Is it possible that if the exhaust manifold has a leak, the engine sounds rough when you start the truck when the engine is cold? Also, how often do you replace them? Or, just needed a new gaskets? Is the replacement easy? Thanks

 

It's very possible. It could be cracked (unlikely, I've never seen or even heard of a cracked D2 manifold, but still possible). It could be the gaskets. The gaskets between the head and the manifold is also unlikely, but not completely unheard of. Most likely would be the gaskets from the manifold to the y-pipe. Replacement is... not horribly complicated. You'll want to soak those manifold studs going into the y-pipe multiple times over the course of a day, then pray they come off without shearing the studs.

Now, by rough do you mean loud? Like it sounds like there's an exhaust leak or it's just running rough?

 

I will take it a step further.
The MAF and the upstream O2 sensors determine the fuel flow, spark timing, etc.
So If you have an intake leak, or exhaust leak before the O2, theoretically, it could affect your idle.

 

Few minutes after starting it sounds like a broken muffler or an exhaust leak and then it gets quiet and smooth after a while. When driven its quiet as well.

 

So, is it only during the first time you start the car and idle? Or, all the time when you idle? Thanks

 

For a few or or after a few minutes?
......

 

Well, normally, your engine runs in closed mode, until the O2 sensors are up to temp, at which time, if all conditions meet the ECU requirements, it goes to open mode. Now if your vehicle is equipped with SAI, (secondary air injection), Air is injected into the exhaust allowing any unburnt gases to keep burning, therefore heating the O2 sensors faster(and reducing emissions). So in a sense you have some sort of combustion going on in the exhaust manifold while the SAI pump is running. The SAI pump, normally runs for a 30 seconds or so, then shuts off.
I am not saying this is the problem, cause or anything. I am just saying it could be.
I would get any exhaust leak fixed and then lets see what is going on.

 

Few minutes, probably around 3 to 5 minutes in the morning after starting or from a completely cold engine (ambient temperature).
Thanks

 

Had two of three of studs missing from one of my manifold to pipe flanges and would do similar. Replaced those sans a new gasket and it decreased. Checking for loose/missing studs/nut good to do, if all present flange gaskets are easy to replace with right tools from beneath.
......

 

Thank you all for explaining it the best possible way. Now it all make a lot of sense to me. Cheers!