I noticed there is a powder-like residue on the coolant hose at the back of the engine bay, and also on the heat shield and the exhaust around that same area. Any ideas what that could be? It feels very dry to the touch.

 

Your 951 may have a problem

 

Normal surface residue from weather changes.its almost minimal. Don't drive yourself nuts.

 

What exactly is it though? I've never seen this before.

 

Former owner used the car to run drugs?

Doesn't coolant form into white specs when it dries on hot car parts?

 

Battery acid?

 

From the looks of it I believe it is aluminum oxidation and is able to be remedied fairly easily.

 

You might have an exhaust leak coming from your #4 exhaust manifold header ?
Its hard to tell from the picture
The headers can leak under the shields or right at the meet of the block and header
Good luck
Regards
Ed

 

Does that usually deposit white powder? I know this might not be clear from the picture but it's very concentrated in the area around that coolant hose. It's quite noticeable and while I'm not sure how long it's been there, it's not normally there (I inspect everything in my engine bay fairly often). But if an exhaust leak could be the cause, that might make sense because I tried to remove that test port a while back, which goes into the crossover just below this area, and I couldn't get it out of there, but with the flare nut loosened it might have started to leak.

 

Its mildew from the treated materials that got weather beaten. It won't cause an issue but you can order a new piece if the visual bothers you

 

I was going to say the test port, but the pipe was there in the picture....
It's probably that ..... I would just remove it and believe it or bot if you have an extra lug nut they fit right on top of that test port to seal it off
Why couldn't you get it off ? Heat the pipe up for a few mins with the engine running then spray some PB blaster and wait a while and go at it. It should come right out . Make sure you have a nut to put over the test port before you do this ....
Regards
Ed
ps btw you need to clean that engine up a bit

 

Yep I have a spare lug nut for that. I loosened the flare nut but the tube just wouldn't come out. It's hard to get a decent grip on it by hand and I didn't want to pinch it with anything that might make it break off...then I'd be stuck with a leaking tube that I couldn't remove

I did use lots of Kroil but I can't remember if I tried it with the exhaust hot...I'll try that next.

 

I hear ya but there really isn't anything to get stuck. Its like you said a flare nut
With a little wiggling it should come free ... Mine came right off. In fact the metal tang at the top broke off on its own...
Good luck

 

This! This is what I was trying to insinuate.

 

I believe aluminum oxidation would be unlikely considering the material where the powder is present (steel, rubber, painted surface of the body heat shielding on the frame rail). I wouldn't worry much about it. I've had many exhaust leaks and have only ever seen black soot with the occaasional tan, nothing white.