my continuing series of articles of the annoying E30 idle problem, I will
now take you through the steps involved with replacing the throttle switch
on the BMW E30 3 Series 325i models with Motronic fuel injection. Keep in
mind that the vehicle in question here is my 325is, however, this article
will apply to nearly every E30 model.
you noticed an erratic idle that seems to stay constantly around 1500RPM
then drop to 600 RPM with no known reason? Is your car constantly running
lean? Well the possible culprit is the throttle switch. This small switch is
directly connected to the throttle butterfly and is used to send a signal to
the idle control valve, which regulates how much air is flowing into the
engine to keep the engine idling correctly. The switch has only two
positions, full on and full off. When the throttle butterfly is closed, it
sends an electrical signal to the idle control valve to open. This allows
air to flow past the throttle butterfly and hold a constant idle. When the
throttle butterfly is opened past a certain point, the switch closes and the
idle control valve closes, preventing extra air from entering the engine and
causing a lean mixture.
the case of my car, I had noticed that the car wasnít running correctly,
the idle seemed to have a mind of itís own. Believe me I went through
everything trying to figure this out, including replacing all the vacuum
hoses, coolant temp sensor (an ambient sensor that controls mixture), and
the oxygen sensor. Had I known about this switch sooner, I would have saved
time and money. But hey, you learn something new every day. A local BMW
mechanic was good enough to point me in the right direction. Sure enough, my
switch turned out to be bad. I ordered a new switch from Pelican and once
installed, it cured all my idle problems.
how you test the switch. First make sure that the battery disconnected. This
is crucial. If you fail to do this, you could send a spike through the
system and fry the ECU. Now open the hood, and look under the throttle body.
You will see a small black box with a plug going into the side. Press and
hold down the metal clip on the outside of the switch and pull it off. You
will now see three terminals on the inside of the switch. These are the
terminals that go to the idle control valve. What you need to do is get a
hold of a multi-meter, set it to check for continuity. It will be a tight
squeeze getting the terminals under the intake air boot, but it is possible.
Now, check for continuity between the left outside terminal and the center
terminal. While holding the test probes in place, slowly open the throttle.
Just after opening, there should be no continuity. Now test the outside
right terminal and the center terminal, and slowly open the throttle to the
full position. As you come close to full throttle, you should see continuity
return. I you fail to see any continuity return, try loosening the two
screws under the switch and rotate it until you have continuity when the
throttle is fully closed. If you still fail to see continuity, the switch is
bad and must replaced. This was the case in my car.
replace the switch, we must first remove the intake air boot. This is done
by first removing the upper air cleaner housing, then loosening the hose
clamp that holds the air boot to the airflow sensor, which is bolted to the
air cleaner. Now unplug the electrical connection to the airflow sensor by
pushing the small metal bar in and pulling it off. Now, pull the air boot
off the air cleaner/airflow sensor off and set it aside. Next, disconnect
the electrical connection from the idle control valve and remove the rubber
sleeve that holds the idle control valve to the bracket. Once the valve is
loose, pull the idle control valve out of the intake boot and the connector
to the throttle body. Now disconnect the breather hose from the valve cover
to the throttle body. Just loosen the hose clamp holding it in place and
pull it off. Next, remove the vacuum connection for the brake booster from
out of the intake air boot, and disconnect the vacuum lines that connect the
other end into the throttle body.
all these parts have been removed, loosen the hose clamp that holds the
intake air boot onto the throttle body and pull the boot off. You will now
see the throttle butterfly. On the side of the throttle body, there are two
coolant hoses that are used to keep the throttle body warm during cold
starts. Loosen the hose clamps that hold them in place and carefully pull
them off. Be aware that you will most likely see some coolant spill out of
these hoses. Now disconnect the throttle and cruise control cables at the
top of the throttle body. Just pull the throttle wide open and pop the
grommets out, then slide the cables out.
are now ready to remove the throttle body. Locate the four 10mm nuts around
the perimeter of the throttle body and remove them. Now pull the throttle
body off, and rotate it so we can gain access to the bottom. You will see
one more vacuum hose attached underneath. Remove this hose from itís
fitting and set it aside. Unplug the connector for the throttle switch. Now
take the throttle body to a clean work area.
this time, I usually like to go over the throttle body with a rag and some
cleaner. This will get off all the crud and grime that have accumulated
inside from years of oil mist from the valve cover breather hose. We donít
have to get it perfect, but a good once over should do.
clean, turn the throttle body upside down and remove the plastic cover for
the throttle switch. You will see that the switch is held in place by an
indexed shaft and held on by a clip on the bottom. Using a screwdriver,
carefully remove the clip. This clip is under tension, so be aware that once
you remove it, it will most likely go flying off, so have a rag or other
means ready to catch it. Now loosen and remove the two screws on either side
of the switch that hold it on and pull the switch off the shaft.
could immediately see what caused the switch to fail. The small soldered
contacts inside seem to have broken free of the switch and broken
continuity. I could just go ahead and re-solder them, however I feel that in
order to solder contacts this small, you will need to have extensive
experience with electronics. However, if you feel up to it, go ahead.
place the new switch on the throttle shaft in the same relative position and
install the clip on the end to hold it on. Now install the two screws on
either end but do not tighten them. Now grab your multi-meter and set it for
continuity again. Connect the test probes between the terminals and make
sure the throttle is in the fully closed position. Now, rotate the switch
just until there is continuity. Now tighten the screws to hold it in place.
Put the cover on and take the throttle body back to the car.
now have to re-install the throttle body. Remove whatís left of the old
gasket between the throttle body and the throttle body and place a new
gasket over the studs. Now, reconnect the vacuum hose underneath and plug
the connector for the throttle switch back on. Put the throttle body in
place over the studs and install four new self-locking nuts. Tighten the
nuts down, but be careful not to strip them out. Once secured, re-attach the
throttle and cruise control cables. Just pop them into the grommets and
thread the cable in and re-adjust them to take out any slack.
re-attach the coolant hoses for the throttle body. Clean off the inlet ports
and slide the coolant hoses on. Once pushed on, tighten the hose clamps to
hold them in place. Be sure to add a little water/coolant into the reservoir
to top up any coolant that may have spilled out. Next, re-attach the
breather hose from the valve cover and tighten the hose clamp.
these connections have been made, place the intake air boot back over the
throttle body and tighten the hose clamp that holds it in place. Now
reattach the fittings for the brake booster into the air boot and also the
fittings into the throttle body. You may find it helpful to put a little RTV
sealant round these fittings to keep them tight inside the throttle body.
Now re-install the idle control valve into the intake air boot while
simultaneously pushing the other end into the throttle body connector. Now
re-connect the electrical plug on the end of the valve. Now take the upper
air cleaner and slide the intake air boot over the airflow sensor, and
tighten the hose clamp that holds it in place.
re-connect the battery and start the car. You should now have a smooth
stable idle that purrs like a kitten.
Well, there you have it - it's
really not too difficult at all. If you would like
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