Hi everyone, I've been noticing an issue with my 04 Bonneville GXP since the weather has gotten colder. The internal vents won't start blowing warm air until the temperature gauge reaches ~210. I'll even start the car, let it sit for about 20 minutes, get in and still have to drive it about a mile before the warm air starts.

Does anyone have any idea what this could be? I feel like it could be the thermostat, but I have no idea.

Thank you all in advance.

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Black 04 Bonneville GXP
9012 HIR Lowbeams

Either low on coolant, or a thermostat stuck closed.

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Andrew - owner/operator of Bonnevilles Unlimited

2004 Bonneville GXP | 60k | White Gold Tricoat | custom built supercharged 3800 hot rod | garage queen
1997 Corvette | 55k | Silver Metallic | Z06 wheels | Date-night Hauler/Parts runner
2014 Town & Country Limited | Cashmere Pearl | 95k | Family Hauler
2002 Ram 1500 Quad Cab Sport | Black| 260k | Official Bonneville Hauler

Would the low coolant prevent the cabin from heating up?

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Black 04 Bonneville GXP
9012 HIR Lowbeams

Yes. If no coolant is circulating, it can't get to the heater core in the dash.

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Andrew - owner/operator of Bonnevilles Unlimited

2004 Bonneville GXP | 60k | White Gold Tricoat | custom built supercharged 3800 hot rod | garage queen
1997 Corvette | 55k | Silver Metallic | Z06 wheels | Date-night Hauler/Parts runner
2014 Town & Country Limited | Cashmere Pearl | 95k | Family Hauler
2002 Ram 1500 Quad Cab Sport | Black| 260k | Official Bonneville Hauler

Alright, thank you. I will check the coolant level on my lunch break.

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Black 04 Bonneville GXP
9012 HIR Lowbeams

That would be odd, though, because I never received a low coolant warning and I just had my car to the dealership in September. I would think that they would check my coolant level.

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Black 04 Bonneville GXP
9012 HIR Lowbeams

Had this same issue with the GXP I bought a few months back. Started like that and within a week it would overheat after driving only 10 miles. I did a ton of research on the caddy forums and almost every thread there that was started due to overheating ended up being head gaskets. Once in a while it would be just low coolant, but overheating and no heat was basically a dead ringer for head gaskets. The cylinders push air into the cooling system the then the air basically vapor locks the water pump preventing it from pumping any coolant.

Check the coolant level and make sure it is at the line in the tank. If it is up there and it still overheats, then you likely have a head gasket issue.

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Ryan

2003 Bonneville SSEi - The Black Mirror SOLD!
2002 Bonneville SE - The Mutt Complete 2004 SLE interior, drivetrain, and body harness swap, ECC swap, HUD swap, black GXP wheels, GXP headlights and tinted tails - SOLD
2003 BMW 540i M Sport, 2001 BMW X5 4.4i, 2010 GMC Acadia, 2017 Grand Design Imagine 3150BH
1982 Cutlass Supreme - The fun one

It started with not being able to heat up as well last March or Feb (would heat up well sometimes, but would take forever other times). I took it to a shop then and they said they couldn't replicate it. I went all summer without making any changes to the coolant; AC would run fine, car never overheated. Never noticed any large oil consumption issues either. I also had the car to that shop two or three times and they replaced the rear main seal gasket and one or two other gaskets (maybe oil pan, but not head gaskets).

I can't quite tell where the line is in the coolant surge tank. I think the tank is dirty. It's either full up to the line or it's empty. When I shake the car there's no visual change in the tank. I really feel like the tank is empty.

I also can't get the cap to open.

And really, shouldn't the dealership have noticed if the coolant was low in September? I got it inspected right at the start of September.

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Black 04 Bonneville GXP
9012 HIR Lowbeams

The coolant should be about 1/2 to 3/4 full in the tank I believe, if you are low on coolant then depending on how far the parts store is, it may be ok to drive, but I wouldn't want to overheat it anymore then I have to. The cap is a radiator cap so you have to push down on it and turn at the same time, make sure there is no pressure in the cooling system first though before opening.

From what I found on the caddy forums, some engines could go on for a year running with mild overheating from head gasket issues, others like mine, would go from minor overheating to can't even drive into town in only a week.

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Ryan

2003 Bonneville SSEi - The Black Mirror SOLD!
2002 Bonneville SE - The Mutt Complete 2004 SLE interior, drivetrain, and body harness swap, ECC swap, HUD swap, black GXP wheels, GXP headlights and tinted tails - SOLD
2003 BMW 540i M Sport, 2001 BMW X5 4.4i, 2010 GMC Acadia, 2017 Grand Design Imagine 3150BH
1982 Cutlass Supreme - The fun one

Thank you for the heads-up. I'll definitely be looking into the situation. I'll try running to the gas station down the street on my lunch and picking some up from there.

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Black 04 Bonneville GXP
9012 HIR Lowbeams

It could be a bad water pump too. If they lock up they will throw the belt off.

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Seth

Bonita 78,000 miles traded on a CTS4

Is that part of the serpentine system? I checked that and the belt looks fine.

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Black 04 Bonneville GXP
9012 HIR Lowbeams

No it's on the drivers side front valve cover, just check and see if there is a belt there.

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Seth

Bonita 78,000 miles traded on a CTS4

Awesome, thank you. I appreciate everyone's help.

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Black 04 Bonneville GXP
9012 HIR Lowbeams

Update: I added coolant to the system. I'm still, however, not getting a timely reaction out of the heat. With the engine fully heated up (about the second small hashmark above the middle of the sweep), it's still blowing cool air. This is a big deal since I don't have heated seats and it's only going to get colder. The car will blow some warm air when it hits the second hashmark, start blowing cool air again and then eventually heat up after about 5 minutes of driving. Bad heater core? I don't want to start throwing dollars at this until I have an idea of what it might be.

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Black 04 Bonneville GXP
9012 HIR Lowbeams

I would not assume it's the heater core at this point, with the engine overheating as well. Check the coolant level again, it may have "burped" some air out again.

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Bye Bye:

RIP sandrock

Okay, I'll be sure to check that. I couldn't find the radiator cap and I know that usually you put coolant in the radiator too. Do we just have the cap on the surge tank or what?

I'm also considering a coolant flush from the dealership. I'm getting close to 100K and I have no idea the last time it was done (or if ever).

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Black 04 Bonneville GXP
9012 HIR Lowbeams

There is no fill on the radiator, the only fill point in that system is the cap on the coolant tank.

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Ryan

2003 Bonneville SSEi - The Black Mirror SOLD!
2002 Bonneville SE - The Mutt Complete 2004 SLE interior, drivetrain, and body harness swap, ECC swap, HUD swap, black GXP wheels, GXP headlights and tinted tails - SOLD
2003 BMW 540i M Sport, 2001 BMW X5 4.4i, 2010 GMC Acadia, 2017 Grand Design Imagine 3150BH
1982 Cutlass Supreme - The fun one

Before you get a flush, have them do a pressure test. That should be able to give you a better idea of what's going on...

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Bye Bye:

RIP sandrock

These engines consistently pass most methods of head gasket failure testing, so a compression test, leak down test, and even cooling system pressure test can come back fine yet still have had gasket issues.

Most reliable method I seen suggested is a coolant sniffing test (don't remember exactly what its called) that detects the presence of exhaust gasses in the coolant, or basically get on the highway after fully up to temp and do several wide open throttle runs from like 60 - 90 and see if the engine cools back down afterwards on it's own.

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Ryan

2003 Bonneville SSEi - The Black Mirror SOLD!
2002 Bonneville SE - The Mutt Complete 2004 SLE interior, drivetrain, and body harness swap, ECC swap, HUD swap, black GXP wheels, GXP headlights and tinted tails - SOLD
2003 BMW 540i M Sport, 2001 BMW X5 4.4i, 2010 GMC Acadia, 2017 Grand Design Imagine 3150BH
1982 Cutlass Supreme - The fun one