No heat while car is warming up

[iamdub]

Same here my 00 had low coolant 1 time in the 5 years I’ve owned it. No obvious leak. No recent work done. Heat did the same exact thing. Topped it off and kept an eye on it. Never went low again and the heat worked great. Weird if you ask me. But maybe a ton of air was trapped in the system for a long time and finally “burped”

The cooling system of the LS engine is known to be a pain to bleed of all air. I wouldn't be surprised if it just took that long for the air to collect and form a large void.

 

[Tahoe97]

Checked coolant level. That is it. Very low. Must be losing coolant somewhere as the water pump was replaced by GMC in September and coolant topped off at that time.

Before start up and cold I added about 1/2 gal Dex-Cool (50/50) up to the Full Line on the reservoir, started the engine and after about 30 seconds it had drawn down the coolant in the reservoir well below the Full Line. I shut off the engine and added the remainder of the 1 gal Dex-Cool and that brought the level about 3/4" above the Full Line. Drove 176 miles on 2/19 and this morning, 2/20, before startup checked the reservoir again (cold) and it was just a bit below the Full Line. Not quite sure what to conclude here other than maybe most if not all of the air is now out of the system. The heat works fine now which was the original issue but now the question is "is coolant escaping the system" somewhere? I suppose I need to keep an eye on this for a couple of weeks and if it continues to drop then I suspect the issue is not air in the system and I am losing coolant.

The water pump was replaced on 9/14/21, 4500 miles ago. I needed to add 1 gal Dex-Cool on 2/19/22.

 

[iamdub]

The water pump was replaced on 9/14/21, 4500 miles ago. I needed to add 1 gal Dex-Cool on 2/19/22.

This puts a better perspective on things. 4500 miles is plenty of time for trapped air to work its way to the surge tank. I just don't see that much sitting in it for that long. I'm really suspecting you have a leak, and it's possibly internal if you don't smell it after a drive.

Now that we have a "baseline" (after adding a gallon), keep an eye on the coolant level. If it continues to drop, I'm betting you have a failed head gasket or cracked head. If you're bored and have an hour or two to blow, you can look under a valve cover or both. Slow leaks might not show in the oil or exhaust, but they'll surely leave a mess in the engine.

 

[tRidiot]

This puts a better perspective on things. 4500 miles is plenty of time for trapped air to work its way to the surge tank. I just don't see that much sitting in it for that long. I'm really suspecting you have a leak, and it's possibly internal if you don't smell it after a drive.

Now that we have a "baseline" (after adding a gallon), keep an eye on the coolant level. If it continues to drop, I'm betting you have a failed head gasket or cracked head. If you're bored and have an hour or two to blow, you can look under a valve cover or both. Slow leaks might not show in the oil or exhaust, but they'll surely leave a mess in the engine.

@iamdub is 100% right - these trucks, particularly if you have a 706 or 862 number on the head like this:



are known to develop a slow coolant leak into the cylinder head. The Castech version are the ones that leak (not the casting mark in the picture above, Castech mark is a little battery like this:



That's a bad sign. 706 heads, especially if you can confirm the battery casting mark underneath, combined with a slow coolant leak, signals an eventual death knell for your heads. And possibly engine, like mine.

Not being a Debbie Downer, but this is a common problem and the slow, inexorable coolant leak is what points you in the direction of the heads.

 

[Tahoe97]

With 286,000 miles on the engine, I am leaning toward the theory presented of an internal coolant leak. There is no external leak that I can see or smell. In fact now I wonder if I needed a water pump as that was GMC's diagnosis back in September, that the weep holes on the pump had signs of coolant leakage which was their explanation as to why our auxiliary fan was running to cool the engine and our coolant was so low. So here we are again, low coolant. I will watch this over the coming weeks. Thanks for the tips!

 

[rockola1971]

With 286,000 miles on the engine, I am leaning toward the theory presented of an internal coolant leak. There is no external leak that I can see or smell. In fact now I wonder if I needed a water pump as that was GMC's diagnosis back in September, that the weep holes on the pump had signs of coolant leakage which was their explanation as to why our auxiliary fan was running to cool the engine and our coolant was so low. So here we are again, low coolant. I will watch this over the coming weeks. Thanks for the tips!

Have checked the Tees at the heater core for evidence of coolant leaks? Its a pretty common problem for our chassis.

 

[Tahoe97]

Have checked the Tees at the heater core for evidence of coolant leaks? Its a pretty common problem for our chassis.

Would I see coolant on the ground from that? Can I visually see the T's in the engine bay?

 

[iamdub]

Would I see coolant on the ground from that? Can I visually see the T's in the engine bay?

They're right at the firewall, to the left (engine's right). You'll see the two 5/8" heater hoses running to them from your water pump.

Although, if they were leaking, you'd probably smell the coolant and not see it. It's often a small enough leak that it evaporates and never makes it to the ground. Depends on how badly they were leaking. Yellowish crud around them would be a sign.

 

[iamdub]

@iamdub is 100% right - these trucks, particularly if you have a 706 or 862 number on the head like this:

View attachment 363785

are known to develop a slow coolant leak into the cylinder head. The Castech version are the ones that leak (not the casting mark in the picture above, Castech mark is a little battery like this:

View attachment 363786

That's a bad sign. 706 heads, especially if you can confirm the battery casting mark underneath, combined with a slow coolant leak, signals an eventual death knell for your heads. And possibly engine, like mine.

Not being a Debbie Downer, but this is a common problem and the slow, inexorable coolant leak is what points you in the direction of the heads.

I was trying to keep the bad juju out of this one, at least until further diagnosing. But, facts are facts.

 

[Tahoe97]

They're right at the firewall, to the left (engine's right). You'll see the two 5/8" heater hoses running to them from your water pump.

Although, if they were leaking, you'd probably smell the coolant and not see it. It's often a small enough leak that it evaporates and never makes it to the ground. Depends on how badly they were leaking. Yellowish crud around them would be a sign.

I did look at what I thought were those T's going into the firewall. I will look more closely at them tomorrow. I do recall 10 or so years ago they were replaced as they were leaking at the time. Looking back through my maintenance notes I found that we have replaced the water pump at 95,000 mi, 172,000 mi, and 281,000 mi. Still runs like a champ but maybe not for long if coolant is entering the engine where it doesn't belong.