I have a 1995 4 runner, 3.0 v6 5 speed. I had a rebuilt long block put in 6 months ago. I have followed all specs per the warranty. It has been using what I would say to much oil. I had 1150 miles on the last oil change and had added 5 quarts. I changed the oil again because the oil on the stick was black. When I pulled the oil plug it smelled burned to me and was as black as coal with only 1150 miles on that oil plus the 5 quarts I added. There is no leak anywhere and I do not see smoke from the exhaust. I don't have what I would call good power, and not good mileage. It seems to be running fine as far as idle, and smoothness. I have not overheated the engine. Any ideas?
If you don't have a leak and you are not burning the oil then it has to go somewhere. If you were burning that much oil in that short period of time you would be smoking out the neighborhood. If it was leaking that bad you would see it all over the engine compartment.
Check the oil pressure switch to make sure it is not leaking. It will only leak when there is oil pressure.
The only thing I can think of is the oil is not draining back to the engine oil pan and staying in the valve covers. The oil is probably black because there is so little oil in the pan and is over used.
When you suspect the oil is low at or before the next oil change remove an oil cover to see if it is filled with oil. If it is then you need to locate where the oil drain blockage is and repair it.
If you can not locate the problem I would get in touch with the company you got the long block from and see if they ran into this problem before. If there is something wrong with the engine it should be in the warranty period. Check your paper work on the warranty policy.
If you installed the heads and that is where the problem lies then it's up to you to fix.
Recheck your valve timing and ignition timing for the power problem.
Try these few suggestions and let us know your results. We can diagnosis the problem further if needed.