[richtazz]

The UIM's are a very common issue on GM vin-k 3800 engines from late-95 to early-99 (and stll prone to failure on later models too). It's due to the area around the EGR stovepipe being too thin and too close to the heat of the stovepipe. This causes the plastic UIM to deform and leak coolant either externally at the bottom of the T/Body gasket, or internally into the cylinders (usually 5 and 6 due to they're being the closest to the leaky area). There are re-engineered UIM's available both through GM and Dorman that will rectify the situation. The Dorman one comes will everything needed to change it (gaskets, new stovepipe, injector o-rings, pcv valve, etc...) and I have used them with success. If you start using coolant and having drivability problems, the UIM is most likely at fault. Don't wait to fix it either, as it can hydrolock a cylinder if it gets bad enough.