The tip is the last surface the gas touches, so what settles there is a mixture of what left the engine unburned, what condensed out of the stream as it cooled, and what the pipe itself shed along the way. None of it is a measurement. It is a record of conditions, and like any record it can be read badly.
Read it cold and read it dry. A tip examined straight after a run is still wet inside, and moisture darkens soot and spreads it into shapes that mean nothing at all. Leave the car standing overnight, then look before the engine is started again.
A thin, even film of dry black that ends cleanly at the outlet is the ordinary state of a healthy system. It forms because the last section of pipe runs cooler than the manifold, so a small fraction of what the stream carries drops out of suspension and plates the wall.
What matters is the boundary. Where the film fades gradually towards clean metal further in, the pipe is behaving normally. Where it stops abruptly in a hard line, something changed at that point: a step in the bore, a joint that disturbs the flow, or a section running at a different temperature from its neighbour.
An even ring around the whole circumference says the gas is arriving symmetrically and leaving the same way. A smear confined to one side says it is not. The usual cause is a tip sitting at an angle to the flow, so the stream scrubs one wall harder than the other.
Before blaming the engine, look at the mounting. A tip hanging low on one hanger, or pressed against a bumper cut-out, will produce a one-sided pattern on a system that is running perfectly. Correct the position, drive the car, and read the tip again before drawing any conclusion from it.
Dry soot wipes off with a cloth and leaves the metal visible underneath. That is carbon, and on a modified engine with a richer calibration it is expected. It is also what accumulates on a car used for short trips, where the system never holds enough heat to burn it away.
Black that smears, feels tacky, and leaves a stain that will not shift is a different material. That is oil or unburned fuel carried through as liquid rather than as vapour, and it points at something upstream rather than at the pipe. The tip is telling you where to look, not what to replace.
A pale grey or ash-coloured deposit usually means the system is running cleanly and hot, with little carbon left to drop out. On a well-maintained engine that is unremarkable. On a car that has just had a major change made to it, it is a useful confirmation that the calibration is not simply dumping fuel.
Grey is not automatically good, though. On an engine using oil, ash from burnt lubricant also reads pale, and it is harder and drier than carbon. Rub a little between the fingers: carbon crumbles, ash feels gritty and does not smear. The same distinction separates a core that is sooting from one that is burning oil when stainless steel exhaust mufflers are opened for inspection.
A tailpipe cannot tell you a mixture strength, and no amount of staring at the pattern will produce one. It cannot distinguish a sensor that has drifted from an injector that is weeping, and it cannot date a deposit. Anyone who claims otherwise is reading more than the metal holds.
Use it as a first look that decides where to test next. If the pattern is even, dry and unremarkable, move on to a proper diagnostic. If the boundary line is hard, inspect the section behind it, which is where a length of exhaust pipe with a consistent wall makes the comparison worth making.
