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Can a bad tensioner cause a misfire?

I have two problems on my 2000 Jetta 2.0 and am just trying to figure out if they are correlated. I have a random misfire at idle, so far I have seen it on cyls 1, 2, and 3. I have replaced the plugs, wires, and coil pack, no luck. The misfire is only at idle (~800 RPM) and if I just touch the accelerator to get it up to ~900 RPM it runs fine. I just had my timing belt done and I think the bearings on the tensioner he put in was bad because I get a high pitched drone/squeel. I listened to the area with a piece of tubing and it is definitely coming from the timing belt cover. Could this bad tensioner be causing the misfire or is it a red herring?

Did you have misfires before the timing belt was changed? It would be good to say one way or the other.

A high pitched noise from the timing belt usually means the belt is too tight.

As to the misfire, could it be the timing belt is installed a tooth or two off?

Other than those 2 things a rough running engine at idle can be caused by a vacuum leak, low compression, faulty injector spray pattern, etc.

Did this problem exist before the timing belt job?
Any history of broken timing belts?

At this point what I would do is connect a vacuum gauge (they’re cheap, easy to use, and worth their weight in gold) and see what’s going on with engine vacuum.
If there’s a problem with low compression, timing belt off a tooth, etc. the gauge should show it instantly.

The problem occurred after the timing belt was changed. I couldn’t hear any vacuum leaks but I haven’t put a vacuum gauge on it yet. I suppose I will do that next. I ran several bottles of fuel injector cleaner through it without much success, also I listened to each fuel injector and I can hear them all firing; though the fact that it is a random misfire would indicate it isn’t any one particular injector.

Given the fact the belt tension may not be right and the shop let it go out the door droning, one could at least expect the timing belt could be off a notch or two.

If you do some work yourself a vacuum gauge is a great investment. They’re cheap and will let you know in seconds if something is out of the norm. Any time an engine is running poorly to any degree a vac. gauge is the first thing I connect as it only takes a minute and can tell you so much in mere seconds.

Just out of curiosity if the timing belt were off a tooth or two wouldn’t that make it run poorly at any RPM?

Not necessarily, because manifold vacuum changes the second the throttle plate starts opening and manifold vacuum pretty much rules all in my opinion.