1999 Chevy Venture v6 3.4 Check engine light on constantly. Oil pressure light is now on. Van drove fine, stopped, put it in park and left it idle. It died. Thought it was a bad battery so i replaced it with new one and now it runs like crap. Engine sounds loud and my van won't stay running when you put it in gear. It will idle fine in park but stalls out as soon as you shift it into drive or reverse.

Answer:
First of all a bad battery will not cause your Chevy Van to have an oil light or cause it to stall only when in gear. You need to get the computer codes scanned to see why the check engine light is on. This may be the cause for the vehicle to not staying running. Although an oil light on means low or no oil pressure, which could be the sign of internal engine damage.
Get the codes scanned at any large national parts store chain for free, and check the oil level. If the oil level is OK, will need to have the oil pressure checked with a gauge. Possibly just a bad sensor.

For stalling out in gear, check the engine to make sure you do not have a vacuum leak. This will cause an engine to run fine in park but die out in gear. That would also cause a check engine light and codes. A lean code, like P0171 would be stored in the computer. Common areas are broken vacuum hose or leaking intake manifold gaskets.

Another problem to check for is carbon buildup in the throttle body plate and bore. This is common on the GM 3.4L and 3.1L V6 engine. You would just need to remove the air intake duct from the throttle and clean it out inside with carb cleaner. Then take it for a drive and stomp on the gas a few times to blow it all out.

Check for a leaking fuel pressure regulator (FPR). when this happens, gas leaks into the hose and then into the intake manifold causing a very rich condition. This will cause an engine light with a code like P0172. Replacing the regulator is the only repair for this condition.

If the engine is loud then you can have internal engine damage. This will cause it to run bad, check engine light, stalling etc. Possible worn bearing, bad lifters etc. all cause by the low oil pressure problem. If this is the case, then engine replacement would be best as your van is over 20 years old. The cost to remove it and do a total overhaul would not be worth it for the age of your Chevy.