03 2500hd running too cool

Mattt · April 20, 2020

Recently bought an 03 2500HD Silverado with 6.0 gas engine. Ever since I've bought it, it runs at 160-170* Since I was not believing the gauge cluster temp gauge, I plugged in my ScanGauge II and it coincided as far as engine temp. Is there only 1 temp sensor on the lq4 6.0 that provides data for the dash gauge & for the EFI management? That sensor is on the front end of the driver's side cyl head, correct?

Initially I was thinking a prior owner dropped in a lower temp T-stat, but I have my doubts now. I have checked temps on this engine with a laser pointer handheld temp reader after an average 30 minute drive and compared the same temps with my 04 Tahoe. The temp on both cyl heads & driver's side near the sensor is ~190*. Seeing same temp where the water leaves the top water pump neck and heads to the top port of the radiator. Checked t-stat housing and it's ~150* so the radiator is doing it's job.

Is there any sure fire way to discern between a lower temp T-stat or faulty thermostat or a faulty temp sensor on the engine? I'd hate to start throwing parts at it in hopes of finding the source of the low temp.

I think the truck may possibly not be going into closed loop due to the low engine temp, fuel mileage is suffering, and the throttle seems touchier than it should be. Also have a CEL that is for low engine temp, but I can't remember the P code. (edit: pulled code again, P0128, low coolant temp)

Thank you in advance for any and all advice.

davester · April 20, 2020

The engine goes into closed loop mode WAY before the engine gets up to even 150F. It takes only a minute or two to switch to closed loop mode.

I would suggest checking for intake manifold leaks, broken off/missing exhaust manifold bolts first, then a good code reader (I HIGHLY recommend getting a knockoff tech II, they are relatively cheap now) to both get codes from all the computers, as well as display live data like fuel trims, see if it's going into closed loop mode, how the knock sensors and the upstream O2 sensors are doing.

Second thing I would recommend is for you to sign up to an online site like alldatadiy.com , to access the full-service manual for the truck, which will tell you the diagnostic procedure for seeing where the problem is with the temp gauge, as well as how to get your engine running better.

SierraHD17 · April 20, 2020

So are you saying you shot the cylinder head with a temp gun and are getting 190 like your Tahoe yet your sending unit is still showing 160 to 170? You have troubleshot it at that point and a temp sensor is a $20 part. If you really want to go the extra mile and can find a resistance chart for the temp sensor you could verify that the temp sensor is outputting the resistance for that 160 degree reading when head temp is 190 and be able to rule it faulty.

https://ls1tech.com/forums/pcm-diagnostics-tuning/1508642-anybody-have-ohms-vs-temp-table-gm-coolant-temp-sensor-40-f-284-f.html

That includes the resistance chart for an 05 gm truck which is identical to your 03 for that anyway...

Formulabruce · April 20, 2020

I put in a 160 T-stat Every summer, and run the stock one in the winter.

This really keeps engine and more importantly the Trans temps way down during towing.

My engine Always runs in Closed loop with the 160 T stat. A/C works better too.

If it has one in it during the winter, it can be a bit too cool running.

In the summer, unless sitting for an hour, mine runs at around 180-185 while moving, with a load.

If you live in the south, or southwest, I would leave it alone as it could save your transmission.

Mattt · April 23, 2020

It turned out the sending unit was the problem. I replaced it with another one and the dash gauge is now reading a normal ~200* temp, which matches the ScanGauge II and the laser thermometer readings. The touchy throttle did seem to subside as well. I suspect with the ECU receiving the incorrect engine temp, it was throwing the driveability calibration off. Thank you for the replies above everyone.