With cold engine, reservoir full, while radiator less than half

  • 3.4L
  • 6 CYL
  • 2WD
  • 160,000 MILES
I went to check the coolant level in my wife’s car listed above yesterday (cold motor), and when I opened the reservoir cap with the attached, funky plastic dipstick, I noticed it was completely full, a condition I have never seen before. As you know, the coolant dipstick has a series of cutouts near the bottom end to gauge the reservoir level. I asked my wife if she had filled/overfilled it, but she said ‘no’. I then popped the radiator cap, and saw that the radiator wasn’t even half full. This seems completely backwards to what I’m accustomed to. All my previous vehicles had the reverse situation: radiator full, and reservoir within a certain range, with plenty of room to spare.

Curious, I wanted to see if the system would “burp”, so I fired up the engine with the radiator cap off. After a few minutes, I could see the thermostat had opened, and the first tiny flow of coolant began. However, while the coolant level in the radiator very slowly began to rise, there was no “burp”, no drop in the level within the reservoir.

I allowed the motor to come up to operating temperature, and waited for the electric fan to kick in. By this time, the coolant level in the radiator still hadn’t reached the top, but the reservoir level hadn’t changed. Puzzled, I turned off the ignition, only to be witness the radiator starting to puke coolant out of the top. I’m guessing that, since the water pump was no longer circulating coolant, the residual heat caused the radiator to “burp”, and about a quart of antifreeze ended up on my garage floor. D’OH!

At any rate, can anyone help me understand what may be going on? I mean, the car runs fine and doesn’t overheat, but why on earth would the reservoir be completely full, while the radiator isn’t even *half* full.

Thanks, in advance. : )
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have the same problem?
Sunday, June 6th, 2021 AT 11:51 AM

1 Reply

The description sounds like a faulty radiator cap. The cap is actually a somewhat sophisticated part with two valves in it. It works as follows, You start the engine up with the radiator full of coolant and the reservoir at the low cold mark. As the engine heats up the coolant expands, on older cars this was just dumped out the tube and onto the ground. This left you with a radiator that would be full only when the engine was warm. Then they added the reservoir system with the intent of providing a safe place to capture that coolant and reduce gasket and seal failures as well. Those would happen because just like the cap allowed the coolant out through the pressure relief valve in the cap (that is the larger spring in the cap) it also would generate a vacuum as the coolant and engine started to cool off. So now the reservoir would catch all of the "overflow" but now as the engine cooled off the small center valve in the cap would be drawn open, and that pulls the coolant back through the hose into the radiator until the vacuum generated is equalized and the radiator is now full. This also allows for better temperature control because the coolant can maintain a level as high as needed based on the engines temperature. So say the engine runs at 180 degrees on a cold day in the winter, but the guy driving through death valley in august is cooking the engine at the 225 degree point. In both cases the coolant will remain full in the radiator. The other thing this system does is help with corrosion by keeping air from getting into the cooling system by maintaining the coolant at the full level, that prevents a lot of the scale and crud from forming in the system.
In your case the pressure valve in the cap is working and the coolant is escaping into the reservoir, but the return valve isn't and the coolant isn't being drawn back in. That points to either the cap being bad or there is a leak in the reservoir that is not allowing the return of the coolant. An easy way to test which it is would be to use a small vacuum pump to apply a vacuum to the return line that connects to the radiator below the cap. If a small vacuum lets you pull coolant through then it's more likely a bad cap. If not there is a leak or the line on the overflow of the reservoir itself is acting like a check valve. I have seen that happen if a bug built a nest in the tube.
As to the replacement cap, I try to use either OEM or Stant to be sure the new one isn't an issue. The Delco part number is 10316828 or RC94
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Sunday, June 6th, 2021 AT 4:07 PM

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