Temperature and Heat Problem

[Cerulean]

Hello all, and let me say thank you in advance for any and all help. It is greatly appreciated.

So, it goes like this... The other day I went outside, (I live in MI so it has been very cold) and I started the truck. I usually let it sit and run for about 5 minutes or so to warm up enough for the heat to kick on.

I have a 1999 Tahoe 2WD 4 DR 350 Vortec.

After a few moments I went outside to get in the truck and I noticed that the heater vents were blowing cold air. I then noticed the Temperature Gauge was just past the halfway line. Usual running temp, even in the summer when hot, is just below the 1/4 (quarter) line.

I first checked my coolant level. It was a little low, but nothing too bad. It has shown no signs of leakage. That didn't help. So next I changed out the thermostat but that didn't seem to do anything either.

So, while driving, sometimes the temp will shoot up to past the halfway mark and then start to creep up even higher. However, most of the time if I put it in Neutral and race the motor the temp gauge drops back down to normal. When the gauge is high it blows cold air but as soon as the gauge drops down to normal temp the heat kicks back in and works very well.

Someone told me this could mean I need a new fan clutch, so I went ahead and changed that out. It is still doing the same thing.

Next I went and had the coolant system flushed and re-filled and it seemed to help a bit. I drove it for the next two days and the temp was running just a line or two higher than normal, but it was steady and the heat worked fine. Today though, it started to do the same thing again, where as I was driving the temperature would start to raise and the heat would go off. Then, after racing the motor a bit, it would drop back to normal and the heat would kick on.

Now someone is telling me I should have the heater core flushed because it could be a blockage. I just wanted to get some opinions from you guys, who know a lot more about this sort of thing than I do, before putting any more money into something that won't fix the problem. This is not only my first Tahoe, but my first Chevy as well. I bought it about 6 months ago and this is the first time this has happened. There were no warning signs before that first day.

In any case. Sorry for the long post but I just wanted to make sure I was as thorough as I could be. Any help would be great. Thanks.

Chris in MI

 

[SunlitComet]

well you still have a radiator that the coolant can go thru so a blockage in the heater core should not cause you to overheat. You can look into a clogged radiator, a weak water pump, or a malfunctioning fan clutch to start. The water valve on the heater core is controlled by the hvac control head and only opens that valve when the temp knob is on any setting other the max cool.

 

[Cerulean]

I suppose I could have just gotten a bad fan clutch but I bought the new one I put on brand new and it is under warranty. It seems to be working though. Is there a certain, (and possibly easy), way to check for the radiator blockage and how well the water pump is working? I did just have the whole radiator flushed out a few days ago so I just kind of assumed that would take care of any blockages.

I'm not exactly what you would call "mechanically inclined". heh

What is the best way to make sure the water pump is working at full capacity?

Also, someone mentioned it perhaps being a bad temp sensor, (The one right above the thermostat). Could that be it? All I really know for sure is when the temperature gauge starts going up, the heat starts to not blow hot.

Thank you for your reply.

 

[SunlitComet]

No specific flow tool really. Lets assume radiator is clear. The fact that you have to rev it up for it to cool down shown that coolant is moving and cooling is taking place. That shows a reduction of flow or cooling fan use. When it gets hot does the fan clutch get tighter so when you flare the throttle you her massive fan noise? You should also be able to hear a howling wind noise from under the hood when flaring the throttle immediately after start up of a cold block and radiator. As for the pump it would really need to be removed to check for impeller damage. If the pump is physically ok and the thermostat has been verified working(Requires 195° version btw), in a separate boiling pot of water and thermometer test for example, then a blockage clutch failure is likely. What does not make sense is the heater core going cold when engine is overheating. With the valve open you should get all the heat in there. There is no input from sensors that tell the control head of hvac to shut the heat off. It is strictly the temp knob position. Make sure the water valve has its vacuum hose plugged in to the valve and firewall mounted solenoid. And the electrical connector on it. Also check the rigid line from the solenoid to the left side of engine where pcv system enters the intake plenum. What coolant are you using? How was the color before and after flush? Did flush push out any nasty? The sender which feeds the dash is mounted on the left cylinder head just above the front spark plugs. It has one wire. To compare and look for anomalies get a scanner to view live data. The data will show coolant temperature as read by the pcm thru the sensor on the thermostat housing. That computer sensor is used by the pcm and does not output to the dash. Look at scanner and dash at the same time. They should generally mimic each other once the engine hits 100° or more. The dash gauge just does not go as low as what the pcm can see itself. So if temp gauge hits say 1/2 way up your scanner should read about 210° at the same time. If there is a big difference something is wrong with one of them. When the engine is say getting really hot does the upper hot get really stiff or hot as well or is it squeezable and cooler then say the lower one(best check with engine not running to avoid moving parts)?

 

[99Vortec]

When you replaced the thermostat, did you use a "Failsafe" model?

Or a regular model?

If the engine is overheating (for whatever reason(s)) the Failsafe thermostat model will be locked into the open position until it is replaced.

I am wondering if the water pump was not initially pressurizing the system properly. (As there were no signs of leakage.) Installing a failsafe thermostat would only compound the problem and make it more difficult to troubleshoot.

I just went through a cooling system rebuild on my Denali. It was a little bit easier to diagnose though. Well, somewhat; The water pump was leaking coolant through the weep hole, so I replaced it. Then After refilling everything, warming up the engine, shutting it down, waiting for it to cool, and topping it off again I noticed radiator leakage/damage that was not evident before the pump was replaced. So I had to drain the system again, swap the radiator, go through the refill process and then all was well.

In retrospect, I might have saved time by simply pressure testing the system before refilling it.

But even then, where could I still be leaking pressure at? The heater core? The freeze plugs? The radiator?

Can a water pump start to fail to deliver adequate pressure and still not leak?

 

[livingez_123]

The water pump impeller can become lose and cause heat loss in the cab. The impeller can slip on the shaft, and you wont have enough coolant flow to move the water where it's needed.

 

[99Vortec]

The water pump impeller can become lose and cause heat loss in the cab. The impeller can slip on the shaft, and you wont have enough coolant flow to move the water where it's needed.

So the system simply bleeds down upon itself.

Interesting. A water pressure gauge would be nice!

(Something I had to deal with on a big block powered olds jet boat....Water pressure could get too high in that case.)

But way off topic.