Q: unknown oil leak upper half of engine on 1991 Toyota Previa

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Had a small oil leakage problem 1 yr. ago. It got worse after we had below freezing weather (engine idled higher). I took it into two mechanics and they couldn't locate it since it was coming from the upper half of the engine. Engine is located under the front seats of the van. Cannot see it from the bottom and cannot see it from the top since Previa's have only a small compartment door just enough to check my oil and transmission dip stick and to add oil. I might have to drop the engine just to find where it's leaking. Which comes to, where in Milwaukee, WI can someone do it and how much?
(2) Answers
Before going that far I would find a shop to add dye to oil and then drive for couple days and then have them look with blacklight for source of leak where dye is coming out. Keep in mind that when drivine the oil leak can be blown around by turbulance to make it look like is leaking at one point but may actually be leaking elsewhere.
It could be coming from your PCV valve. The PCV is located on top of the valve cover gasket. If the PCV valve isn't working properly, the engine oil will spray out and spread all over the top of the engine.

You'll need to remove the passenger seat and engine cover to get to it. PCV is cheap but it take awhile to get to it.