Occasional death rattle on startup – but not during day – chances engine dies during day?

My question is: Assuming my engine does not destroy itself during the death rattle in the morning, how likely is it to do so during the day?

This might be an odd question because I am wondering when (time of day) my engine is going to fail as opposed to how to diagnose / fix something. My 2011 Prius with 198k miles sometimes has a minor death rattle on cold start in the morning. It is a very minor rattle and resolves itself within 5 seconds. Car runs fine for the rest of the day, until the next morning, when the cycle repeats. Yes, it appears to be burning coolant, so there is likely a head gasket issue. It takes a little over a year for the coolant to go from full to low (supposedly .5 liters) which in my mind is a very small leak. For purposes of this question, just assume there is a small head gasket leak.

The death rattle only occurs in the morning. My typical drive cycle is that my drive to work is long enough that the engine comes up to normal temperature. Some days I take a short trip after the engine has cooled to get lunch, where the engine does not get up to temp. Other days I don’t get lunch so the engine stays cool for the full work day until I go home. My drive home gets the engine back up to normal temp. The only time the death rattle ever occurs is in the morning after an overnight cold soak.

If the engine blows up in my driveway at home, I am perfectly ok with that. I will just get in my other car and go about my day. If the engine blows up while I am at work/driving, it is a much bigger problem. First, I park in an underground parking garage in a high rise building in Chicago. Getting a tow truck in there is difficult and expensive. Second, if my engine blows up in the underground garage, they are going to be upset about 5 quarts of oil being dumped on their clean garage floor. I am sure they will charge me a huge cleanup/environmental fee. Third, if the engine blows up while I am driving on the road, I may block traffic on a major highway, inconveniencing an uncountable number of people.

To anticipate some questions / comments:

1. I am aware of the multiple techniques for confirming head gasket leaks. The majority involve some level of disassembly of the engine compartment (wipers, cowl, etc.). It is November and I am in the Chicagoland area. It is cold and my only place to work on the vehicle is outside. I am not doing that until next May at earliest

2. I don’t want to pay for repairs. Local Toyota dealer quoted $5176.00 for head gasket replacement. Another chain shop was $3600.00+. I can’t imagine an independent mechanic would be less than $2500.00. I feel I have gotten my full value out of this car and don’t want to put more money into it.

3. Some are going to say the prices quoted for repair are too high. Although I might agree with that statement, these are the prices where I live. I don’t necessarily live where you live.

 

Thank you all for you responses!

I completely understand that no one can tell me for sure when it will fail. I am looking for probability based on symptoms. From the searches I have done, the most common scenario is that the death rattle in the morning gets progressively worse over multiple days/weeks, then the engine completely dies one morning. I am sure it has happened, but I have not come across a post where the engine started up with a minor rattle in the morning, then destroyed itself 8 hours later. Also, I don't understand why you say I am risking other driver's safety. You said that no one can tell when a head gasket will completely fail (which can happen in a brand new car), so that means every vehicle on the road is risking other driver's safety? Aside from the possible issue of a piston being shot out of the engine block onto the road, how is a blown head gasket putting other driver's safety in any more risk than someone running out of gas? Also, why would I move onto a non-hybrid (which I have not)? This is an ICE only problem, hybrid has nothing to do with it.

Looking back on my original post, I think I might not have been clear. I am going to drive this Prius until it has a failure that renders the car undriveable (see reason why below). I understand the head gasket issue will cause the car to eventually die. I would just prefer that it die as close to my house (as opposed to 30 miles away at work) as possible. Who knows what the failure will be though? I am still on the original 15 year old traction battery. There is also the ABS booster failure that has a $4k repair that I keep hearing about. Maybe those happen first? What should be understood is that I am not going to get anything preemptively fixed.

I am not going to use any of these types of seal products because I don't want to screw the next guy down the line. Maybe the traction battery dies first and I sell the car to an informed buyer. He may want to legit fix the engine. Dealing with that sealant stuff is going to make his job so much harder, if not impossible. I don't want that kind of karma.

Multiple commenters have suggested selling with full disclosure. I wish I could do this, but I can't (see below). It is way too easy to hide a Gen 3 Prius blown head gasket if the car still moves. Car flippers are out there and they are garbage people (see my reason below). I am going to drive this vehicle until it dies in such a way that it is very clear you will need to do major repairs before selling.

TLR version of see below

Early this year I decided to replace my 22 year old Nissan Pathfinder with a 2025 RAV4 Hybrid. The dealer didn’t want the Pathy as a trade-in and said to just sell it myself on Marketplace. I went home and wrote a two paragraph description that fully disclosed every single thing that I knew was wrong with it. I even finished my description by saying that if you don’t have the skills to do the repair work yourself, I would strongly advise against buying the vehicle because mechanic’s costs are going to eat you alive. The car had less than 100k miles and the drivetrain ran like it was brand new. I needed it gone within two weeks, so I priced it at the real low end of the Blue Book value.

A guy came to see the vehicle Saturday afternoon saying he was looking for a car for his daughter. For even more full disclosure, I reiterated verbally everything I knew that was wrong. Two of the things (ABS system and front shocks) were things that I considered safety concerns that should really be addressed before you let your kid drive it.

Sunday morning (less than 18 hours later) I look on Marketplace and see my car up for sale again (this time at the high end of the Blue Book value). You know what was missing? My two paragraph description of everything wrong with the car. I felt so bad that this dude was trying to flip this car onto some unsuspecting person who might just see the low mileage for a 22 year old car and think it was great. The only things that assuaged my guilt were that 1) it is very difficult to hide ABS faults by resetting the codes because unlike emissions faults, they come back right away and 2) a quick test drive / pushing down on the front end would easily reveal the shock problem.

A small Prius head gasket leak is just too easy to hide. Keep the coolant full and don’t let the potential buyer hear the first start after an overnight cold soak. No seller is going to let you dissemble half the engine compartment to get a scope in the cylinder.