Bad Manifold Crack

My truck suddenly is running very bad. First time in twenty years. I can feel lots of air coming out of the back corner of the manifold. Im about to take it off. Question could this leak make it run very bad. Enough to make check engine flash when revving at forty mpg. I am also betting a battery light

 

If you have a major vacuum leak then yeah that could definitely cause it to run rough. Would fix the leak first and foremost, and pull the codes with a scanner to see what it's throwing (flashing CEL means its misfiring, hence the rough running). But the severe vacuum leak could be causing that.

 

Yes... a bad (exhaust) manifold will make your truck run terribly. My manifold cracked at the exhaust flange last year. Ended up replacing the manifold with OEM and then replaced the entire exhaust system. Everything essentially went out within a 3 month period. There are a variety of methods to find for a cracked manifold: Soapy water on a cold engine, leaf blower in the exhaust pipe, etc.

 

What ever I have under there is blowing like a fan out the back side. Do you think something that bad could make it run terrible. Tow truck is on the way. Im a pretty good mechanic, but this truck is hard to work on and diagnose