Help diagnose a truck anyone?

[CubDieselFan]

I bought a scan tool that you can see the coolant temp. My 1998 Chevy was not acting right. It helped me figure out what was wrong and lets me see the temp the ECU is seeing from the sensor.

[J-Mech]

Originally Posted by ol'George

Some Gm thermostats have a wax "pellet" inside that when overheated, melts and renders the thermostat junk, replace thermostat just to be safe.

I think you are referring to a fail safe t-stat. When they overheat, they stick OPEN though.

I always replace a t-stat if I change a water pump.

Originally Posted by CubDieselFan

I bought a scan tool that you can see the coolant temp. My 1998 Chevy was not acting right. It helped me figure out what was wrong and lets me see the temp the ECU is seeing from the sensor.

Just as a note, there are two different sensors. One for the gauge, and one for the ECM. Just because one reads, doesn't mean the other one does.

[jimbob200521]

Originally Posted by J-Mech

Is the cooling system full? In all this you never stated if you checked the coolant level....

If the tank looks full, then (DO THIS WITH THE ENGINE COLD) take one of the coolant hoses loose that runs to the heater. I usually do it up next to the water pump. Might be air locked.


I really hate reading threads like this. I really try to avoid it. All the answers that have to do with other vehicles that are not a 2001 Chevy..... a 350 motor and a 5.3 aren't even made anywhere near similar. The T-stat on a 5.3 is mounted to the water pump and is on the lower side of the system, not the top side. In other words, when the t-stat opens, it lets water circulate into the bottom of the motor, not out the top. The water pump bypass circulates through the heater, so if the t-stat was stuck shut the heater should have been so hot he couldn't stand it, not just warm. I'd say either the water pump went out, or it's low on coolant. Some of these had plastic impellers on the water pumps. Not all that uncommon for them to come off the shaft. I've even seen steel impellers do it.

Check the coolant level, then see if the water pump is circulating. If not, replace it.


Tim, I don't know where you bought bad t-stats at the you need to check them before installation. I've probably changed at least 600 thermostats. Probably only 10 or 15 were bad that were new. Not very often has that happened.

So after checking the coolant level (there was coolant in the resivour but it was a little low, not so low I thought it'd cause a this issue but hey I've been wrong before) remove a heater core hose from the tstat/pump side to verify flow as the vehicle warms up? Also, from what you're saying I interpret it as saying even if the thermostat was bad I'd still have blazing heat, correct?

Edit: so there's two temp sensors, I get that. Without buying a reader is there a way to verify engine temp? Such as taking an infared thermo and gathering readings from the block? Could a failing lower hose collapsing cause this issue as well?

Btw thank you for taking the time (and patience) to read through this and help out, I appreciate it.

[J-Mech]

Originally Posted by jimbob200521

So after checking the coolant level (there was coolant in the resivour but it was a little low, not so low I thought it'd cause a this issue but hey I've been wrong before) remove a heater core hose from the tstat/pump side to verify flow as the vehicle warms up? Also, from what you're saying I interpret it as saying even if the thermostat was bad I'd still have blazing heat, correct?

You don't quite understand, or I didn't explain well.

Don't check for air with it running. Top the level off in the reservoir, then leave the cap off, take the clamp off of the highest heater core line, and I usually just grab the hose with a pair of pliers and twist it to break the seal. (Be careful not to damage the hose.) Then once it's loose, stick a flat blade screwdriver into the end between the hose and the pipe. Just enough to let out air/coolant. If coolant comes out, then it's fine. Put the clamp back on. Then, IIRC, there is a small hose running from the top of the radiator back to the coolant tank. Pull it off and check to see if it has coolant at it. Should be a small,,,, maybe 1/4" hose. (Can't remember)

On most models of anything really, if the t-stat sticks closed and the engine is super hot, yes. You should have scalding heat. If it is stuck open, or opening too soon and not getting to operating temp, you will not get hot air out of the heater. The fact you have cool air is a good indication that the water pump isn't moving coolant, either because it is air locked, or the pump is bad.

[J-Mech]

Originally Posted by ol'George

Also some vehicles collect a tremendous amount road debris between radiator and AC condenser mounted in front of it.
Remove cover retaining radiator, and check between them.
I always remove the radiator and clean the junk out between,
and back blow the outside of radiator with air pressure after washing with water several times. It is unbelievable what gets stuck in the front side of a rad.
it is also a good idea to back blow the AC condenser to remove debris and carefully straighten bent fins with a fin comb.
Do not use high pressure water from a pressure washer, as you will destroy the fins.

This is the best advice/ description of a real problem I've read. I have dealt with this very issue so many times. Every summer, when it gets really good and hot, we always had a huge rash of overheating cars/trucks. Primary cause: Exactly what George says. Plugged up exterior cooling system. Sometime literally the area between the A/C condenser and the radiator was completely full. We always just pulled the vehicle into the wash bay, drained the system and removed the radiator completely. Power washed it, and the A/C condenser. Sometimes if it was an easy removal, we also removed the condenser. Completely cleaned them, went ahead and back flushed the cooling (internal) system with water with the t-stat removed, then refilled it with new coolant. That may sound like a lot of work, but we had a set price for the actual cleaning. So you paid the same labor as a radiator R/R, but the bill was far less than the cost of a radiator. Obviously, if it needed a hose or something we would take care of that while out..... but back to the original statement: It is VERY true that a lot of vehicles cooling systems are plugged up with road debris, and bugs. Good post George!


You're welcome Ryan.

[jimbob200521]

Originally Posted by J-Mech

You don't quite understand, or I didn't explain well.

Don't check for air with it running. Top the level off in the reservoir, then leave the cap off, take the clamp off of the highest heater core line, and I usually just grab the hose with a pair of pliers and twist it to break the seal. (Be careful not to damage the hose.) Then once it's loose, stick a flat blade screwdriver into the end between the hose and the pipe. Just enough to let out air/coolant. If coolant comes out, then it's fine. Put the clamp back on. Then, IIRC, there is a small hose running from the top of the radiator back to the coolant tank. Pull it off and check to see if it has coolant at it. Should be a small,,,, maybe 1/4" hose. (Can't remember)

On most models of anything really, if the t-stat sticks closed and the engine is super hot, yes. You should have scalding heat. If it is stuck open, or opening too soon and not getting to operating temp, you will not get hot air out of the heater. The fact you have cool air is a good indication that the water pump isn't moving coolant, either because it is air locked, or the pump is bad.

Well, some progress at least. I did as you suggested and cracked one of the heater core hoses off and with no flow I completely removed the heater core hose with the reservoir cap off. I had very little to quickly no flow out of the hose I removed. So I replaced it and tried starting the truck. This time, I looked in the reservoir once the truck was started and noticed that once running, it was sucking the fluid in the reservoir such that it was pulling air. So I ran a mile or so down the road and grabbed some distilled water and topped the reservoir off. Feeling somewhat optimistic about this, and early for work, I drove around a bit to see if the problem repeated. Sure enough, once it got to operating temp and sat there for a minute, it suddenly began to quickly rise. I once again tried blasting the heat and this time, boy did I have heat! So that's a start I suppose

I'm wondering if once the truck get to operating temp and the thermostat opens allowing a fresh batch of coolant into the engine, it's sucking air from somewhere therefore shoving air rather than coolant into the engine causing the spike. At this point, I'm pretty convinced the pump is good as well as the thermostat but I have the "simple" problem of air trapped in the system somewhere.

Does this theory sound logical to you guys? Any good ways to burp the system?

[J-Mech]

Just take the cap off and fill it with it running. If you got the air out of the water pump, then it will self bleed from there.

Now, the question is, where did the coolant go? I bet the water pump is leaking.....

[CubDieselFan]

Originally Posted by J-Mech

Is the cooling system full? In all this you never stated if you checked the coolant level....

If the tank looks full, then (DO THIS WITH THE ENGINE COLD) take one of the coolant hoses loose that runs to the heater. I usually do it up next to the water pump. Might be air locked.


I really hate reading threads like this. I really try to avoid it. All the answers that have to do with other vehicles that are not a 2001 Chevy..... a 350 motor and a 5.3 aren't even made anywhere near similar. The T-stat on a 5.3 is mounted to the water pump and is on the lower side of the system, not the top side. In other words, when the t-stat opens, it lets water circulate into the bottom of the motor, not out the top. The water pump bypass circulates through the heater, so if the t-stat was stuck shut the heater should have been so hot he couldn't stand it, not just warm. I'd say either the water pump went out, or it's low on coolant. Some of these had plastic impellers on the water pumps. Not all that uncommon for them to come off the shaft. I've even seen steel impellers do it.

Check the coolant level, then see if the water pump is circulating. If not, replace it.


Tim, I don't know where you bought bad t-stats at the you need to check them before installation. I've probably changed at least 600 thermostats. Probably only 10 or 15 were bad that were new. Not very often has that happened.

No one said it was a 5.3 and a 350 were the same. Or that a 2001 chevy is the same. Why would you even say that? It must really suck being in a pissy mood all the time.

[J-Mech]

Originally Posted by CubDieselFan

No one said it was a 5.3 and a 350 were the same. Or that a 2001 chevy is the same. Why would you even say that? It must really suck being in a pissy mood all the time.

Post #4. Adam talked about changing the intake on his 350 and said it air locked. He was saying that because Ryan changed his intake. The intake on a 350 has water jackets in it. The 5.3L motor has a dry intake. Don't even have to drain the coolant to remove it. So, in essence "comparing" the two jobs/motors.... yet they are worlds different.

[jimbob200521]

Originally Posted by J-Mech

Just take the cap off and fill it with it running. If you got the air out of the water pump, then it will self bleed from there.

Now, the question is, where did the coolant go? I bet the water pump is leaking.....

I'm going to tackle that tonight after work. If that doesn't do it, I nabbed a new thermostat while out for work just in case. I figured it was less than $15 for the actual thermostat so if I need it, great to have it. If not, I will either change it since I have it or return it (hint: I'm going to change it ).

Then in a few weeks (if tonight takes care of it) I'm going to do a flush on the cooling system since I will have added water throwing the ratio off.