TL;DR
A downstream detergent injector only draws soap when the black low pressure tip is fitted, which is why most people conclude their soap function is broken. Apply the detergent from the bottom up, give it five to ten minutes of dwell time out of direct sun, then rinse from the top down with a wide tip. The chemistry does the cleaning; the water only rinses.
Two kinds of injector, and why it matters
Pressure washers introduce detergent in one of two places, and the difference decides what you can safely run through the machine.
A downstream injector sits after the pump. Soap is siphoned into the water line on the outlet side, so it never touches the pump seals. Nearly every consumer and prosumer machine works this way.
An upstream injector feeds the soap into the water before the pump. It gives a stronger, more consistent mix and is common on commercial equipment, but it also means the detergent passes through the pump, so only products approved for that machine can be used.
The practical consequence: on a downstream machine you have more chemical latitude, but the mix reaching the wall is heavily diluted. On an upstream machine the mix is stronger, but a harsh chemical will destroy the pump.
Why the black tip is required
This is the single most common pressure washer complaint, and it is not a fault.
A downstream injector works by suction. Water rushing past a small port creates a pressure drop that pulls detergent up the siphon tube. When a high pressure tip is fitted, back pressure at the pump is far too high for that suction to happen, so nothing is drawn at all.
Fitting the black soap tip -- the one with the widest opening -- drops the pressure enough for the injector to work. It is not a cleaning nozzle. It is the nozzle that turns the soap function on.
If you fit black and still get nothing, check the siphon filter for clogging, check the tube for cracks or a loose fitting, and check that the detergent is thin enough to be pulled. Thick, undiluted soap will not siphon. More on tip roles is in nozzle tips by color.
What the injector does to your dilution
Downstream injectors typically pull at a fixed ratio, often somewhere in the range of one part detergent to ten or twenty parts water.
That means the strength in the jug is not the strength on the wall. If you want a specific working concentration on the surface, you have to account for the injector ratio, not just the label dilution.
Source: pressure washer manufacturer injector specifications
Two things follow. Running a detergent at full strength in the jug is normal on a downstream machine and does not mean you are using it neat. And a longer hose lowers the draw, so the same setup on 100 feet of hose delivers a weaker mix than on 25 feet.
Because of this dilution, many people apply exterior detergent with a separate pump sprayer and use the pressure washer only to rinse. That gives full control of the mix and is the standard approach for soft washing and when it applies.
Applying the detergent
Work bottom to top when applying. A run of soap over dry siding leaves a streak that is hard to remove; a run over already-wet siding does nothing. This is the opposite of the rinse direction, and the reasoning is laid out in whether to work top down or bottom up.
Pre-wet plants and grass around the work area with plain water, keep them wet, and rinse them again when you finish.
Cover or close anything that should not get wet -- windows, doors, and vents, along with any exterior electrical fixture, which needs the power cut rather than a hope that the gasket holds. Apply in sections small enough that the first part is still wet when you come back to rinse.
Dwell time is the point
Detergent needs contact time. Applied and immediately rinsed, it accomplishes very little; left for five to ten minutes, it does the actual work of breaking down algae, mildew, and greasy film.
Sun and heat are the enemy. A solution that dries on the surface stops working and can leave a residue that streaks, so work shaded elevations, or work early and late in the day. If a section starts to go dry, mist it lightly with plain water to keep it active.
Do not scrub a delicate surface while the detergent is on it. Let the chemistry work, rinse, then decide whether a soft brush is needed.
Rinsing without streaks
Switch to a rinsing tip -- white for anything delicate, green for hard flat surfaces -- and rinse top down so dirty water always runs over surfaces you have not finished yet.
On siding, keep the stream angled downward and stand well back. Rinsing upward drives water into the lap joints, which is the failure mode covered in washing vinyl siding without forcing water behind it. Overlap each pass and keep a steady distance; streaks after a detergent job are almost always missed rinse coverage rather than a chemical problem.
Finish by flushing the injector: drop the siphon tube into a bucket of clean water, fit the black tip, and run it for a minute. Detergent left in the line corrodes fittings.
Safety with exterior detergents
Most exterior house wash products are either bleach-based or surfactant-based. Never combine a bleach-based product with an acid such as a rust remover, masonry etcher, or descaler -- that releases chlorine gas. Never combine it with an ammonia-based glass cleaner either.
If you switch products mid-job, flush the injector and rinse the surface thoroughly before the second product touches it.
Wear eye protection and gloves. Bleach-based solutions are hard on fabric and on plants, and they will discolor a lawn quickly if allowed to pool.
Do not run a wand from a ladder, with or without detergent. Use an extension wand from the ground or hire out the height.
FAQ
Why is my pressure washer not pulling soap?
The most likely reason is that a high pressure tip is fitted. A downstream injector siphons only when pressure drops, and only the wide black soap tip drops it far enough. After that, check the siphon filter for clogs, the tube for cracks, and whether the detergent is thin enough to be drawn.
What is a downstream injector?
It is a detergent siphon placed after the pump, so soap enters the water line on the outlet side and never passes through the pump. It protects the pump seals and allows a wider range of cleaning products, but it dilutes the detergent significantly compared with the strength in the jug.
Can I put any soap in a pressure washer?
On a downstream machine you have broad latitude, though bleach-based products still corrode fittings if left in the line. On an upstream machine the detergent passes through the pump, so only manufacturer-approved products should be used. Never run a bleach product together with any acidic cleaner.
How long should detergent dwell before rinsing?
Generally five to ten minutes, and never long enough to dry on the surface. Sun and wind shorten the window, so work shaded areas or cooler parts of the day. If a section begins to dry, mist it with plain water rather than letting it set and streak.
Do I apply soap top down or bottom up?
Apply from the bottom up so runs land on already-wet surface and do not streak, then rinse from the top down so dirty water always flows over unfinished area. Reversing either direction is the usual cause of visible streaks after an otherwise correct wash.
The whole-house equivalent of this routine is covered by house cleaning services.
Chemistry, dwell time, and rinse direction decide how an exterior wash turns out far more than the machine does. If you would rather have the mixing and the sequencing handled, get a quote and we will look at what your exterior needs.