Asterix, you’re right. Sorry I misspoke. The cam shaft “top” spoke is aprox. 25 - 30 degrees past tdc, not before. And no there was no backfire and yes this is weird, it should have damaged the belt but that wasn’t the case. Everything does logically point to some erratic behavior of the tensioner, maybe an occasioanally wonky spring?
I have a lot of confidence in my mechanic and if he’s stumped?? I’m going to talk with him again Monday morning.
Well, both ways are correct, no problem, the fact is that it jumped “backwards” if Your mentioned tdc is also on the crank. Is Your car with manual transm.? Did You park on a hill, nose up, and just put it in 1*st gear without using the handbrake? If you did, the backwards motion could have turned the engine a little and very little is all it takes for the belt to go slack.
In my experience: If the teeths on the belt could withstand the force of being pulled over the crank, then the belt would have snapped. The increase in radius on the bottom wheel would be much more than the belt (correctly fitted) can tolerate.
Could be a missing cotter pin on a pulley.
a missing cotter pin on WHAT pulley?
By no means have I worked on all vehicles that use timing belts, but I’ve never seen a setup that uses cotter pins to hold pulleys for the cam timing . . . not on a vehicle with a timing belt, FWIW
I’m not saying it doesn’t exist, but I haven’t seen it or heard of it, and the idea sounds kind of strange to me
It’s a manual and was parked on flat ground when it happened. Everything seems to point to a temporary loss of tension.
I meant to say woodruff key.
Yes, but then the cam would be delayed compared to the crank, not ahead. That’s impossible. Unless it jumped a full 360 degrees minus 5 teeths, and unless it’s freewheeler, that engine would not start again.
I was reading that a belt will skip teeth if moved backwards, especially if it is an OHC engine. I came across this thread while trying to do more research and now that’s where my head is at. Especially when hearing that everything was in good condition otherwise, I’m wondering if that’s what happened here. Like if you were parked in a gear that was too high for the incline you were on, or if you got bumped backwards (again while parked). I’m only just starting to learn about cars (and how to drive for that matter), maybe there’s a more obvious scenario where this could happen.
I know this is old but I’m curious if I could be correct!
Exactly! I am always a nervous wreck when changing the timing belts on interference engines. I usually gently clamp the top sprockets together so they cannot turn before removing the belt. I just clamp them so they are tight but you don’t want to over muscle it either. I have seen people lock them together using a tool made from an old timing belt as well.
I wonder if the belt had been done using cheap parts in the past. This is why I always buy the highest grade of parts store brands or just get OEM. Random cheap parts on the internet make me very nervous. It is one thing if it is cosmetic but skimping isn’t good on something critically important like this.
Anyway, glad it survived. As mentioned, the engine doesn’t have to be running to do damage. A starter has more than enough power. Accidentally turning a sprocket while doing work is also bad.
It’s a good question. It seems unlikely for a recently installed belt. I suppose if the belt were close to worn out it might skip when rotated backwards, b/c the wear pattern on the belt teeth might make it easier to slip. For a belt in good condition I expect the engine would really have to rotating backwards at a significant rpm before the timing belt would slip. Skips more likely caused by incorrect belt tension, so if the belt tensioner didn’t work correctly in the reverse direction, such a thing might happen.
That is a possibility if the car is a manual transmission.
Once on my 4.6L Ford I had the manual transmission in a forward gear with the front parked uphill. It rolled downhill backwards for a bit after I turned it off, essentially turning the engine backwards. It rolled with no resistance vs. if I had it in reverse and still wonder if I wasn’t accidentally in 3rd instead of the intended 1st gear.
It took forever to start and ran crappy for 30 seconds or so once I got it started. Of course this one is chain driven but I was told I likely bled the tensioner down and introduced all kinds of slack in the chain by doing this. The tensioner also took a while to pump back up with oil and that is why it ran crappy for a while. Another bad thing about this is that someone told me if you suddenly start an engine with slack in the chain like this, you risk snapping the chain on this engine which is also interference by the way.
Being that this is an old engine with many miles and I don’t want to snap the chain, plus the danger of a roll off, I am always careful to put it in 1st or reverse, depending on how I am parked. It it were to try and roll forward based on the slope, it goes into 1st. If it was to try to roll backwards based on the slope, it goes into reverse. If the slope is steep, I put it in low-range for even more benefit.
Also, on many of the manual tensioners, they will have the appropriate notch for a NEW timing belt vs. one that has already been in use.