I have the same problem on my 99 sienna. White smoke at random times. Also leaking oil but have recently replaced the valve cover gasket in front and are waiting to see if that does the trick. Hope someone can weigh in on this issue soon.

 

Replacing the valve cover gaskets will give you the opportunity to look and see if you have any sludge issue going on.
The front valve cover is not so much work.....that rear one is a bit of a job to get to for removing.
A light coating of varnish, tar, or deposits is not an issue but a buildup of crud IS an issue.

 

Sludge

Yes, I saw a little sludge build-up but nothing major. Didn't take a look at the rear valve cover since that seemed a bit too much effort for my ability. My 99 sienna has 2 problems. One is the oil leaks which I am waiting to see if the front valve cover gasket replacement addresses. I know it was leaking so it should help. Just not sure if there were other leaks. Checked underneath and could see oil as it was all over the oil pan. Cleaned the pan and it doesn't appear to be leaking, so it may have come from somewhere above the pan. The other is the seemingly random white smoke emission from the tailpipe which I have yet to resolve. Any advice is appreciated.

 

If the front valve cover gasket is leaking, it is most likely that the rear one is leaking also.
Another thing to check would be the oil filter location.
When I remove my oil filter, oil runs down onto the oil pan, so a leak there could show up on the oil pan.
I don't see these causing smoke out the tail pipe though.

 

oil leaks in 99 sienna

Right now i am waiting to see if the front valve cover gasket replacement has done enough to stop the leaks. I've got to clean the engine to check if other leaks still exist. As you indicated, the rear valve cover gasket may need to be replaced also, but I don"t want to go back there unless it's absolutely needed. I checked the oil filter area and it does not look like it's leaking. Thanks for the input.

 

If you end up going back to the rear valve cover....here is a link to a folder of pictures about my 2003 Sienna......I have a series about the rear valve cover gasket job.
You remove the cowl....and the upper intake manifold with the throttle body attached.........
http://rides.webshots.com/album/576572371kcnZsT

 

Thanks for the link. Not sure if I can handle this job but may take a stab at it and see how it goes. Do I need to use sealant on both the upper intake manifold gasket and rear valve cover gasket, and if so, does it go on the bottom or top side of the gasket? Also, how much coolant should be drained during the process? Thanks for the help.

 

The factory had a small amount of sealant on the hump ends.....to the corners.
They did not have ANY on the flat surfaces where the bolts go.
FELPRO says to install the gaskets with NO sealant.....which is what I did.

 

Any resolution to the smoking?

Did you ever figure out what was causing the smoke? Our '02 Sienna with 120k miles is exhibiting the exact same symptom.