Coolant Overflowing But Not Overheating Silverado 4.8

[sktyrhrtout]

I've been dealing with a tricky coolant overflowing problem. Some backstory: 

 

Truck was running fantastic with no overheating/coolant issues for 8+ years. Took it in for an oil change about 3 months ago and the water pump is leaking from the weep hole. No problem, I buy a water pump but it kinda slipped my mind to replace it. Pull in the driveway about 3 weeks ago and I see the coolant coming out the overflow tube. I didn't notice the temp gauge overheating but it's possible it did. Definitely didn't have anything on the dash go off so it couldn't have got too hot. 

 

So replace the water pump & thermostat and test drive. Seems fine so we take it camping about 4 hours away. No problem going over a mountain pass and up the hill but when I pull over to park it bubbles over. I was cautious having just replaced the water pump so I definitely know it didn't overheat (gauge parked just a tick under 210 like always). We camp and I figure I'll try and refill and baby it home. No dice, it keeps puking coolant on the way down. I figure I'll get it towed to a NAPA autoparts down the hill and see if I can swap out another thermostat just in case that's the problem. I doubt it but it's a shot in the dark. We get down there, I swap out the thermostat and cycle it from idle to 3,000 RPMs to get it warmed up and overflows again, no overheating again as well. I say oh well and decide to let it cool and we'll drive it to a local shop and get a ride home. Well we let it cool down, I refill it and we drive it over to the shop. I have the heat on to see if we lose heat and I'll know we've lost coolant. Seems fine for the 10 minute drive over there and no coolant loss when I park it. I give it one more shot and decide to drive it hard with the A/C on around the area and see what happens. Well it decides to run just perfectly, cool as a cucumber. We decide to set the next shop as our destination about an hour away. Long story short it takes us all the way home, through 105+ temperature and back over the grade. Doesn't lose a drop of coolant and we pull in to the driveway with the A/C on. 

 

I check the coolant reservoir and there is a ton of black gunk/corrosion/etc. I think maybe the radiator was plugged and the 3X bubble overs may have acted as a flush? Who knows. I decide I'll flush it and then replace the radiator as it was about 14 years old. I flush it a few times and then decide I'll put it under pressure to confirm no leaks. 

 

Instantly the connectors at the heater core start pouring out coolant. I go to grab one and give it a push and it just breaks off in my hand. I end up replacing both quick connects and one of the hoses along with the lower radiator hose and radiator. I get it filled up and "burped" and drive it and seems to run fine. I drive it that night on a round trip for 30 minutes each way, gets up to temp and no problem. 

 

I drive today the same round trip and when I go to park it pukes out the coolant again, no overheating. I'm there for a few hours, fill it back up and drive it home and it gets home no problem, I have heat the whole way and when I park at home the coolant reservoir rises about 1" but I figure that's normal as there is a lot of heat when you shut off the truck. 

 

 

Where do I go from here? I don't notice serious bubbling in the reservoir on startup, no oil in the coolant and no coolant in the oil. No white gas coming from the tailpipe. Thermostat is confirmed working with a pot of boiling water and the upper radiator hose gets hot while the lower is cool to the touch.

[richard wysong]

Try a new radiator/reservoir pressure cap

[dna9656]

Richard, could this be air trapped in the block expanding and driving the coolant out?

[richard wysong]

Not likely, There may be 2 different water pumps for that engine though with opposite rotations. My $ is on a bad cap, if it won't hold pressure the coolant will boil in the hot spots of the engine and burp the steam causing the overflow problem

[64BAwagon]

Sounds like a head gasket to me. 

[Grumpy Bear]

[64BAwagon]

Just for reference I chased a head gasket issue on my 2005 Duramax for quite some time. I even had it at the dealer my son worked at for a week and two of the better techs couldnt definitively say that the HG's were blown. I first noticed pressure in the Degas (overflow) tank even after it had sat overnight and was stone cold. This along with a stiff upper radiator hose is a definitive sign of bad HG. I first noticed that the temp gauge was low during normal driving, it took forever to warm up in the colder weather. The only time I had any issue with it pushing water was when I towed AND the engine was under a towing load AND was heat soaked for at least 30 minutes. It then progressed to pushing out coolant when towing only. I wrapped a white rag around the overflow tube on the degas bottle to confirm this. By this time I had replaced the Degas tank and cap along with the EGR cooler. I had pretty much concluded it was at least one HG so the next time I towed our trailer I checked under the hood with the engine running on an 80 degree day. It was pushing air into the Deagas tank because I could see the system pushing air bubbles into the tank. 

It ended up being both headgaskets but it took me a bit of convincing before I dropped the coin on the repair. I also had tested the Degas tank for combustion gasses and it came up negative since I hadnt been towing and the engine wasnt heat soaked. An oil analysis showed no coolant in the oil. The failure was like a reed valve and it would only push combustion pressure one way but wouldnt allow coolant to flow into the cylinder. 

 I really hope thats not what your issue is.  

 

Scott

Edited August 8, 2023 by 64BAwagon

[BigTex4547]

Replace temp transistor on head and thermostat at intake.  Common coolant repair by gm shops.