QuestionQUESTION: so I have a 99 mustang GT (V8), Its had a clicking in the engine for a while now. most people told me to just ignore it cuz its not that bad sounding. so the other day, I really stomped on it and it sounded great, and it drove home just fine from there. the next time I started it (the next day) it started for a second and made a "clunk" noise and died. now when I try and turn the engine by hand, it will only turn 90 degrees back and forth and hits something (apparently inside) I took the belt off to see if it was an accessory, that didn't help. and I pulled the valve covers to see if there was a rocker that jammed in the cam, nothing wrong there. and I pulled all the plugs and felt down there to see if all the pistons were moving when you turn the crank and they seem to be. I also took all the rockers off to see if a valve stem would come out without the valve, and they're all fine.
any ideas would be much appreciated! thanks!
ANSWER: I would be suspicious of the timing chain having failed, possibly because of the tensioner or guide failing first. This would have been the noise that you heard. These engines are not prone to failure of these systems, but they are not unheard of, either. Since you have the cam covers off, does the cam turn, in both directions when you rotate the engine? If it doesn't, or you can see that it is loose (grab it and give it a yank), then that is most likely the problem. there is always a chance something happened with the clutch or transmission, which are directly connected to the engine, but it's more likely the chain.
---------- FOLLOW-UP ----------
QUESTION: thank you so much for your reply and help. the chain seems tight and the cams looked like they rotated in both directions with no slack but I'll double check. but you're right the whole thing where it does turn 90 degrees is leading me away from piston/crank problems or piston to valve contact etc. the car has a brand new (1000 miles) automatic trans so no clutch etc. I'm hoping the problem isnt in the trans cuz my plan is to keep tearing apart the engine.
there's a little more to this story too. I was suspecting the noise may be an exhaust leak or spark plug leak so I decided to change the plugs. I pulled all the plugs and changed them. one of the plugs had an adapter on it because the previous owner had blown a plug out of the head (common on these) and they drilled and tapped a larger hole. anyway, I replaced the plugs and that's when this happened. I removed all the plugs though and none are bent or damaged in any way so its not like the pistons hit into them or anything.
Answer Hmmm. I would try to get a good look into that cylinder where you have the adapter. If part of it broke off, or if screwing in the new plug caused it to protrude further into the cylinder, then it might be keeping the piston from going past top dead center, which would keep the engine from turning. The 90 degree rotation thing makes me think you have something stopping one of the pistons. They sell, and you might be able to borrow, little fiber-optic scopes for looking into combustion chambers and other places that eyeballs and mirrors won't fit. They come with a light source and everything and are super handy to have.
