When you have an engine-related problem, you have to list the engine size. Without knowing that, I can only suggest that in general, most Asian import engines are "interference" engines meaning when the timing belt breaks or even jumps just a few teeth, the pistons will hit the open valves as they coast to a stop, and bend them. It's a good bet that has happened but there's easier ways to tell for sure. Your mechanic wants to install the new timing belt so he can perform a compression test. The problem is if all the bent valves are bent real slightly, the compression could still appear to be okay.
A better test is a cylinder leakage test. Normally that involves turning the engine by hand to place a piston at top dead center on the compression stroke so all the valves are closed, then you pump in compressed air through the tester, and observe the percent leakage. This test can be done without installing the timing belt. The crankshaft is turned so no cylinder is at top dead center. That is just to allow the camshaft to be turned freely by hand. You don't even have to know where to place the camshaft. Simply turn it while watching the gauge. As the valves close, the percent leakage will go down. By turning the camshaft one complete revolution, you will have hit the point at some point where the valves are all fully-closed and leakage will be at its lowest.
Typically we'd like to see less than ten percent leakage, but the advantage to this test is you can listen in four places for the cause of the excessive leakage. If an intake valve is bent, you'll hear air rushing out at the throttle body assembly. If an exhaust valve is leaking, you'll hear it at the tail pipe. For worn piston rings, you'll hear the air at the oil cap or dip stick tube, and for a leaking cylinder head gasket you'll see bubbles in the radiator. I wouldn't be concerned with excessive leakage past the piston rings. We normally do this test when the engine is warmed up and parts have expanded to fit and seal properly. That won't be the case with your engine. Also, if the engine was running okay before the timing belt broke, nothing was damaged related to the piston rings.
$700.00 seems like a real lot for a timing belt and tensioner so you might want to check around for a second opinion. I'm more familiar with Chrysler products, and $700.00 would be about right to cover new head gaskets, and the timing belt and tensioner, and maybe new valves.
As far as is it worth repairing, the car is hardly worn out at 120,000 miles, and head gaskets can start leaking on any car. While head gaskets aren't uncommon, it IS customary to replace the timing belt and any tensioning devices at the same time to insure the quality of the repair, so if you look at it that way, you need a repair that is fairly common and not a reason to give up on the car. If you DID scrap it, what would you replace it with? You may end up with another interference engine in a car that's unfamiliar, with an unknown maintenance history, and it will develop the same problem soon.
Beyond that, I personally will never own an interference engine, but since you already have the car, consider that it is going to have a real lot fewer computers than are found on newer cars. Having a background in tv / vcr repair and car repair, I see all the time the expensive repairs and the highly-technical diagnostics and equipment required for systems that do not need or benefit from hanging an unnecessary computer on them. This is the main reason my daily driver is still a very rusty trusty '88 Grand Caravan. It has automatic transmission, air conditioning, front and rear heaters, front and rear delayed wipers, power seat, power mirrors, power steering, power windows and locks, a whole pile of interior lights on three different interrelated circuits, head lights, a sliding side door, dimming rear-view mirror, power lift gate opener, backup lights, horn, and get this, ... An electric starter motor to start the engine by simply turning an ignition switch! Not one of those features requires a computer, and as such, those systems are so reliable we rarely have to work on them.
For the last ten to fifteen years the insane engineers have seen fit to hang a totally unnecessary and unreliable computer onto every single one of those features I just listed. Instead of a simple, inexpensive, reliable backup light switch, now they use the switch to turn on a circuit inside a computer that turns on the lights. (Remember the game "Mousetrap" and how overly complicated it was)? This is the same thing. The starter is run through a computer too. No one knows what problem they found a solution for, but I have enough '80s minivans saved up to last the rest of my life. I'd love to buy another new van, but not when I know I'm guaranteed of having to spend thousands of dollars on repairs that should never be considered "normal". Plus, most replacement computers now have to come from the dealer and be programmed by them to your specific vehicle ID number. You can't even use a good used computer from a salvage yard. The manufacturers have figured out all kinds of ways to separate us from our money after the sale, but as a result, I know quite a few people who won't buy any of these vehicles. I would consider the positive things about your car and look at the repair cost as being less than two or three monthly car payments on something newer. If you sell it, someone else is going to perform the needed repairs and get another 150,000 miles out of it.
Tuesday, April 7th, 2015 AT 11:50 PM