The timing chain just broke on my 2005 Mercury Mariner. I know it is an interference engine…is it worth replacing the chain or is the likelihood of engine damage so great that it just isn’t worth it?
Smart money says do not try to fix it. There is always a slim chance you had minor damage, but not worth to risk of the time and money you will invest. If the car is in good shape otherwise, see if you can get your mechanic to put a used engine in ( with a new chain)
You doing the work or paying someone??? Parts are a couple hundred bucks…labor could easily reach $1000 or more.
If it were my car I’d tear down to the valves and see if they were trashed. But I can do that for free on a Saturday afternoon in my garage. If I had to pay someone to look? Nah. Overwhelming chances are that the engine is toast, so it’s not worth putting cash into it unless you really like long-odds gambling.
You could toss some short money at it and do like Shadowfax said except pay a mechanic to do it. Expose the chain area and valve covers, remove plugs. Rotate the crank and cam(s) so valves are closed and do a leak down test on the cylinder. Lather, rinse, repeat for the other cylinders. This may also uncover the reason the chain snapped and make the decision for you (e.g. something seized up). If you can’t do yourself, then maybe get an estimate for this effort from an independent mechanic.
You could test the valves without major surgery by manually turning the crank until each piston is at the top of the compression stroke and testing to see if the cylinders will hold air pressure. The crank should turn pretty easily with the plugs out.
Oops, I’m sorry TT, I just noticed that you suggested exactly that! I guess I should change my post to say “I agree with TT”!
I also agree withTwin Turbo. I bought a '98 Jeep Cherokee for a couple of hundred dollars a few years back because it had a snapped timing chain. I just pulled all the sparkplugs and used a compression tester on the engine. All cylinders were in the normal range so I decided to replace the timing chain. The engine ran like a new one and it’s still running today.
The vehicle is over ten years old so it probably has a lot of miles on the engine. And being an interference engine you know valve damage has occurred for sure. And who knows what damage has occurred to the cam shafts/pistons?
So to repair the engine will cost more than the vehicle is worth.
I had a customer with a 2001 Infinity QX4 where the timing chain failed. And just to tear engine down to the point where it could be determined what was damaged was going to cost more than a $1000.00 just in labor. They decided to replace the vehicle.
I have a question for @missileman.
How was the engine able to produce any compression with a snapped timing chain, and the valves weren’t opening and closing?
Tester
Just curious OP, what was happening when the timing chain broke? Idling in the driveway? Driving to 7-11? Or freeway driving?
Often timing chains tend to snap right at start up. If that was the case, there’s a better chance of only minimal damage to the engine. If it snapped on the freeway, more likely than not you’d be looking at a replacement engine or a replacement car.
BTW, it’s sort of unusual to hear reports here on Car Talk Forums of broken timing chains on 11 year old cars. Are there anything that happened to your car in the past that might have made this more likely to happen? Like loss of oil pressure?
On today’s I4/V6/V8 engines that utilize timing chains, plastic timing chain guides are also used.
Here’s timing set for this engine.
https://www.rockauto.com/en/moreinfo.php?pk=3561942&cc=1431307&jsn=538
And when these plastic timing chain guides fail, and it’s an interference engine, you’re going to bend valves.
That’s how it happened on the QX4. Started the engine, it ran for like 5 seconds and shut off.
After that, the engine spun over with no compression.
Not all vehicles are as antiquated as your 70’s Toyota/Ford.
Tester
So it’s really a Ford Escape with nice seats, right? 2.3 L 4-cylinder engine…it’s worth 5 to 7 grand depending…A decent repair shop can determine whether any valves got bent before replacing the chain…Even if the head is shot, a rebuilt head can’t be THAT expensive…As it sits, it’s not worth squat…Shop around, including a Ford dealer…There are places that specialize in this sort of repair…
Not all vehicles are as antiquated as your 70's Toyota/Ford.
Antiquated? I prefer the term “classics” … lol …
It worked for me. Try thinking out of the box Tester. Hint: How does a bicycle pump create pressure without valves?
Missleman:
I must be missing something. Bicycle pumps have valves inside.
WRT to Tester’s question, on an engine at rest, some number of cylinders will have intake and/or exhaust valves open. With a broken timing chain or belt, rotating the crank will not get you to a state where both valves are closed.
I’m curious. How are you saying it’s done?
Geez, you have to move both the crank and cam(s). It takes careful rotation to get them in the right position but trust me, it’s been done before. And, interference engines have survived when belts/chains snap. Perhaps a small minority but worth at least checking, for me anyway. Throwing it out on an assumption is a (relatively) rich man’s solution…
TT:
Based on the earlier description,
I just pulled all the sparkplugs and used a compression tester on the engine.
I assumed this was done without opening up access to the camshafts. Hence my confusion.
If the cam(s) are rotated in 2:1 sync with the crank, yes it’s doable.
Missleman:
Is that how you did it?
I simply rotated the cam by hand until the valves were in the closed position for each cylinder. I’m sorry that I left out that little detail.
Sorry @missileman, that doesn’t work with a compression test.
If both valves are always closed, no air getting into and out of the cylinder to be compressed. As a matter fact, when performing a compression test, the throttle plate must be held wide open to make sure the correct volume of air is entering the cylinder. Otherwise the compression reading will be skewed.
And if you turned the cam shaft by hand, it means the timing chain cover had to be removed. So why didn’t you just replace the timing chain at that point?
Tester
I screwed up but apparently got lucky… nobody caught it! I suggested the OP turn the crank by hand until both valves were closed… impossible with a busted chain! I should have said to turn the camshafts… but, oh well, I can live with myself. 
The timing chain broke on start-up. The car has had some issues with oil in the past, but I had just changed the oil the day prior, so it was definitely full of oil. The shop I towed it to knows about the oil problems and they seem to think it’s worth the $1000 it’s going to cost me to fix it. I’m good enough to change the oil, but not to do a job like this.