Good morning all,

I’ve got a suburban with an overheat issue, that I’m going to start into today. I’m asking mainly for quick troubleshooting tips. I don’t want to drag out a common thread like this, but I also want to make sure I’m not missing any information.

The problem:
After you start it, the temp gauge climbs all of the way up. It’s a steady, slow climb.

The plan:
I’m not 100% sure it is a legitimate overheat, so I will use an infrared thermometer to cross check the gauge. I’ll start by checking and clearing codes. I will then pressure test the coolant system to check for leaks. If the system can hold 15 psi, I’m then going to pull the thermostat. After I reassemble the housing, I’ll pull the upper radiator hose and briefly start the truck to verify water pump is working and flowing well through the engine. Beyond that, I don’t know what else to check.

The story so far:
The owner let me know that she kept up with all regular maintenance like brakes, rotors, oil and tune up, also replaced both cv axles, oil pressure sensor, air filer, tires. Since this has happened, she has replaced the thermostat, the water pump, the radiator, and the coolant temperature sensor.

Summary:
I don’t know what changes GM has made over the years, so I don’t know where to focus my time if the above checks out. Any help would be appreciated.

 

If you’re still trying to figure this out, here’s some info that may help:

1) The temp gauge is logic controlled. The coolant temp signal is received by the PCM then sent via data circuit first to the BCM, then on to the cluster.

2) The PCM supplies a 5v reference to the ECT. With the connector unplugged, Check this voltage to see if it’s present with the engine running - a range of 4.5-5.5 is acceptable but anything above or below would be reason to suspect circuit problems.

3) One of the easiest tests is to unplug the ECT sending unit with key on, engine off. This should cause the gauge to drop below its minimum reading as the PCM interprets an open circuit as -40*F. If the gauge does go full cold, try starting the engine with the ECT unplugged - if the gauge slowly rises as before, and all the other gauges/indicators are working, the cluster is likely at fault.

 

Thank you for the feedback. It is always appreciated.

This one ended up being a simple fix. The new replacement thermostat was bad from stock. I pressure tested the coolant system (I had to bypass the reservoir) and it held pressure. My next step was to pull the thermostat. I started the truck and felt the upper radiator hose for fluid flow (I could feel the bubbles pushing past my fingertips). Once the temperature stabilized, I took it for a test drive. The rest is history.

 

Great !