It's been one week (without a radiator cap)
July 7, 2019 3:47 PM   Subscribe

The mechanic left the radiator cap off when doing a tuneup about one week ago. How worried should I be and what's the best steps?

2018 Subaru Outback
~280 miles without the cap, mostly 55-70mph for 20-30 mile stretches
ambient temperature has been 80-95ish

The only dash warning I received was last night and said to check the coolant and I got off the road about 15 minutes later (I was in a non-ideal place to stop and the prose of the warning was along the lines of "check your manual" rather than "get off the road now"). The engine compartment is splattered with evaporated coolant. The reservoir is at about the 90% mark between the minimum and maximum markers. I have not checked how much is actually in the radiator yet.

How worried should I be and what steps should I take? It does have comprehensive insurance - should I notify them? I have photos of the cap sitting nestled next to the radiator. The work was done by what I assume is a local franchise of a national chain.
posted by Candleman to Travel & Transportation (8 answers total)
You should call your mechanic immediately and report the error, and ask for their insurance carrier. Or yes, call your insurer and let them start that process for you. Don't drive your car or even start it. Your mechanic has Garage Liability that covers errors in doing maintenance and repairs that cause damage to your car. Start there.

IDK if you're still on the road but calling road service and having your car towed to another mechanic (maybe the one your insurer sends you to) would not be out of line, and should all be covered by the mechanic's Garage Liability, since it's your duty to prevent additional damage to your vehicle, once you discover the nature of their error. Your insurer should be able to get you a rental car, and all of this should be covered by your mechanic's carrier, or else your carrier will do it and then recover from them.

/former commercial auto insurance adjuster
posted by toodleydoodley at 3:54 PM on July 7 [13 favorites]

Assuming the 2018 is like earlier Subarus and most other Japanese cars, if the radiator cap is off, the reservoir level isn't going to reflect the level of coolant in the engine/radiator at all. (It needs a sealed radiator cap to draw coolant back in from the reservoir.) So the fact that the reservoir is fullish has no bearing on how much coolant was circulating through the radiator and engine.
Also, almost all modern cars run the coolant at temperatures near or above the zero pressure boiling point. (The pressure in the system prevents actual boiling.) So, anything which prevents the system from building pressure is likely going to result in localized boiling/hotspots/ loss of coolant through evaporation.
Your engine might be (or seem to be ) "OK" if the radiator is refilled, but there's a non zero chance of head gasket damage, warped heads, etc. which might take a while to surface.
posted by Larry David Syndrome at 5:14 PM on July 7 [2 favorites]

Re reading your comment, the saving grace might be the relatively short stretches you drove. I initially misread it as you drove 280 miles continuously.
posted by Larry David Syndrome at 5:22 PM on July 7

Check your manual. Does the car have a CLT (coolant temperature warning light) as such, and if so did it illuminate? Does the car have a coolant temperate gauge and did it climb to the red/warning zone? If yes to either/both, then you may have a potentially serious problem.

Step 2, fill the radiator with coolant. How much did it take? How much does the cooling system hold, and by subtraction, how much was in the system when you filled it? From visualisation of those volumes, does it look like the coolant was below the level of the top of the engine proper? If yes, you may have a potentially serious problem.

Loss of some coolant is no big deal. The critical question is did the engine overheat, either because the coolant got too hot (not enough left to absorb and transmit the engine heat out through the radiator), or the level dropped so far that hot parts (ie cylinder heads) had no coolant passing over them. Above diagnostics will give you some idea if either was the case. Document, through photos and notes, especially the coolant splashed over the engine bay. Then contact your mechanic.

We don't know just what a 'non-ideal' place to stop was, but driving for 15 minutes after a warning/advisory may have handed your mechanic a 'get out of jail free' card.
posted by GeeEmm at 8:06 PM on July 7

Before you do anything, don't add water or fluid to the radiator without first checking the level very carefully. You need to know how far below the cap the fluid level has dropped because that is the key to any possible damage.

A typical water/antifreeze mixture will not boil below 220 degrees and most engines do not run hotter than 220 degrees. So boiling fluid isn't really the problem, most likely, although sometimes the fluid can get much hotter in specific locations like around the exhaust ports.

What is a problem is if the water level was below the level of the cylinder heads and intake manifold, leaving them exposed to the air. Fortunately you have a Subaru which has a flat-four engine which means the heads and intake manifolds are a bit lower than many other cars.

So first find out how low the water level got in your radiator, maybe by using a bristle from a broom as a dipstick into the radiator passage through the cap. If the level is not too low, you should be okay. If it is very low, there could be permanent heat damage to the heads or manifold gaskets.
posted by JackFlash at 8:29 PM on July 7

Don’t take responsibility for this. Either call the mechanic or get your car towed to their garage and have them deal with it.
posted by freya_lamb at 12:44 AM on July 8

A late thought ... when you pulled over and opened the bonnet, what did you see/hear? Was there coolant gushing out, lots of gurgling and hissing noise and steam evident? This would suggest a boiling coolant situation, the seriousness of which would depend on just how much coolant remains (ie how low the coolant level got to). The absence of such sound and visual effects might be grounds for optimism, but until you know the coolant's minimum level you won't know enough about how the engine may have suffered.

Your mechanic needs to do a compression test and a leak down test. If those values are in spec, the engine is probably ok. If you only put a little coolant in to get the radiator filled (say less than a litre), that might also suggest you have dodged a bullet. But I would be looking to move the car on ASAP anyway, unless you like rolling the dice, and can afford a second-hand engine swap down the track. If the values are not in spec, you are going to have some serious talking to your mechanic.
posted by GeeEmm at 12:53 AM on July 8

While the first answer is probably the best one to follow in case there IS a problem, I really don't think this is a big deal. It's a belt and braces line to being covered if there was a big issue, but most likely this is no big deal (albeit incompetent on the mechanic's part).

Before you do anything, don't add water or fluid to the radiator without first checking the level very carefully.

In most systems the only way to know is to fill it up. Just measure what you put in. That will tell you enough.

s for my answer - there is a good chance this is non-issue. Water flows through the radiator at all times, even when low in volume, and no part of the pressurisation process causes an issue with coolant flow. So the key is how much coolant you had to add. The coolant expands as it gets hot and the pressure cap allows a constant volume to be used. The worst thing that happens with an unpressurised system (as long as it stays out of the 'big problem' range) is you lose some volume of coolant as it expands, overflows out the open cap and then just have a partially filled system when it cools again. This means you have less cooling capacity and so the car can be more prone to overheating. However, this DOES mean the system remains full of water. It just means it can't cool as efficiently.

So, if you are adding a gallon of coolant, we will likely have an issue. If you just needed to add a quart or two, then you are likely completely fine. If it were me, I'd do the following: top it up with the correct proportion of coolant/water and see how much it needs. If it is 2 quarts or less I'd put the cap on and utterly ignore the issue from now on.

However if the following occurs:

More than two quarts required? Potential problem. (potential only)

Engine showed signs of overheating? (as in more than 3/4 of the way across the water temp gauge)? Possible issue because enough coolant had been lost to mean some parts of the engine were likely running dry or partially dry.

More than a gallon required? Higher likelihood of localised hot spots and engine damage as the coolant gets to very hot parts of the engine and locally boils. Water pump bearings in particular are prone to damage in this case, as is head gasket and (in extreme cases) possible head cracking. However, I think the coolant would have to be low AND the car overheat noticeably for these to happen. In extreme case enough coolant can be lost that the temp sensor goes dry and so you'd only notice if you were paying attention - the gauge would show hotter and hotter then drop to somewhere near normal or 'not as hot' as it then measures air temp not direct contact water temp. But with short journeys this is less likely.

So, summary. If this is a small amount of lost coolant it is likely a zero issue. If it is more, there is a sliding scale from 'probably still ok' to 'not good'. Which is why the uber-cautious legal approach in the first answer has merit.
posted by Brockles at 10:52 AM on July 8

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