2009 Silverado 1500 5.3l flex overheating in traffic

Trevor Jones · February 12, 2020

Hello, I have a 2009 Chevy Silverado 1500 5.3l flex fuel that has 135k miles and I cannot figure out why it is over heating. I have flushed/changed the coolant, changed the spark plugs/spark plug wires, changed the thermostat/reservoir cap, changed the Mass Air Flow sensor/cleaned the k&n air filter. The radiator is about 3 years old aswell as the hoses/belt. I have tested the radiator fans by turning on the a/c and they run but seem to not want to run with out the a/c on but they both do kick on. The engine won't start to run hot unless I am in drive and in stop and go traffic, putting the truck in neutral and running the heater usually stops the heat from creeping up long enough for me to get moving again which quickly gets me right at 210. The truck has never fully over heated more than an 8th above 210 but I have had it for 11 years and it has always stayed right at 210 so I know something is not right. The passenger side wheel well is showing signs of coolant leaking but not enough for my reservoir to drop below the cold fill line. I'm thinking water pump or something to do with the radiator fans but I figured I would come here first as it's really starting to impact my wallet. The first time I noticed the temp running hot was about 3 months ago driving to los angeles from dallas, the coolant in the reservoir somehow expanded so much it filled the reservoir to the top and was leaking out of the bottom so I replaced the reservoir cap as I assumed the pressure was failing, seemed to do the trick as I had no more issues from new mexico all the way to Los Angeles. Also this is my first post so if this is in the wrong location or if I did something wrong please let me know. Thanks in advance to any suggestions.

Edited February 12, 2020 by Trevor Jones
Added total mileage

swathdiver · February 12, 2020

There are three relays for the fans, are they all working? Thermostat could be stuck open. Pump impeller blades could be wore out but I would not think so with those miles, I've got 20K more on mine with the original pump!

BigBadSierra · February 12, 2020

The entire GM product line has been known to have premature water pump failures. So check the things above first but it could still be a pump. Allot of pumps usually have a weep hole though start to leak when they fail.

So this may be your leak like I said they weep so not allot comes out but the pump is compromised.

The simple thing is to test the cool-lent. Modern engines actually rely on it to run cooler.

Trevor Jones · February 12, 2020

Thank you guys, I really appreciate the suggestions. It's gonna be a few days before I can get to it but this weekend I'll get back to you with an update.

Edited February 12, 2020 by Trevor Jones

It's Tim · February 12, 2020

Verify temps with an IR heat gun.

xranger90 · February 13, 2020

9 hours ago, BigBadSierra said:

The entire GM product line has been known to have premature water pump failures. So check the things above first but it could still be a pump. Allot of pumps usually have a weep hole though start to leak when they fail.

So this may be your leak like I said they weep so not allot comes out but the pump is compromised.

The simple thing is to test the cool-lent. Modern engines actually rely on it to run cooler.

We're going to work on buddy's new Silverado next right after we finished installing the brake kit and continental tires on the Wrangler project. The previous owner of Silverado said it got intermittent overheat reading and could be due to pump acting up. Gonna test the coolant as well.

carnau · February 13, 2020

18 hours ago, Trevor Jones said:

Hello, I have a 2009 Chevy Silverado 1500 5.3l flex fuel that has 135k miles and I cannot figure out why it is over heating. I have flushed/changed the coolant, changed the spark plugs/spark plug wires, changed the thermostat/reservoir cap, changed the Mass Air Flow sensor/cleaned the k&n air filter. The radiator is about 3 years old aswell as the hoses/belt. I have tested the radiator fans by turning on the a/c and they run but seem to not want to run with out the a/c on but they both do kick on. The engine won't start to run hot unless I am in drive and in stop and go traffic, putting the truck in neutral and running the heater usually stops the heat from creeping up long enough for me to get moving again which quickly gets me right at 210. The truck has never fully over heated more than an 8th above 210 but I have had it for 11 years and it has always stayed right at 210 so I know something is not right. The passenger side wheel well is showing signs of coolant leaking but not enough for my reservoir to drop below the cold fill line. I'm thinking water pump or something to do with the radiator fans but I figured I would come here first as it's really starting to impact my wallet. The first time I noticed the temp running hot was about 3 months ago driving to los angeles from dallas, the coolant in the reservoir somehow expanded so much it filled the reservoir to the top and was leaking out of the bottom so I replaced the reservoir cap as I assumed the pressure was failing, seemed to do the trick as I had no more issues from new mexico all the way to Los Angeles. Also this is my first post so if this is in the wrong location or if I did something wrong please let me know. Thanks in advance to any suggestions.

Ok, so first lets discuss how your cooling system works.

The engine cooling fan system consists of 2 electric cooling fans and 3 fan relays. The cooling fan relays are arranged in a series/parallel (S/P) configuration. This allows the engine control module (ECM) to operate both fans together at low or high speeds using 2 fan control circuits. The cooling fans receive positive voltage from the cooling fan relays, which receive battery positive voltage from the underhood fuse block. The cooling fans are supplied a ground.

For low speed, the ECM applies ground to the coil side of the cooling fan low speed relay. This energizes the coil and applies voltage directly to the left cooling fan through the switch side of the low speed relay. The left cooling fan is connected in series to the right cooling fan through the de-energized series/parallel (S/P) cooling fan speed control relay. The result is a series circuit that operates both fans at low speed.

For High speed, The ECM applies a ground to the coil side of the cooling fan low speed relay, the S/P cooling fan speed control relay, and the cooling fan high speed relay. When energized, the high speed fan relay applies voltage directly to the right cooling fan through the switch side of the relay. Simultaneously, the low speed fan relay and the S/P speed control relay provide ignition voltage and a direct path to ground for the left cooling fan. During high speed fan operation, both engine cooling fans have their own ground path. The result is a parallel circuit with both fans operating at high speed.

Now, When should the ECM command the fans on.

For low speed.

When engine coolant reaches and exceeds a calibrated value. On at 226°F and off at 219°F.

When A/C pressure exceeds a calibrated value. On at 185°F and vehicle speed less than 35 MPH.

For High speed.

When engine coolant reaches and exceeds a calibrated value. On at 235°F and off at 226°F.

When A/C pressure exceeds a calibrated value.

When Certain DTC's Are set.

Now, The thermostat (if not changed) Should begin to open at 188°F and be fully open at 206°F.

With all that said, I said that to say this.

Your vehicle is working as designed. You may have not noticed it in the past, or the style of driving you do has changed and now you are noticing it more than normal. An 8th above 210°F is probably in the 216°F to 220°F range and that is perfectly within specs of the vehicle.

Hope this eases your mind. It's great that you are taking such great care of your vehicle.

Edited February 13, 2020 by carnau
Not low side... Low speed!