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2011 FORD SUV Mysterious coolant leak

Hi everyone,
My car has been losing coolant slowly for months without visible leak anywhere, overheating, oil in coolant, milky oil, sweet smell and bubble in recovery tank. The coolant level in tank droped about 1cm every 30-40 miles driving in average. Coolant level kept lowering even after it was below MIN marking.
There is minor condensation on under-side of oil fill cap, it always reappears few days after I clean it, does this show a very minor water or coolant keeps going into oil? But oil from dipstick looks clear and clean.
3 minutes after cold start, much damp white smoke and a few drips keep coming out from tailpipe for about 15 minutes then completely disappear after engine completely warms up.
Every time when I remove the overflow tank cap with completely cold engine, I can hear a popping sound that seems like some air in coolant system escaping out. Does this show a small pressure from engine is leaking to coolant system?
For pressure test, the coolant system can hold 18 psi pressure very well for more than 5 hours with cold engine. When I kept connecting pressure tester to overflow tank and started engine without pumping pressure, the pressure on gauge didn’t increase quickly, stayed at 5 psi after engine warmed up, then I pumped it to 15 psi, it slowly went up to 19 at most after 1 hour idling, and finally when engine got cold I saw coolant level was 0.5cm lower (or about 5oz less) than it was before this test. But I still did not see any external leak.
Moreover, the recovery tank cap has been changed, the dealer also checked transmission pan and spark plug, and did TK test (combustion gas detection) , they couldn’t find the leak too.
I’m at a loss. Any suggestion? Could it be a very small and early stage blown head gasket or head crack without obvious symptoms?

There comes a new clue. As I mentioned, after 1 hour high pressure test with engine idling yesterday, coolant level droped 0.5cm. Today I started my car and just leave it idling for half hours without pumping additional pressure, after engine was cold, I saw the coolant level still droped about 0.5cm. So no matter the engine was running for 1 hour or half hours, and no matter there was high pressure or normal pressure in coolant system, my car lost the same amount of coolant. It seems the car loses coolant only in the first half hour after engine cold start. What can be the cause?

Head gasket leak seems likely to me.

Have you yet put the car on jackstands and crawled underneath?

Some leaks just can’t be seen without getting under the car with a good light and maybe a small telescoping and swiveling mirror

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Actual experience: a very slow leak that lasted for years – turned out to be coming from around the shaft of the water pump in very small amounts.

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The dealer inspected it for nearly one hour and couldn’t find any leak sign.
Also if it is a external leak, I think the pressure on gauge should drop after a long time pressure test with both cold and running engine .

Do you actually mean 50 km , i.e. just 30 miles? Or do you mean 50,000 miles? If the latter, just monitor the coolant level and top up as necessary. If that’s all you are losing, nothing else needs to be done until/unless it gets worse.

The same symptoms and ended up a head gasket?

Drops 1cm every 50 km or so. After 1 or 2 weeks, coolant in the recovery tank all gone.

hmmmm … 1 cm drop in the coolant level in the recovery tank every 30 miles. Thanks for clarifying. Definitely requires investigation what’s causing it. It sounds like your shop has done all the right things. The spark plug tips show no signs of being steamed cleaned? And there’s no mayonnaise looking stuff on the under-side of the oil fill cap? Are you able to drive for long distances at freeway speeds w/no signs of overheating? And no overheating in stop and go driving and while waiting in line for fast food? If so you’ll probably just have to monitor the situation. Eventually whatever’s broke will fail more, and in a way that will provide a clear diagnosis. Hopefully it’s an external leak that’s currently hidden for some reason, and easily repaired.

Another idea might be to do a shop test for combustion products in the coolant.

They only checked spark plugs and found nothing but didn’t look down into cylinder. Only some condensation on under side of oil fill cap, but it is strange that even if I clean the condensation it always reappears after driving. Does that mean there is always a very minor water leaking into oil? And the engine is never overheating both when freeway driving and city driving. It does not really matter that I need to add coolant constantly because of a small external leak, but I am worried the engine would be slowly damaged if it is a internal leak. Do you mean the test for combustion products is to use a solution to detected combustion gas in coolant? The dealer guys call it TK test, already done. So I think maybe a inernal leak is so small that there is not enough combustion gas leaking into coolant system to be detected.

Yes. The combustion of gasoline produces nearly as much water as there was gasoline burned. Most of the water exits the tail pipe as water vapor, but some liquid water gets into the crankcase and mixes w/the oil. So there’s always a little water in the oil, unless you drive mostly long distances on the freeway. That sort of driving evaporates most of the water out. Suggest to read this week’s “Ask Car Talk”, which Ray talks about this very subject. Click upper left. I’ve never noticed any condensation on the underside of my oil caps, but then again I’ve never looked specifically for that. I’m checking for anything really unusual looking there, something that looks like mayonnaise, which is a clear sign of large amounts of water or coolant in the oil. You don’t seem to have that problem. Which is good news. If you are able to, open the radiator cap (not just the expansion bottle) and see if you notice any oil floating on top. Given what you say, I doubt you will. Which again is good news.

Yes, that tested for exhaust gasses in the coolant. Negative means more good news.

Since nobody has any idea where the coolant is going, you’ll just have to continue to monitor the situation. If the coolant loss starts to get worse, or you notice other symptoms, take it back to the shop.

Another idea, some people – especially women – have a much better sense of smell than the rest of us. One time after I had installed a new gas range I had a co-worker that I asked to come to my apartment and use her nose to test if she smelled any natural gas odor. I thought I might be smelling something, but not sure. She said right away “no, everything is fine, no odor”. And she was correct. So if you know someone like that, ask them to smell for any coolant odor after you take your car for a drive. In the engine compartment, passenger compartment, or elsewhere.

I checked 3 other cars today, all of their oil fill caps do not have any condensation on the under-side, and all of their recovery tank caps do not make any sound when removed. I will keep an eye on it. Thank you.

[quote=“9969517_142053, post:12, topic:124893”]
I checked 3 other cars today … and all of their recovery tank caps do not make any sound when removed.[/quote]

On some cars the recovery tank is designed to be pressurized, while on other cars it isn’t. On my older Corolla and truck for example the recovery tanks aren’t pressurized.

Yesterday I started engine and left immediately for a 10 miles nonstop driving, after engine cold I saw the coolant level didn’t drop any!

Last week I started engine and just left it idling for half hours, then after engine cold I saw coolant level dropped 0.5cm.

So it seems that the car is losing coolant only when engine is idling or running at very low rpm, especially in the first half hours after engine starting. What could it be?