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Coolant system issues any ideas?

I have a 2003 Chevy impala 3.4L engine, on my way home the other day I noticed my car was overheating and that the heater was blowing cold air, I pulled over to let it cool off and turned it back on and continued driving, at this point my tempature gauge was all over the place , spiking all the way to hot and then shooting to halfway or below, rinse repeat. When I got home I turned the car off and noticed a glugging sound for 5 seconds or so and it stopped. I changed the thermostat and it didn’t help, I brought it to a shop and they bled the lines for me and said they had success. On my way home the tempature was stable right in the middle, had heat, life was good. Then it happened again. Anyone have any ideas? Faulty thermostat? Water pump? Intake gasket? Need to be bled more? I’m having an anxiety attack thinking it’s a blown head gasket but it has none of the traditional symptoms that go with it other than over heating. Any ideas? Thanks

It could be a faulty thermostat, I think you are blowing coolant, a pressure check is in order, check the radiator level when cold. A new radiator cap or plugged coolant return hose can cause issues like this along with a leak in the system.

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Pull the oil dip stick out and check if the oil is contaminated with coolant.

If it is, the engine has an internal coolant leak.


Fill the radiator with the proper coolant mix. Park over clean dry pavement and look for drips.

With the engine and engine compartment cool enough not to burn you, check the plastic tanks on right and left radiator ends, especially the top driver’s side opposite the radiator hose connection.

Also, see if you can hold a small mirror on an angle down below the water pump pulley and shine a flashlight right into the mirror. The mirror will reflect the light onto that area and you could see evidence of a leak.
,Leak/leaks could be puddles, drips, dampness, or just a crusty stain from dried evaporated coolant.

I have a Stant pressure tester I’ve had for many, many years. With a cool engine, it installs in place of the radiator cap and allows the operator to supply the correct operating pressure to the cooling system. Then, what is noted is whether or not the system can hold that pressure or if it starts falling too rapidly, indicating a leak in the system.

The leak will either be external and coolant will be noticed and fairly easy to pinpoint or internal and not visible. You may have to have a shop do a similar diagnostic on this vehicle.

Do you have the special adapter for your Stant pressure tester?

Because since this GM radiator doesn’t have a pressure cap, you need this adapter to test cooling system at the reservoir.


My 3.8Ls (I have 3 cars with them. Only the Grand Prix is here.) have radiator pressure caps and a reservoir vented to the outside. My Stant tester is in a black plastic molded case and I’m pretty sure, with different adapters, but it’s 1500 road miles away right now. There is one that fits the radiator cap neck. I’ve not worked on a 3.4L.

Also, had it been a 3.8L I would have suggested the plastic coolant bypass elbows, but it looks like the 3.4 does not have them.


Sounds like the system isn’t maintaining pressure for some reason. Probably some sort of leak. Barky’s idea above for a coolant system pressure test is where to start, presuming there is no obvious visual signs of a leak in the engine compartment, no water under the car in the morning, etc. It doesn’t take much of a leak at all for this symptom to develop during freeway driving. Does it seem to not overheat much , on the freeway, but as soon as you get off the freeway it starts to overheat? That’s definitely consistent with the cooling system not holding pressure.

Make sure the radiator fan is working correctly of course. Easy enough to check. When it overheats, pull over and pop the hood, leaving the engine on. The radiator fan should be spinning like crazy.

If the engine has ever severely overheated in the past, or if the cooling system routine maintenance (drain/refill with fresh coolant every 2-3 years) has been long deferred, definitely have to consider the head gasket.

Before i bled the lines it happened 10 minutes into a ride going about 35 mph, after the lines were bled it was perfect for about 12 miles from speeds ranging from 30-45 mph but reverted back to overheating and cooling irregularly and a bunch of pressure came out when the radiatior cap was taken off.

If there’s a leak, that itself will defeat any bleeding just performed, as the leak will allow air back into the system. The system should be pressurized when the coolant is hot, but not so much when it has cooled overnight. What was the coolant temp when you noted it was pressurized?

Both hot and cold it seemed hyper like pressurized

Cooling system is hyper pressurized even when cold? I’m not familiar with the cooling system design used on your vehicle, just a diy’er driving classic cars, but your car may have an incontinent head gasket. Cooling system pressure test, head gasket tests look to be in your future. With a conventional radiator a diy’er can sometimes discover a head gasket leak by looking for streams of bubbles coming up through the coolant in the radiator. Often they’ll increase rapidly in number the higher the rpm. But not sure if such a test is possible the way your cooling system is configured.