The engine uses alot of oil. The head gasket and valve seats were replaced. And the problem stills exist.it uses a half to a full quart within 100 miles

YOU HAVE COVERED MOST ITEMS, BUT YOU MIGHT HAVE A CRACKED OIL RING ON A PISTON. A QT A DAY IS ALOT

Depending on how long since the overhaul, and how much oil is being used, you may not have any worries.
Engines built in the factory are within very close tolerances. Engines rebuilt in the shop are not.
So like it was 50 yrs ago, your rebuilt engine may need some break-in time to wear in. The piston rings have to seat or wear into the shape of the cylinders, and so on.
Most of the time you can expect to use some oil for the first few thousand miles then it should stop. You have to drive the car at various speeds and not under much strain for the first thousand or so.
After the first oil change at 3,000 oil consumption should drop off to normal.

The engine can burn alot of coolant before you see any smoke.
And having a cooling leak in the engine could cause it to run hot at times.
If you are adding coolant every week and don't see any leaks, the engine has to be burning it. Most of the time it is a blown head gasket and cracked head or block.

Hello and welcome to FixYa!

The cylinder head is located on the center of the engine. Yes it can be a worn out head gasket or piston rings. If the piston rings are worn out the oil is burnt that's why it emits white smoke. You will need to replace the head gasket and piston rings.

Hope this helps and thank you for using FixYa! Have a good one!

According to GM 1 quart per 1,500 miles is an acceptable oil consumption tolerance. 1 quart in 3k isn't bad.

I've had several gm 3.1 and 3.4 liters that have had there oil consumption issues remedied after the lower intake gaskets where replaced due to coolant leaking.

There are a couple of common problems that can cause oil consumption. Valve seals, bad rings, and PCV valves.

Valves seals would require a valve job which would requirethe removal of the heads. Rings would require a motor rebuild. Both of these are expensive options and oil is still pretty cheap.

Replacing the PCV valve might help. On the 3.8 it will be under the MAP sensor on the passenger side of the intake plenum.

As for as the intake gaskets, its a possibility but I don't have any hard evidence this could be the problem on a 3.8. Eventually the gaskets will start to leak oil. If you decide to replace them at that point I guess you will find out. I wouldn't do it on a hunch, its not a cheap repair.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

most likely it is the valve cover gasket. it covers the valve train where there is alot of oil to lube the top of the motor. if the head gasket was leaking it may have more signs of running poorly. also having coolant leaks also. so i will have to say valve cover gasket. now usally you may have a oil burning smell with it depending on the area of the lek but not always.

the valve cover can easily be seen when the hood is open. it is right on the top and where it meets the clyinder head is where the gasket is and the leak can be seen.

if you have more question please ask.

Ouch!
White smoke means water (steam) in the engine cylinders, coming from the cooling system....
broken head gasket or cracked cylinder wall.

1998 - you might want to temporarily use an engine sealer. AutoZone can advise which is best for plugging a head gasket leak.
It will need to be replaced ($200+) and get valve job done ($100) at an ato parts store that has a machine shop, since the valve head must be removed to replace the gasket between thr head and the engine block.

Not likely, but it could be water in your gas tank. Did you just fill up?
or was the car driven with an over-heated engine?

Look at the opil dipstick. There will be a milky substance in the oil. That's water mixed with oil.
--- sorry ---

sound like intake gasket leak.it will leak oil into the cooling system.

The toyota previa uses a formed silicone gasket for the valve cover and the cover is held in place by metric shoulder bolts ( 10mm). The valve gasket leaks because you can't get enough preload on the bolts to re-seat the old gasket when it gets old and shrinks. The shoulder bolts only allow you to tighten the gasket enough to seat the shoulder on the bolt. If you continue to torque down the suspected bolts, you will snap the bolt off ( don't ask me how I know this ). You can either 1.)replace the old gasket with a new one or 2.) remove the existing bolts around the leak area and put in new metric bolts that do not have a shoulder. You may want to put in a small sleeve into the existing bolt hole but shorter than the thickness of the valve cover to account for the smaller diameter bolt. You can now torque down the bolt and get enough preload to seal the valve cover without removing the old gasket. Make sure you use a torque wrench ( in-lbs) or be very careful not to snap these small valve cover bolts.

Reseal the valve cover. Don't use a gasket, use RTV sealer for the best results.