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I took off the heat shield from the exhaust manifold on my 2003 Nissan Primera while looking for an exhaust leak, and I noticed this yellowish poweder / dust on it:

enter image description here

Any ideas what it is?

EDIT Sept. 19th 2017

So I wiped a section of the yellow powder with my finger and this was the result:

enter image description here

There seems to be another layer of something under the powder which got wiped away at the same time, and the yellow powder itself seems grainy.

I also verified ( as well as I could ) that there is only one exhaust leak which is from the base of the O2 sensor. I stuck a funnel in a length of hose and had my wife block the exhaust with a rag and this was the only place I heard anything, and the change in sound was quite pronounced when she blocked the exhaust.

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  • How does the heat shield looks like on the inside?
    – Martin
    Commented Sep 17, 2017 at 18:49
  • What is the heat shield and manifold made out of (what metal - if you can tell)?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Sep 17, 2017 at 19:32
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    No one has asked this: Is it loose dust that you can wipe with your finger? Is there a layer of "stuff" embedded on the surface and then also loose dust? It looks like there is sharp line of difference in the top left of the photo, leading away from the sensor. What's on the other side? Which way is gravity pulling? It looks much thicker on the "top" (of the photo).
    – Zach Mierzejewski
    Commented Sep 18, 2017 at 13:53
  • @Pᴀᴜʟsᴛᴇʀ2 I can't tell what it's made out of. Maybe stainless steels as it has no signs of corrosion of any kind on it.
    – Robert S. Barnes
    Commented Sep 18, 2017 at 19:55
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    Could it be possible that the previous owner used fuel that he bought from another country that has some "relaxed" regulations regarding sulfur content in fuel?
    – Martin
    Commented Sep 18, 2017 at 21:28

4 Answers 4

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IMHO that powder is either:

  • Dust, that got "burned" by the hot exhaust manifold and transformed into some yellow-looking substance.
  • Sulfur, as the color has a striking resemblance to sulfur. The presence of sulfur could indicate a leak and a faulty emission treatment system. There are test strips available to test for sulfur. Should you decide to try a test please get informed about the correct procedure and the risks of this test (lead acetate and Hydrogen sulfide are poisonous)
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  • I apparently have an exhaust leak somewhere in that area although I haven't located it yet.
    – Robert S. Barnes
    Commented Sep 18, 2017 at 5:48
  • When I sprayed soapy water on the O2 sensor connection I got bubbling, so I assume I've got a leak there. I also got some bubbles in the area right about the sensor where the four pipes join together; there's allot of yellow powder in both areas. Makes me think you're right and this is sulfur from an exhaust leak.
    – Robert S. Barnes
    Commented Sep 18, 2017 at 8:14
  • How are you pressurizing the exhaust system? If the car is running are you sure the bubbles are air leaks and not boiling?
    – self.name
    Commented Sep 18, 2017 at 15:21
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    That's a lot of sulphur, if that's what it is. Most countries now have 50ppm or less in petrol and diesel. Also I wouldn't expect it to be deposited as elemental sulphur, which burns well if unpleasantly. The deposition also doesn't look like it's from a single point leak.
    – Chris H
    Commented Sep 18, 2017 at 15:34
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    @ChrisH I agree, that's what is making me wonder about the whole issue
    – Martin
    Commented Sep 18, 2017 at 15:35
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If your engine was running / exhaust manifold hot; the yellow is then zinc oxide. It would result from a galvanized steel manifold which has naturally oxidized. At room temperature the ZnO is white , I forget the temperature where it turns yellow.

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  • the manifold was completely cold when I took this picture. it had been at least 6 hours since the engine ran.
    – Robert S. Barnes
    Commented Sep 18, 2017 at 5:47
  • @RobertS.Barnes - blacksmith37 is actually stating if the exhaust gets too hot while running it may turn yellow, not at the time the picture was taken.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 9, 2020 at 14:54
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The answer by @Robert is correct.

This yellow powder is Hexavalent Chromium CR(VI), the most toxic form of chromium. It is found on exhaust manifolds as a result of heating stainless steel to 600°C. Do not get it on your skin or inhale it.

Here are additional references:

Regarding the other answers here:

  • Zinc Oxide becomes yellow only when heated. It returns to powder white when cool.
  • Any dust would be burned off on the exhaust
  • Sulfur is also yellow, but would have burned off the exhaust as well

This is not dust, ZnO or S. The yellow powder is Cr-6.

Decontamination

According to a Technical Information Bulletin from Caterpillar and a 2004 Article:

In the event that hexavalent chromium is discovered, Caterpillar recommends following all local guidelines and wearing the correct PPE during the decontamination and removal process. There are multiple methods of cleaning material that can be considered. Once such method is to utilize a solution consisting of 10% citric acid, 10% ascorbic acid and 80% distilled water to convert the hexavalent chromium powder to a trivalent chromium state. Special care should be taken to prevent agitating the powder and creating airborne dust.1

(Ascorbic acid is vitamin C.)

The results indicate that vitamin C could be used in effective remediation of Cr(VI)-contaminated soils and groundwater in a wide range of pH, with or without sunlight.2

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Be wary, this could be hexavalent chromium, which is extremely toxic and a new hazard that has been recently identified with stainless steel and high temperatures, particularly in the presence of calcium-containing insulation.

https://www.eneria.fr/en/hexavalent-chromium-or-chromium-6-cr-6-on-engines/

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