Lawn Sprinklers Dry Spots in Lawn

Troubleshooting Dry Spots

See the glossary on my web page for an explanation of any words or
terms used below that you don’t know.  Example; if your not sure what
the difference between a lateral pipe and a mainline is, look up
those words.

Check the valves to be sure they are fully opening.  First, make sure
the flow control setting (if there is one) on the valve is in the
full open position.   Next, manually open the valve using the bleed
screw or the open/close lever on the valve.  Does the sprinkler
system work better?  If so either the valve is bad, the controller is
bad, or the wires are bad.

Get three of the standard box-shaped 9 volt radio batteries (the ones
with the snap connections on the top).   Make a “battery chain” by
snapping all three of them together end-to-end.  This gives you 27
volts (9×3) between the unattached end terminals.  That’s enough to
power the valve.  Disconnect the valve wires at the point where they
are spliced into the wires leading to the controller.  Cut out the
old splice and throw it away.  Attach the valve wires to the open end
terminals of the battery chain.  Does the valve fully open as it did
when manually operated?  If so the valve is OK.  If not the valve is
bad, replace it.

Next test the voltage output from the controller for each valve
stations.  It should be at least 24 volts and over 0.5 amp.  If you
don’t have the equipment to load test the controller for amperage you
can remove the controller from the wall and take it out near the
valves.  Then run new wires from the controller to the valve wires.
Get a long extension cord so you can plug the controller in and then
turn on the valve manually using the controller.  If the controller
is working correctly the valve should fully open. If the controller
checks out, then the wires between the controller and the valves are
bad and need to be replaced.  If not, then the controller is bad and
needs to be replaced.

If the valves, controller and wires are ok, the problem is a
restriction in the piping.  It could just be that the pipe is too
small, causing excessive friction loss.   It could be sand or muck
buildup in the pipe.  It could be a piece of metal, a rock, a rag, or
whatever stuck in one of the pipes. I’ve seen everything from fish to
toys in pipes.

Remove the last sprinkler heads and the sprinkler head riser on each
valve circuit.  Cut the ell off the pipe where the last sprinkler was
connected to the lateral pipe.  Install a new 45 degree ell on the
end of the pipe and a short length of pipe extending above ground.
The purpose of this is to get rid of the restrictions that a 90
degree ell and the riser create so you can get a good high velocity
flow to flush out the pipes.  The outlet pipe needs to be above
ground to prevent dirty water from being sucked back into the pipes
after flushing.  Turn the water on and allow the water to flush for
at least 5 minutes.  After everything dries out, cut the flush outlet
off and reinstall the sprinkler.  Test to see if that cured the

If not, there is probably something wedged tightly in the pipes
somewhere, so the next step is to isolate where that restriction is.

Prepare to get a wet while doing this test!  You will need a 0-100
PSI pressure gauge.  If you have rotor-type sprinklers get a “pitot
tube” to attach to the gauge.  You can get one at an irrigation
store, call first, not everyone will have one in stock.  The pitot
tube goes into the sprinkler nozzle so that you can measure the
pressure  while the sprinkler is operating (doesn’t work for spray
heads, only rotors).   If you have newer spray type heads with
industry standard female thread nozzles you can get an attachment
that allows you to install a pressure gauge on the sprinkler in place
of the nozzle.  The attachment you need is a Rainbird #PA-80 adapter.
If you have male thread nozzles (Toro) or a non-standard nozzle
spray heads you will need to remove the the sprinkler head and attach
the pressure gauge directly to the sprinkler riser pipe where the
head was attached.  This will require a few fittings and a short
length of threaded pipe.

Measure the sprinkler pressure at each sprinkler head location
starting with the last head and moving back toward the valve.  Write
down the pressure for each sprinkler.   If there is a large drop
between any two heads then that is where the problem is, drop down a
few paragraphs for instructions on how to fix the problem.  If not,
then the problem is either someplace before the first sprinkler.

Tap into the mainline pipe just before the valve and install a tee on
the mainline and your pressure gauge.  Turn on the valve and take a
pressure reading with one sprinkler circuit running.   The pressure
should be around 5-10 PSI higher than the pressure you measured at
the first sprinkler after the valve.  If it is more than that there
is probably something stuck in the pipe between the valve and the
first sprinkler.  However, note that the pressure drop through
different brands of valves varies, so it might just be a high
pressure loss valve.

Next check the mainline for excess pressure loss.  Tap into the
mainline right after the point where it connects to your water supply
and install your pressure gauge.  Again, take a pressure measurement
while one of the valves is operating (it needs to be the same valve
as with the previous measurement).  The pressure difference between
this point and the pressure reading you took at the valves should
generally be not more than 5 PSI.  Again this varies dependant upon
the length of the mainline.  If you have over 100 feet of mainline
the loss may be higher.

If you still don’t find an unusual pressure drop the problem is
probably just a poorly designed sprinkler system.  The pipes are
likely all too small.  There are no good solutions at this point
other than to trash the sprinkler system and start over.  You can try
to divide up the larger valve zones into two new valves to reduce the
flow and thus the pressure loss.  To determine if this will work,
remove and cap off half the sprinklers and test it.  If it works
good, you can install a new valve and pipe to the capped off
sprinklers.   But such approaches are usually less than satisfactory.
Consider seriously if it would be less work to just install a new
system that will work efficiently and correctly.  My experience is
that you wind up paying more over the long run if you try to hack
together a cheap fix for a bad system.

If you think you may have an obstruction in the pipe:
Once you find the general location of the obstruction in the pipe
using the procedures above, the only solution is to start digging out
the pipe.  Start by cutting out the fittings under the sprinklers,
most often if something is stuck in the pipe it will be wedged into a

If you cut out a pipe and need to replace it I suggest using the next
pipe size larger than the old pipe for the new one.  It will not hurt
the sprinkler performance to use a larger pipe and it may help it.
It is worth the cost of the larger pipe.  Where the new pipe connects
into the old ones use a reducer fitting to change sizes.