Timing chain jumped. Probably bent a piston. Is my engine worth fixing?

Question

I have a 2007 Mazda 3 2.3L with about 155k km on it.

I was attempting to replace the AC pulley bearing, and had to manually turn the crank pulley to get the belt back on the AC pulley. After making sure the AC pulley was tightened, I started the engine to see if the AC worked. When starting the car, I learn a bang/clank sound, and went back to see that I forgot to remove the torque wrench I had on the crank pulley. The starter would no longer start the engine, and it seemed like it was spinning faster than normal.

I was told that most likely the wrench caught when I started the car and stopped the crank pulley from turning, and subsequently caused the timing chain to jump, which is why it wouldn't start anymore. Also, I turned the crank pulley a couple more times and then it would no longer turn. I was told that means that the chain likely bent a piston when it jumped, which is a very costly repair.

Just wanted to see if this all makes sense, and if it's worth taking the engine apart to fix, or if I'm better off getting a used engine with equivalent mileage to swap in.

Thanks in advance.

Update: As per Howie's suggestion I took the valve cover off to check the timing chain, and it looks like it's still on. And now the crank pulley is turning again. I turned it for one full revolution of the chain, so I guess it's not a bent piston/rod etc. I'm not sure what could have caused it to get stuck before, but I guess it wasn't the chain. As I was turning it the chain got some slack on it at one point but then got tight again. I'm guessing that doesn't make a difference. Now I'm kind of confused as to what the cause of it not starting is, but I'm guessing since it's turning, the piston/valve isn't bent so it looks like I don't have to replace the engine just yet...

Update 2: As per Ben's suggestion, I got it at TDC (sticking a wooden dowel down the shaft, turning the crank pulley and stopping when it's at peak with the lobes pointing up) . The picture of the chain at TDC is below. There is some slack on it at that point but I'm guessing that's not an issue. There's also a close-up of what I believe is the balancer, and I can only see 19 teeth off the chain.

However, what I noticed when taking the spark plug out to try to get it at TDC was that one of the spark plugs was mangled. Seems like it was hit by the piston. Would that cause it to not start, or for the piston to be bent? At this point I'm thinking I should just replace the spark plug and give it another shot.

Update 3: I replaced the spark plug but am hesitant to start it just yet lest I also break the new spark plug. I shined a light down the shaft where I pulled the spark plug, and it looked like the piston had hit the spark plug from underneath, or something got in between the two and damaged both. There was a small gouge carved into the top, as shown below. I'm not sure if that helps, but it's just another clue to solve this mystery.

Answer 1

Before jumping to conclusions, you could take the rocker cover off and see whether the chain is still connected. The fact that you were able to turn the engine over for a while quickly without it jamming may suggest that the chain snapped. The valve gear may be in a safe position and it could possibly just be the chain that is now jammed.

Answer 2

IIRC on the 2.3 there is no keyway to keep the crank gear and balancer aligned. If the engine is spinning and has compression you probably spun the balancer and it's out of time.

Set piston 1 at TDC and line the balancer up as indicated.

I think this is an M6x1.0 bolt.

This should be 20 teeth off the blank space on the balancer.

Here's an example using a scope, this came in after a head gasket replacement and you can see the balancer is slightly off.

Yellow: Cylinder 1 Compression
Blue: Crank Position Sensor
Green: Cam Position Sensor

To find TDC without special tools

Pull all the spark plugs and remove the cylinder head cover if it hasn't already been removed. Put a long screw driver or similar in the #1 spark plug hole and rotate the crank clockwise until the screw driver is at it's peak.

On the rear of the camshafts there will be slots that should line up with the cylinder head.

If you want to make sure it's dead on use these tools.

https://www.amazon.com/Assenmacher-ASMFM2330-Piece-Mazda-Timing/dp/B009SB70N2

The pin goes in a slot on the left side of the engine and locks the crank shaft at TDC.

Note that on these engines the crank bolt holds the balancer with a diamond washer and needs to be replaced if removed.

Either way compression check the engine.

Answer 3

I don't see how the timing chain could have jumped or broken in this situation. If the wrench was on the crank pully, and this is what stopped the engine, there was little to no force on the timing chain because there's little resistance on the cams. I'm guessing you probably damaged your starter gear or broke some teeth off of the flywheel, so the faster spinning sound you're hearing might just be the starter spinning in place and not actually cranking the engine. First, make sure the engine is actually turning with the starter and go from there.

Answer 4

The good news is that everything happened at low speed, because you never actually got the engine running. So whatever the damage is, it will probably be fairly limited.

The bad news is that with this sort of problem, you don't know what you are going to find until you take the head off at a minimum, and just doing that isn't cheap even if there you find there is no collateral damage. If the valves are obviously damaged you really need to check the pistons and bearings as well, and that's even more expense on taking things apart.

IMO taking a chance on just replacing the chain, retiming the engine, and performing your local customs to ensure good luck is not a good idea. If you have bent a valve stem or cracked a piston crown, you want to find out when the car is in a workshop, not when the engine spins up to 2000 RPM just after you started it, and most definitely not while you are driving!

If you can find a local car breaker with an old replacement engine that doesn't look obviously past its sell-by date, that would probably be your cheapest option.

Answer 5

If the chain jumped or broke while turning the engine with the starter, what could happen (if you are unlucky) is that a piston hits and bends some valve. The piston would probably survive. If so, it would not be quite as expensive to fix as you have been told.

Answer 6

If the engine is turning now I think there's a fairly good chance you avoided any serious damage. See my question and self-answer from when something similar happened to me: https://mechanics.stackexchange.com/a/4470/426

Try realigning the timing, using the procedure I did if possible: turn the engine until all pistons are centered or at least not at the top of their cylinders, then adjust the crankshaft to match, so that you don't have to worry about getting into an intermediate configuration where the pistons hit the valves. If you manage to get the car to start, it's probably okay, but it wouldn't hurt to take it to a professional to have them check it out. When I did this the timing was still off by a notch or two and the mechanic diagnosed and adjusted it for a very reasonable price: Low power, bad gas mileage, smelly exhaust after engine work

Answer 7

You guys really have no idea what yous are talking about get the head off and you will find the head of a valve gouged into the cylinder head, which is the only cause of the spark plug damage

Answer 8

If the damage is what you think, most likely the repairs would cost more than a replacement engine. You might need some machining done in addition of replacement internals.

So I would go for a full replacement.