Hi, I'm new here. I'm hoping maybe someone can help me figure this out. My 98 Deville has been getting really bad milage, it runs rough about half the time, the coolant is low, and it's running just a little hotter than usual. PCM P0171 and P0174 are the codes that come up, most of time it says they are history, only sometimes does it say they are current. Whetheer it reads as current or not doesn't seem to depend on how well the car is running, as I've checked it when it was running really rough and had it say history, and I've had it read current when the car seemed to be running ok. I checked the vacuum lines and the plenum with an unlit propane torch, and there doesn't seem to be any leaks.
I bought this car about a year ago. About 7 months ago, I started having some cooling system problems. I ended up putting in a new radiator, new overflow tank, and all new hoses because they were all rotted out. I'm thinking the problems might be due to the head gasket, but I was wondering if anyone had any other ideas. If it is the head gasket, has anyone had any luck with any kind of stop leak? Any input would be greatly appreciated.

Thanks,
Duder

 

There are many threads regarding overheating and head gaskets - search them including FAQ section. P0171 and P0174 related lean exhaust - vacuum leaks or low fuel pressure. Vacuum leak does not have to be at intake. A co test when engine is hot should tell you if head gasket is blown?