It could be the air filter. How long has it been since it was changed? Do you travel on gravel roads often? You can check your filter by removing it and shine a trouble lamp behind the filter. You should be able to see light through the filter. If in doubt, replace it.

The battery would not cause this. If it is strong enough to start the engine, the alternator will provide power to run the car & charge the battery.

 

Had this issue with a Buick. When I really pushed it hard, I got a check engine that read as "random multiple cylinder misfires". I did a tuneup - plugs, O2 sensors, fuel filter, etc - no different. Finally took it in, and dealer found a plugged catalytic converter.

 

Like pkshaver, I had a Chevy that "all of a sudden" had a plugged catalytic converter, had to drive from Milwaukee to Detroit not being able to go faster than 55....and if there was an incline....my speeds would drop to 40 mph. Replaced (actually removed) the catalytic converter, and it was like a new car!

 

My 2006 Sienna seems sluggish and similar to yours. I've noticed also that when I take my foot off the gas , the enjine holds it back. All the other vans I've had let the van coast without holding it back. I was told that the van has a "Jake Brake" on it??? Anyone know anything about that? Also have you noticed a drop in mpg when you had the plugged catalytic converter? And are the catalytic converters covered by somekind of warranty? I heard that there is a warranty for the catalytic converters for 100 k miles.

 

You don't have a Jake Brake, it is a completely different concept. What you are seeing is engine braking by cutting fuel to the injectors. It is a gas savings technique. You can shift to neutral if you want to keep the effect of coasting with "holding it back" but there's nothing wrong with it, it's simply a property of the van's ECM.

If you had a plugged cat you wouldn't have just a drop in MPG, you'd have a lot of issues with your van. Cat's don't plug on their own, a plugged cat is a secondary effect of a bigger engine issue causing it to run too rich. It probably would not be covered under warranty for an issue of plugging since it's not a failure of the cat.

 

I think I understand that now . No one could explain it clearly to me. Thanks for the reply.

 

Hello to the original poster, its been over two years. Was the problem fixed? Please report back. This make the forum more valuable. Thanks