Over the course of the past few days my Yaris has started having tremors when I reach a speed of 45-50 miles per hour. The tremors are less noticeable at a higher speed, but today in heavy start and stop traffic, I noticed it seemed to have more tremors with the sudden changes in speed. It has 55,000 miles and I am supposed to replace the timing belt at 60,000. Could the timing belt be the problem and if so, should I have it replaced as soon as possible.
I recommend that you follow your manufacturers maintenance intervals, at 60k miles you are due to replace your timing chain. If you have about 55k miles on your Yaris that is about one oil change away. I suggest you contact your servicing dealership and have a fuel system service, and request your spark plugs to be inspected as well. This may be a costly service, but not as bad if your timing belt failed.
The tremors your talking about, sound like a misfire to me. Because this condition is more noticable at low speeds, I am leaning toward a vacuum leak.
Misfires can also be related to spark plugs and wires, or an ERG stuck open.
Does your engine light come on? If it does, your computer needs scanned and a code will really point you in the right direction.
If you do not have an engine light on, well... a good mechanic will be able to point you in the right direction.
As for the timing belt, that is not related to the tremors. However, I do suggest replacing the belt at the manufacturers recommended mileage.
〉 Answered on Apr 6th, 2011 by Audra Fordin, Owner at Great Bear Auto Shop
A timing belt basically keeps the cylinders and valves moving at the exact moment they should. If somehow, the belt skipped out of time, you would have engine problems at any speed. Without experiencing the 'tremors', I can only take a guess at the problem. It's possible you have a bad tire or a tire out of balance. I would suggest taking a ride with your technician to help diagnose the problem.
〉 Answered on Apr 6th, 2011 by Suzanne Grego, Technician at City of Philadelphia Fleet Management