1997 K1500 5.7 Won't hold heat

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2 weeks 5 days ago #201213 by Brocktoon
1997 K1500 5.7 Won't hold heat was created by Brocktoon
I've been fighting most of the fall and winter with the cooling system and heater core on my 97 K1500 Silverado, 5.7 manual with 133K on the clock. Around end of summer, I swapped out the water pump (with the help of Eric's 98 Tahoe video) because of bearing noise. I flushed and refilled the coolant. After the weather cooled down, I noticed the truck wasn't heating up well, so I changed the heater core. A month or so again the new WP (ACDelco) started leaking, so I pulled it back off and replaced the included gaskets with Fel-pros, which seems to have taken care of that leak. Got one of those no-spill funnels and bled the system. Was getting a leak where the overflow tube meets the radiator, but eventually got the temp in the cab to about 100F. The next day it was back to not putting out heat.

Then last Tuesday the lower radiator hose slipped off on the water pump side while I was parking in town. (That's what I get for trying to parallel park on a hill on poorly snow-cleared streets.) Anyway, I ordered a new hose and t-bolt clamps to replace the worm-gears I had put on when I did the pump the first time. I changed those out this afternoon and checked the thermostat, which I had to pry off of the housing because it was frozen and maybe stuck closed. I refilled and re-bled the cooling system (still leaks at the overflow tube when full). Thermometer in the vent read 102F when I took it out for a test drive.

On the highway not 10 minutes later, the heater started blowing cooler air again. Checked the thermometer, which now read 76-77. Overheat is gone and t-stat opens as it should, but at this point, I'm totally flummoxed by the heater. There's low air flow coming from the vent to begin with, even though the blower fan has all speeds, and I can hear all the actuators and the high speed resistor engage with a stethoscope, and it just won't hold heat. I suck at all of this, and I don't know what I'm doing wrong. I want to like this truck, but it's getting harder and harder to. Does anyone have any suggestions? And sorry that I'm always long-winded with these posts.

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2 weeks 5 days ago #201215 by nightflyr
Replied by nightflyr on topic 1997 K1500 5.7 Won't hold heat
Have you check the heater control valve for proper operation?
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2 weeks 5 days ago #201217 by Brocktoon
Replied by Brocktoon on topic 1997 K1500 5.7 Won't hold heat
Mine doesn't have one of those, just an inlet and outlet directly to the heater core from the manifold.

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2 weeks 5 days ago #201218 by nightflyr
Replied by nightflyr on topic 1997 K1500 5.7 Won't hold heat
If that is the case and considering you have a new heater core installed along with a new water pump.
Assuming the engine is reaching proper operating temperature.
That narrows the possibilities.
Air entrapment
A partial blockage to the heater core or on the return.

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2 weeks 5 days ago #201222 by Brocktoon
Replied by Brocktoon on topic 1997 K1500 5.7 Won't hold heat
The only other thing I can think of is when I replaced the heater core, I broke a couple of pieces off the lower cover that holds the heater (stock photo below -- green outline where it broke) and the support bolt that holds it up. I plastic welded the cover back together the best I could, but I don't know if it there's a very good seal, especially in the back towards the firewall. I don't know if that's enough to cause the temp change and/or low air flow all on it's own. Air flow was never very strong on this truck compared to other vehicles I've driven.


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2 weeks 4 days ago #201228 by nightflyr
Replied by nightflyr on topic 1997 K1500 5.7 Won't hold heat
Easy enough to check with a point and shoot temp gun.
With the vehicle at operating temp check around the enclosure.
Might also try getting a reading off the heater core itself to see exactly how hot it is getting.

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2 weeks 4 days ago #201230 by hawkins
Replied by hawkins on topic 1997 K1500 5.7 Won't hold heat
did you check to make sure both heater hoses were hot on the firewall? they should be as hot as the upper radiator hose. If one is hot and not the other then theres a restriction and you would need to flush the heater core. Disconnect both hoses and flush it right on the firewall. Sometimes radiator stop leak can get in there and clog things up. As far as the air flow problem i would pull out your blower motor and make sure theres no leaves or crud in there as well as check the air inlet on the windshield (plastic cover where the wipers are). That can also get clogged up with leaves and will be obvious when you pop the hood. You may need to remove the plastic cover and clean leaves and dirt out from the inside as well.

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2 weeks 18 hours ago - 2 weeks 18 hours ago #201293 by Brocktoon
Replied by Brocktoon on topic 1997 K1500 5.7 Won't hold heat
Ok, so on Monday I did another bleed of the system, this time with the truck parked further down my driveway facing up, where there's a pretty steep incline.

- Upper rad hose and both heater hoses got hot, not stovetop hot -- I could grab them for a few seconds before having to let go -- but none of them were cold.

- I pulled the blower motor before I wrote the first post and didn't find anything.

- The inlet seemed ok, although I couldn't get the middle section of the cowl off and didn't want to push my luck and risk breaking it. There were small pieces on each side, about 6" long. Just a couple twigs and pieces of leaves that probably got there wherever the original owner parked it.

- As for stop-leak, I avoid that stuff. I did dump a bottle of Blue Devil in when I flushed it last summer, but only the radiator flush (#00204).

I took it out for a bit on Tuesday before most of the snow dropped here in PA, but forget to hit the recirc button until I was almost home, so couldn't tell much. Vent temp didn't get much above 55. Heater core temp varied pretty wildly (around 60-90) depending on where on the box I pointed my temp gun.

Took it back out for a solid hour and a half this evening to give it plenty of time to get warmed up. Pretty much the same as above, vent temp stayed mostly around 55-57, and remembered to put recirc on. I stopped for gas, and as I was pulling in, I thought I felt the vent get just a little bit warmer, thermometer read 64. Heater core temp still varies by where I point and angle I pointed the gun....
Last edit: 2 weeks 18 hours ago by Brocktoon.

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2 weeks 12 hours ago #201294 by nightflyr
Replied by nightflyr on topic 1997 K1500 5.7 Won't hold heat
First things first..
Do you know for certain the engine is at operating temp via temp gauge or thermal gun?
If by chance you have a stuck open thermostat, no matter how long you drive the engine will not come up to temp.

I'm assuming when you stated you replaced the heater core you used a brand new part or did you install a used part?
If it was a used part it maybe clogged.

Now if all the above is working as it should, My thought is you still have air entrapment.
Sometimes it can be a royal PITA.
If normal bleeding doesn't cure the issue.
Raise the front end via ramps or floor jack and secure the vehicle.
Hook up your spill free funnel, start the engine and set it to 2,000 rpm with the heater set to hot and the blower switched off and let it run for 15 - 20 minutes.

Worse case.. if that doesn't solve the issue.
I would install a PRESTONE flush tee on the heater hose and fill that section of the system first.


Last resort, if everything is functioning properly but still getting no heat would be to vacuum bleed the system.
www.google.com/search?client=firefox-b-1.......0i71.nhZfVe2N6EQ
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1 week 4 days ago - 1 week 4 days ago #201347 by Brocktoon
Replied by Brocktoon on topic 1997 K1500 5.7 Won't hold heat
The heater core was new. I pieced together the lower cover with fiberglass resin since the plastic welds didn't hold up. I struggled with getting it to fit with the upper shroud, but eventually got it in and sealed any remaining cracks I could find with weatherstripping.

I put some coolant in the rad, fired it up, turned the fan off and the heat to hot, revved to 2000 rpm....and within 2-3 minutes, the coolant was streaming out under the truck. It looked like it was at the inlet and/or outlet, but can't say for sure. Or maybe the heater core was punctured.

So now I'm back to square one, I'm fresh out of coolant, and I'm rethinking my life choices....
Last edit: 1 week 4 days ago by Brocktoon.

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