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Let's say for some reason you install a new timing belt and it's undertensioned. First, is this even possible? Let's say it is. What are the symptoms when accelerating and driving? A poster recently had no power in 1st and 2nd, and when he changed the belt it fixed it. What I want to know is that if the belt is under tensioned, if it could also feel as if there is engine movement due to the belt having more play than it should. The feeling would be similar to the engine moving iin the motor mounts.

 
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I doubt it, engine movement is strictly motor mounts. Whats your problem exactly?

 
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I have actually been through the whole timing belt change myself. Your engine mounts could need some changing if they look bad. (try looking for tears or separations inside the mount itself)

And if your timing belt is under tensioned, it would slip and cause your car to run like crap or completely stop running.
The only thing i can think of is if your timing belt did slip but only by one tooth, your engine is going to noticeable ride like crap. Considering the timing of your valving is now retarded by a little.

In terms of accelerating...well it wouldnt accelerate fast at all if any.

Start by completely changing the tensioner or readjusting it. I know its a lot of work but it has to be done.

I had the rear engine mount completely rip out of its position. And the only time I could notice a difference in engine movement was during shifting.

ps: if your feeling up to it. Check on your cam pulley and crank shaft pulley. there should be a line on all three of those pulleys. Make sure they all aim at the same direction. (not aiming at each other)
for example-they should all have the slit at the very top. Off by only a little and you found your problem.

I hope this helped out and good luck.

 
Rep Power: 266

Thanks for the replies. No, there's nothing wrong with my car. I was just wondering because I believe the rubber timing belt does naturally expand and contract, and the driver feels this while accelerating and such. Conversely, a timing change probably provides more consistant driving.

My mounts are all new btw.

 
Rep Power: 253

all a worn timing belt would cause is shitty idle. And maybe in extreme conditions even some midfiring.