Air in cooling system (not head gasket or intake manifold gasket)

2002 Pontiac Montana Minivan (3.4L GM 6-cyl). 3 different mechanics, can’t find where air is coming from. All of them say it’s just residual air from recent work (intake gasket replacement). No other symptoms other than air in system & overheating. No smoke, no leaks, no oil in coolant, no coolant in oil, etc.

Initial diagnosis was faulty intake manifold gasket. However, after taking it apart, our mechanic discovered gasket was fine, but some previous person had worked on manifold and failed to tighten any of the bolts!! They had worked their way completely loose, no wonder air was leaking in. Went ahead and installed new gasket.

Also had coolant system power flushed (to try to push out any air pockets, as well as any gunk). But problem persists - if the engine is warm, I need to bleed the air every 2 minutes, or it will overheat.

Finding most of the air in bleeder valve for the upper hose (return hose). I point the nose of the vehicle in the air with ramps (the bleeder valves are pretty low in the engine, I try to force the air to the front). Then I bleed the air (takes a good 5 minutes, lots of air). Then try to run the vehicle. 2 minutes later, more air clogging the system, overheats again, another 5 minutes of bleeding the air.

Have repeated this process 50 times, yet problem persists. I’ve burped the system, massaged the hoses, cracked the radiator cap - each time lots of air. Seems like a massive amount of “residual” air. I believe it’s taking on air somewhere, but no symptoms. No coolant leaks, etc. I’ve been parking the vehicle on a clean sheet of cardboard, so I can monitor any leaks whatsoever. But there are none. Also no symptoms of head gasket (no smoke, no coolant in oil, no oil leak, etc). I also replaced the bleeder valves themselves.

My guesses: The interior heat is barely luke warm, I’ve been wondering if one of the heating cores is cracked (the vehicle has two). I’ve been told that if there was a crack I’d see coolant leaks, but maybe the crack is on top? If so, I may rework the hoses to bypass both heater cores. I also wonder if the water pump is sucking in air somehow.

This is our previous family minivan, now I use for work. We’re done with the vehicle, I was ready to sell before this started. I’ll have a much easier time selling it if I can get this problem solved first.


When you bleed the air from the cooling system you must displace the air with a fluid. If you are adding perhaps 2 quarts of coolant each time you bleed the system, where is the coolant going?

If you are opening the bleed screws when the engine is very hot you are just boiling water. You need to open the bleeders and refill the system at a temperature that is below boiling.

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Classic blown head gasket.


Good posts above. Besides those, here’s my three ideas – guesses really …

  • Faulty radiator cap. Replace it and see if that helps. Unlikely to help, but inexpensive.

  • You actually do have a head gasket leak. Suggest you test the coolant with a chemical test. When exhaust gas gets into the coolant it contains CO2 – and besides producing a lot of gasses in the cooling system – it forms carbonic acid and makes the coolant acidic. The coolant chemical coolant test will show if that is happening. Test kits available at most auto parts stores.

  • There’s something wrong w/your bleeding procedure. It sounds correct, esp about putting the front end of the car as high as you can on ramps. But maybe there’s something else you are doing incorrectly. As posted above, it is done with the coolant cold to warmish, never when it is boiling hot. Suggest to secure a copy of the manufacturer’s bleeding procedure. Factory service manual, All Data, or a dealership might print one out for you, no harm to ask.

Hey, thanks all for the great ideas.

Did the chemical test yesterday, repeated 3 times, negative each time. Also performed a compression test, which was fine. Those 2 tests, combined with the fact that there are no secondary symptoms (no smoke, no fluid contamination, etc), conclusively rules out head gasket, correct?

(FYI, something helpful for others reading this forum: both O’Reilly & Advanced Auto offer free rental of the test, you just have to buy the chemical formula, which was $7.99 and is enough for about 12 tests).

To answer a previous question, no it’s not losing any coolant, and I’m not really topping it off. When it gets warm, it pushes the coolant into the overflow tank. A couple times, the overflow tank spilled over (and yes I did have to top it off). After I bleed the air, coolant goes back into the system.

In terms of bleeding…I already had a pressure flush (should have gotten rid of air), and the mechanic also performed an air bleed as per manufacturer specs. What I’ve been doing is trying to catch the alleged remnant air pockets.

Radiator cap is in fact a little worn, I’ve replaced it, & will keep you posted if that helps. Any further ideas appreciated…

Those 2 tests, combined with the fact that there are no secondary symptoms (no smoke, no fluid contamination, etc), conclusively rules out head gasket, correct?



I’m with @insightful on this.
There is only one source where air can overcome the pressure of the cooling system and continually push its way in.

Cracked block??

That would be a zebra when a horse (the head gasket) is more likely. If the crack is down low in the block, combustion gasses could be forced in when the piston is near the bottom of its stroke so a compression test would read OK.

The chemical test doesn’t rule out a head gasket problem completely, but it seems unlikely at this point. If the head gasket was breached and exhaust gas were getting into the coolant and causing an air bubble to appear in the coolant, the chemical test would almost certainly catch it. Unless the coolant was continually being replaced in large quantities, which might dilute the coolant enough to fool the chemical test.

Seems that more detective work is necessary.

  • Perhaps the chemical test you did was faulty for some reason. Maybe try it again, using a different source for the chemicals and the test equipment. Do a little experiment. Try it on some carbonated soda. Does it test positive? Do you have some pH test strips? If so, try those. What’s the pH of the coolant?

  • When the engine is running and radiator cap is off, look into the coolant at the top of the radiator. Wear safety goggles of course. Do you notice any air bubbles coming up through the coolant and to the top of the radiator?

  • Any signs of coolant in the oil? Look around the oil fill cap for grungy, spongy looking stuff which doesn’t look like oil residue.

  • Pull up the front inside carpet near the heater core to see if there’s any coolant that’s leaked out hiding under the carpet.

  • If you have a loose connection in the cooling system somewhere, that could allow air into the system as the engine went from warm to cool. I suppose it might not leak coolant as the engine went from cool to warm due to some weird one way valve effect. But it seems like that can’t be the explanation as you’re noticing air entering the system as the engine heats up, right?

  • If it is an easy job to replace the water pump, might be worth a try.

  • I wouldn’t expect an intake manifold gasket leak to push air into the cooling system. It might pull coolant into the intake manifold though. The pressure would be in that direction, with the coolant at a positive pressure and the intake manifold at a negative pressure. I think the intake manifold gasket is probably a red herring.

It may be that the new radiator cap will help. If not, and nothing else seems to help, probably your best bet is to take the car to a radiator specialist. They can start by doing a pressure test on the cooling system.

We have Chevy Uplander vans in our fleet, pretty much the same thing as OP’s vehicle

What I’ve noticed is that the cooling system is set up, such that you can NOT remove the air out of the system without using one of those devices which pulls in new coolant under a vacuum

Such as this

Interesting. I’ve never heard of that before @db4690 . Seems a pretty complicated requirement to do what should be a simple job, filling the cooling system. A little like if you wanted to pour yourself a beer, with certain beer brands you’d have to get your vacuum pump out to make the transfer from the bottle to the glass … lol … But I guess if you have to do it that way on that vehicle, that’s what you have to do.

The OP’s comments seem to verge on violating the laws of physics. I’m wondering if there’s something more basic involved that is being overlooked.


There are a few vehicles out there, they are extremely difficult, if not impossible, to remove air pockets without such a device

I have an infinity fx35 that I have the cap off cold with a clear funnel filled half way for bleeding air out of the system, currently the thermostat is removed, no bubble form if I have the car idle to 3k rpm, if I gas it to 5-6 k then the level of fluid goes up and then big bubbles show up and the level drops back to where it is, is it possible that the graphite, porcelain gasket at the water pump is acting like a one way valve at the shaft? After driving the car normally it will push fluid into the reservoir, but not suck any back because when it is cold and I release the pressure bubbles go into the reservoir. I have done the block test and there is no issue with the head gasket, there is no leave in the system, I remove the water in the overflow and put it back in the radiator. Any ideas as this is the same symptoms the OP is having

When you did the compression test, did you test to see if the cylinders HELD pressure (a leakdown test) or only to see that it could build it up?

When you purge the cooling system, are you doing so with the heating system ON? That’s necessary to properly bleed the heater core.

Just realized I never shared the solution on the Pontiac cooling system issue…

It was a worn/cracked hose in the rear of engine, hard to reach, difficult to see. It was so deteriorated it almost crumbled in my hand, but it was mostly worn on top (closest to the engine heat).

That, plus the old radiator cap, was probably enough to suck the air into the system. But a bit ticked at the mechanics who missed this, which really should have been noticed during the radiator flush.

Nonetheless, thanks to everyone who actually had some constructive advice instead of just saying head gasket. (If you haven’t noticed, it’s more than a “pet peeve” when people blame the head gasket as an excuse to give up on a problem)

Excuse us for believing you. Your first paragraph states “no leaks.” But congratulations on getting your car repaired.

you don’t happen to have the exact tube this ended up being do you>?

…and this should have been caught by “Mechanics” 1 or 2… These things make me shake my head at some “mechanics”. Hell a piece of cardboard would have done a better job than these 3 Stooges.

Glad you got it sorted out without causing a bad head gasket…because that is exactly where you were headed.

Hi all,

I’m having the exact same problems as the OP. The car is a '92 BMW 325i convertible with the engine out of a '91 535i sedan. Similar I-6 engine by 3.5L instead of 2.5L. The cooling system is a blend of the two powertrains: radiator and all engine components from the 535i, expansion tank and heater core from the stock 325i chassis. This means that the thermostat housing (highest point on the engine where the bleed screw and temp gauge senders are located) and the expansion tank neck are at the same height, while the radiator is approximately 2" lower

I’m constantly getting air in the cooling system. I’ve used chemical (block tester) and pH (strips) methods for checking for combustion gases and they always come back negative. I’ve pressure tested the system and it holds perfectly steady at 16psi (expansion tank cap is rated to 1.4bar/20psi). I even purchased a vacuum fill setup and used that. I can get all the air out of the system (as far as I can tell) and then go for a drive. Within ~10 miles, depending on temps and speed, I begin to see my temp gauge quickly rise (I even installed an aftermarket VDO temp gauge so I can monitor it very, very closely). If I crack the bleed screw (after letting it cool some so that I don’t cause boiling) I find air, which is why the temp gauges are reading so crazy high. Once the air is gone they return to normal and I can go another ~10mi before it happens again. I’m at a complete loss for what could be forcing air into the system. I agree with @JoeMario that there’s only one way air/gas can overcome the pressure of the cooling system, yet every test shows no signs of hydrocarbons or acidification. No coolant loss, no coolant in oil, no leaks, no coolant smell from the heater core, nothing that points to any causes; just symptoms. It’s been this way for years, hence why I rarely drive it. But I’d love to get to the bottom of it once and for all.

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The service data recommends a compressed air/venturi vacuum-filling tool be used to eliminate air pockets as part of a cooling system refill. Have you tried that method @Darin_S_1 ?