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I have a toyota Sienna 1999 v6 3.0 157000 miles and I have a problem when I press the gas pedal around 3000 rpd its start to have ups and downs in rpm and loose power. It does not have any check engine light. I have change the spark plugs and oil, oil filter and air filter.  

does the check engine light work????? it should come on for 2 seconds when you first start...

96 and newer cars  are all OBD2... (on board diagnostics 2)  they all can use a standardized scan tool for emission and engine diagnostics..   and they can display live data to scan tools that are equipped to display it...   some scan tools only display codes..  but you can look at the live data that the computer is seeing..  really makes them easy to fix..

the first thing i would probably start with after your description.. is the fuel filter...  it will not set a trouble code...

fuel filters clog...  this restricts flow...    before it is totally clogged... it will do like you describe...

change it..   blow through the old one.. when you get it out...   post what you find...    it is important... on that model... that you use 2 wrenches to loosen the fuel filter bolts..  one on the filter body.. one on the bolt that goes through the loop at the end of the line...     there are also copper washers that come with the new filter...  2 on each end... one above the line.. one below...

too many cars have burned up when these fuel filters were improperly installed..  remember.. there can be almost 60 pounds of pressure in the fuel system... a tiny leak...  bad really bad....

trying to loosen the factory torqued fuel filters with only one wrench is impossible... takes two... make sure you have a wrench that fits when you buy it...   the banjo bolt head size is 17MM or 19MM.. i don't recall what size the hex on the ends of the filter are...