I recently started a repair project on my 89 Maxima which was focused at replacing a couple fuel injectors and fixing a coolant leak. The engine DID get really hot from a blown water pump gasket that dumped all my fluid while i was driving. I shut it off and let it cool completely before limping it home on side streets - what actual damage happened from the overheating is a mystery.
All in all, I replaced the water pump, thermostat, fuel injectors, plug wires, PCV, and all appropriate gaskets for each part and intake manifolds as well as valve cover gaskets. After all of this, the engine runs much much better but I still have light smoke coming from what looks to be the exhaust manifold as well as a slight surging in engine rpm's during idle.

My question is: How do i know whether or not the smoke is from an exhaust manifold leak or if it's a leaky head gasket ?
One of the leaky head gasket symptoms I DID have, there was a slight milky residue on inside of oil cap.....but other than that nada. The smoke doesn't smell like burning coolant or oil, more like unfiltered exhaust in my opinion. It's just tough to tell for sure because of how close the exhaust manifold is to the head gasket on the block.
Has anyone had engine bay smoke from a blown head gasket ? or is that not very common
Also, if the head gasket WAS blown, would that cause the engine to surge like that? probably only a 50-100 rpm change at most - it's really slight.

Any help would be greatly appreciated, I just want to stop ripping my hair out trying to guess. Thx guys

so after letting it sit and starting again, it seems like the smoke has decreased dramatically, and it definitely seems to be coming from the exhaust manifold when it was smoking. Also the engine rpm surging calmed down, but it drops to around 500 rpm when idling in gear......seems a bit rough for waiting at a stop light......

Is it possible that the leak in the exhaust system would cause the idle to be rough when in gear ? ie. would the lack of back pressure cause the engine to operate rougher than normal ?

An exhaust manifold leak could possibly cause a rough idle and issues with the vehicle running.

With it overheating though, it's hard not to worry about blown head gaskets as well. Smoke from the engine bay could also be from a valve cover gasket leak too. I had a burning smell coming from mine when the valve cover gaskets were leaking and it would puff out a little smoke from the hood while sitting at a stoplight.

Overheating can do bad things, but usually you will see more symptoms if it were a blown head gasket. I would check around the engine for leaks and see what kind of residue is built up. A leak may not explain the erratic idle, but it may help you track down the problem.

After starting it up and running it for at least 15 minutes 3-4 times a day for the last 2 days, Almost all of the smoke from the exhaust manifold has disappeared. I'm thinking alot of it was likely residue from the leaky rear valve cover (which was fixed), and the rest is leaking from a SMALL little spot on the exhaust manifold - so that's just a gasket replacement I also found exhaust leaks from the Intermediate pipe ....which isn't a cheap piece to replace unfortunately A lot of the kinks were worked out just by letting it run and circulate the fresh fluids, as well as bleeding air from the coolant system.

The ONLY issue that I'm a bit confused about is it seems like the engine is burning a little rich on gas, mainly because of the smell in the exhaust, is there something I can do to test this and/or adjust the mixture manually ?