What would cause a timing belt to jump.

[jim90TSi]

I recently replaced my timing belt because my old one was eaten up by a hose clamp that fell down in there, no big deal. So i throw a new belt, goodyear from autozone. Car is running good for about 2 weeks. I park it one night and go out to start it later that night, no start....when i first cranked it i heard clanking.......thought ah crap. Looked around for obvious stuff then went staright to a compression test. Found 0,0,20,50. Bent valves...all to be exact. What would have caused the timing to jump like that? It jumped about 7 teeth on the crank, none on cams. My autotensioner was still in good condition, no leaks and very stiff to compress. To tension the belt, all i did was pull the pin on the tensioner and let it be for about an hour. Put it all back together and it ran great. What would have caused the random timing belt jump? When i checked the slack on the belt after the jump( between cam and tensioner pulley) it was loose. By the time it got towed home, it was tight again..........any suggestions.

 

[FullyBuiltDSM]

Maybe it wasnt tightened correct or put on wrong somehow??? not really somthing you wanna play around with though....valves can be expensive

 

[Old Mitsu Tech]

Setting the autotensioner is critical in the 4g63. If you followed VFAQ instructions to the letter, then perhaps you could rule that out. However, if the tensioner was fully extended when you checked then probably the improper installation theory could hold.

I've seen the belt job done and the auto tensioner was not compressed. That usually lasts only a few days though. You know the rest.
Check it over, something should stick out.

Good lUck

 

[jim90TSi]

on the tensioner setting with the drill bit and special tool i didnt do. i tightend the pulley then pulled the pin. I was told by a good mechanic to do that, so i trusted him. So, i bet the tension was wrong. If my autotensioner is still stiff to compress could i use it again? I have a new head and i don't want to bend these valves also.

 

[jim90TSi]

and where should i buy an autotensioner if i have to replace it.

 

[GST with PSI]

i was extremely cautious when i replaced my timing belt just for this reason. i wanted to make sure the belt was properly tensioned before i put the car back together. after installing the timing belt i ran the car with all of the accessories off (power steering alternator, etc.). i just let the car run for a minute to basically simulate an actual running condition on the belt rather than just turning the motor over by hand. i, like you, was planning on using all of the old timing components but was scared the hydraulic tensioner may be bad. i followed the vfaq procedure when installing the belt making sure rotate the engine and then again check the tension after waiting the allotted 15 minutes after initial tensioning. i still wasn't happy however, and let the car sit over night. i came back the next day to check the tension and the belt was limp. i stared the engine again knowing due to the relaxed positions of the cams and crank slack would occour temporarily in the belt over night. i was expecting the slack to disappear after the engine had been run but it didn't. turns out my hydraulic tensioner was bad. by re compressing the tensioner rod it restored some of the hydraulic pressure in the tensioner. it was only a matter of time before it lost the initial prime i gave it after re installation. i put on a new tensioner and did the same procedure above once again waiting overnight and didn't have any problems.

i would say above is probably what happened to you. keep in mind your belt is pre- tensioned as well as auto-tensioned. if you can remember, your timing belt was pretty tight after you initially pre-tensioned it, so even if you hydraulic tensioner does totally fail, the belt's pre tension will hold the belt on for a while. the problem is in the mornig after your car sits out in the cold you hit the starter and jolt the belt. your engine block is the same temperature as the weather surrounding, as is the oil and coolant contained in the engine block. all of this abnormal load caused by cold contracted moving parts and thick oil puts a wicked strain on the timing belt causing it to slip. the majority of all timing belt failures occur during initial startup form what i understand.

 

[sherwinz28]

This I have a feeling is what happened to me as well. My car was driving fine for like a couple hundred miles after my head/timing belt job and then one day after I drive it in the rain with no air filter and no timiing belt cover and an improperly tensioned timing belt. I was definately asking for it. I didn't have the tool for the timing belt tensioner as I thought I would just put it on and pull the grenade pin which seemed to work but I believe my belt was not properly adjusted and after a couple hundred miles it slipped and I checked compression and got 150 150 150 30. I then checked the timing marks and could not get the marks to align at all as well as being very difficult to manually turn the crank by hand. This time I'm doing it word for word on the vfaq on how to tension it properly.