noticed that the car seemed to warm up very fast - like 20 degrees by the end of my driveway.

I swapped in a new ECT sensor and the numbers are the same. I then checked the circuit and I the resistance at the sensor vs at the PCM are within 10-15 ohms, which is pretty small given the overall numb er (~3400 at ambient). I also swapped in a spare PCM and it is the same I checked the signal voltage at the sensor connector ant is was 4.99 volts.

So at this point I am stumped. I can adjust the fan temps, that is no big deal, but I would really like to figure out what the heck is going on.

I would appreciate any ideas and sorry for the novel.

flame

iTrader: (13)

Just to make sure I understand you. The scanner reading is 15-20 higher than your IR reading? Or the dash gauge is reading higher?

rao

iTrader: (18)

The discrepancy is with the scanner reading. For example on cold startup this morning when the outside temp was 40, immediately upon starting the car the IAT was 40 and the ECT was 60. The IR reading of the thermostat housing, water pump, radiator and the ECT sensor itself was also 40.

joecar

Measure the resistance of the harness wire between the ECT connector and the PCM connector.

jimmyblue

iTrader: (11)

The ECT sensor reads water temp. IR thermometer
reads the surface temp of the block / heads. The
surface temp is bound to be lower than the water
jacket because the air takes away surface heat.

Might try shrouding the outlet radiator hose to
eliminate air cooling, and point the IR gun at that,
right where it comes out the thermostat.

rao

iTrader: (18)

The resistance of both wires measures 0.2 ohms on the most sensitive scale on my meter - touching the probes together reads 0.1 ohms.


I appreciate the thoughts, but as I mentioned the ECT sensor reads 20 degrees over ambient temps after the car has been sitting for 14 hours in 30 degree weather. Actually the coolant goes into the thermostat housing, not out of it.

The IR gun is consistent with the temps at the coolant outlet, upper and lower hoses and I have checked it against known coolant temps on other cars and it is fairly accurate, not to a tenth of a degree but certainly closer than 20 degrees.

eallanboggs

iTrader: (1)

This is a high failure item for GM over many models. Aftermarket ECTs as well as OEM sensors have the failure. This is a thermistor that outputs a voltage based upon the water temp. Check your reference voltage(5Vcd) as well as the connector pins. Do a drag test. I've been taught that GM says the connector should be replaced once it has been removed and reinstalled 5 times. I guess it fails the drag test at the point, but that seems to be overly conservative.

rao

iTrader: (18)

I have checked the reference voltage and it is right on the money - 4.999 volts. I also checked 2 sensors and they report the same temp.

Excuse my ignorance but what is a "drag test"?

Thanks.

flame

iTrader: (13)

It checks the female terminal to make sure that there is some drag when the male terminal is removed. Sometimes the female terminal opens up and doesn't make enough contact.

rao

iTrader: (18)

Got it. Makes sense. I will check that out.

The strange thing is that if thee was a loose connection, the resistance would be higher and/or the voltage lower which would result in colder readings not warmer.

eallanboggs

iTrader: (1)

You might also want to monitor the sensor output as the engine warms up by backprobing the connector. Does the sensor output voltage changes as soon as you start the engine(it shouldn't). The output should change gradually as the engine slowly warms up.

rao

iTrader: (18)

I will have to backprobe it to know for sure, but from scanning it the voltage doesn't change upon startup and the temp reported by the sensor rise gradually - it all seems to work normally, except that it is 15 degrees too high.

Thanks for the ideas

eallanboggs

iTrader: (1)

That's why I suggested you look for a rapid change when the key is turned to RUN. The voltage change should be gradual as the temp changes. Don't forget that a scan tool is not nearly as accurate as Moses stone tablets. Before you see that value the process is very involved. It goes from the sensor to the ECM to the network DLC where your scan tool interface reads the GM protocol and uses your software to finally display the value on your laptop. Judge Judy would call that 4th hand information. Backprobing is much more accurate. You can even connect the probe to your oscilloscope to see a very precise value.

rao

iTrader: (18)

I hear you.