Coolant Leaking Issues

Hi all, I am glad to join the AF community. For the past couple of days I have been reading a lot regarding the problems that the 97-2003 GM 3800 SII are having with the coolant leaking issues (I just knew about this unfortunately:frown:). I am the guy who changes the oil and filters for it's cars, keep them pampered with a little TLC, put only OEM parts when replacing and like to keep them on top condition. Now the real thing after my intro.
My 98 Grand Prix GT (3.8L - 235ci 90,xxx miles) started with a Low Coolant Light back in the summer, never really overheated and I discovered that the radiator was the one to blame since it was leaking on one of the side caps. At the time, I replaced, coolant level sensor, radiator, both hoses, rad. cap, thermostat with gasket and water pump with gasket. Leaking is gone but now and then the car flashes the LCL and I have to keep adding coolant to the reservoir tank. I checked the oil, does not seem to be mixed with coolant and is not over the "Full" mark. I did saw an "oily substance" on top of the UIM, but it does not smells like coolant and seems more like engine oil, which is weird for me. :banghead:
Question is, what do you reccomend and what to buy new/replace given the circumstances? Any info will be greatly appreciated and all questions welcomed.

Thanks!!

I would look under the throttle body for signs of external coolant leaks, and check the plugs in cylinders 5 and 6 for coolant contamination. These are signs of a UIM failure, or at least a UIM gasket failure. I would suggest buying a replacement UIM, as your's is in the years where the EGR stovepipe eroded the UIM and causes the internal leak. I would also replace the coolant elbow that goes from the LIM to the belt tensioner and the LIM gaskets while you're in there. These are two other weak points that need to be addressed while you have it down. The way the 3800 is designed, the coolant doesn't go into the oil like it does on the 3100/3400's, it gets drawn into the combustion chamber, usually cyl's 5 and 6, due to they're the closest to the leak point.

Thank you for your reply Richtazz. When you mention plugs, you mean the spark plugs? Also, any website reccomendations where to buy and proper name of the parts you suggest?

Take an oil sample and have it analyzed at Blackstone Labs or other reputable place and they can tell if there is coolant leaking into the oil. Sometimes you just can't tell by looking.