Direct relationship betw compr test & blown head gasket?


#1

This is for a 93 subaru legacy coupe, automatic. AMA (against MY advice, i.e., I am the (so-called “wise” (interfering), “experienced” (know-it-all, pompous) (you pick the correct adjectives) father; the impatients are my son & his girlfriend, both early 30s), they decided to buy this car for a 1-time-only cross-country trip (~4-5 K miles) in 3-4 wks. rather than rent a car & pay the higher fees to leave it “there” (end of the trip, which ain’t here), & sell car for some bucks back at end of trip. They decided to save further $$ by not paying the extra $50 or so for the shop to add a compression test to the checkup of the car, & declined for me to test it (yes, I rebuilt my cars for 14 yrs. or so in my own foolish youth, & still have all the tools/scars to prove it). So within 20 miles, after they bought the car, it checking out ok, including radiator, oil & electronics, it blew up literally – a blown head gasket, which AAA (yes, I bought it for them as a parting gift/warning) tow person said the pre-buy check could not have foreseen.

I think that while a compression check only sees low compression per se, it might have spotted this problem. But I’ve never thought about the direct connection much. I never had a blown head gasket on my cars, replacing whenever I opened the engines. I know uneven compression may indicate an already failed head gasket, but what about a compression test foreseeing a head gasket failure that has not yet happened? I also know that uneven compression is of course in general a very bad sign & if I had seen it I would have said “no” to the car. But what if the readings were low but even, as specifically regards head gasket? (I did look thru a few pgs. here before posting, but didn’t see a direct answer.) Thank you.

FYI, they did rent a car after, & continued happily enroute.


#2

I’m unclear on why you even need to ask. You already have an authoritative answer from a tow truck driver. Why did you have to rebuild so many engines?

More importantly what was the mileage on the car?

Good for you for buying the AAA membership.


#3

“You already have an authoritative answer from a tow truck driver.”

I don’t know about the tow truck drivers in your neck of the woods, but most of them in my area are most definitely NOT mechanics. These guys know how to change tires, jump batteries, and pour a few gallons of gas from a jerry can into someone’s car, but their diagnostic pronouncements are more likely to be random guesses, rather than skill-based answers.

I would not want to base my decisions about repairing a car on statements made by most of the tow truck drivers whom I have met.


#4

A compression test can tell you if a head gasket problem exists (other tests should be run concurrently as verification) but it cannot predict the future.

Head gasket failure is, and has been, very common on Subarus and it could be that the seller of this car overheated it, got cold feet about the possibility of a blown head gasket(s), and decided to unload the car.
Your son and his girlfriend were simply the beneficiaries of something that was bound to happen.


#5

I imagine a compression test would probably have shown uneven compression, but it may not have. The warning sign? It was an 18 year old unknown car, just about the last thing I’d pick for a cross-country trip…but you already knew that.


#6

Here’s a story. My BIL and I were talking about the old days and the cars we had. He had a 55 Desoto and was trading it in on a Buick the next day. Took it out drag racing and spun a bearing on it. He and his buddy pulled a bearing out of a junk engine and put it in. Towed it to a block from the dealer and drove it in. Had to go back to the dealer a week later and the guy said, funny thing the boss took that car home and blew the engine up on the way. Who woulda thought? I guess you never can tell the history on a car for sure.


#7

Unless blown totally out, compression will not be affected by a blown head gasket. But often, pulling the plugs and cranking the engine over will make the problem evident. If the leaky gasket allows coolant into the combustion chamber the spark plug will be discolored and often cranking the engine will throw vapor or liquid out the spark plug hole. Believe it or not, there is usually a distinct odor under the hood or out the tail pipe when a head gasket fails. That odor has never misled me.


#8

“VDCdriverVDCdriver July 1 Report
"You already have an authoritative answer from a tow truck driver.”

I don’t know about the tow truck drivers in your neck of the woods, but most of them in my area are most definitely NOT mechanics. These guys know how to change tires, jump batteries, and pour a few gallons of gas from a jerry can into someone’s car, but their diagnostic pronouncements are more likely to be random guesses, rather than skill-based answers.

I would not want to base my decisions about repairing a car on statements made by most of the tow truck drivers whom I have met."

Wow. Nothing gets past you, does it?