Pontiac Repair: Car stalling, oxygen sensors, vacuum leak

Question
QUESTION: Hello, first off I have a 99 pontiac grand am gt with a 3.4 lt engine. The car overheated on me pretty bad a few weeks ago and I had to replace one of the heads. Now after all this was done my service engine light came on and it is having a random stalling problem. The car will try to stall at stop lights sometimes but if I give it some gas it will most of the time recover. So I took it to advance auto to get the codes. The reader came up with a miss fire and system lean codes then I erased them. The engine light hasent come back on but it seems to be getting a little worse. I have checked all vac lines, wires, plugs and injectors. I did replace the o-rings on the injectors and fuel rail when I did the head. Any help in this matter would be a real releif. Thanks, J

ANSWER: Hi Jason:

Strange one.... These engines are no known for stalling. Lets go back to the start. why did it over heat?? as for a misfire/lean condition they could be related. if the engine is running lean enough it can cause a misfire. Does the engine idle high? That would indicate a vacuum leak thus the lean condition. If this engine was burning coolant when it overheated it very wel lcould have damaged the heated oxygen sensors Mainly the one in the rear manifold but both of them could be damaged showing a lean condition when it reality it's running okay. Does the engine feel like it's misfiring like loss of power under heavy accelaration? If si I would do a compression test you may have cooked this engine bad enough that it's low on compression causing the misfire. hwtayou really need is a scan tool to see what the fuel trim is doing on this engine. did you do this repair yourself? I have seen things where an engine was apart and put back together on these cars where someone put the mas air flow sensore in backwards causing a lean condition. Ther is an arrom on the side of it that shows the direction thatthe air passes through the sensor make sure it's in the right way. Check tomake sure that the throttle body is clean and not all carboned up. If so clean it up really good with throttle body cleaner or carb. cleaner disconnect the battery and reconnect it. Before yo ustart the car cycle the key from off to run 2 times do not start the car. when the key is off wait 30 seconds before you key it on again. This will reset the idle control valve. just a few things to try out. Good luck :)


---------- FOLLOW-UP ----------

QUESTION: Thanks for the fast answer. Yes I did all the work myself except for the porting and polishing on the new heads. The reason it overheated was a blown head gasket and I had to stop at every other gas station for 20 miles to get water. I did take it into the red one time to the point that the engine shut down causing one head to warp. The engine has a miss that I can feel only sometimes like it wants to stall till I hit the gas then it kinda hesitates a little but recovers. Thanks

Answer
That's not good that it overheated bad enough that it shut off from over temp. I'm leaning more to damage to the lower end meaning rings or a scored cylinder wall. Water in the engine is not a good thing especially for the bearings. Like I said before you need to get your hands on a scan tool to look at the fuel trim and O2 sensor readings. The fact that it was burnin the water/coolant I'm sure damaged the sensors. That will throw off your fuel trim numbers for sure and it could throw them off enough to make it run to rich or lean and cause the misfire if it's not an engine mechanical issue. Next time something like this happens to you don't drive it have to towed it will save you head aches like this one down the road. Good luck and happy holidays :)