How to fix a floating idle on a Toyota 4A-GE motor

[jondee86]

Full credit to the original poster for taking the time to describe how he
found and cured the floating idle problem on his car However, I would
like to make the point, that there are some other things you can look at
before you consider replacing or modifying the TPS.

These are a couple of quick checks you can do to narrow down the likely
cause of a floating idle problem. First get the engine up to operating
temperature. From a cold start, any 4AGE bigport or smallport (with the
factory intake and thermostatic idle-up valve) should first idle at around
2200 rpm, and gradually drop to around 900 rpm as the engine comes up
to temperature. Once the engine is warmed up, do this...

1. FIRST CHECK
With the engine running, unplug either the TPS or the MAP sensor. See
if the idle stops floating. If it does, then your hot idle is too high. Check
for a vacuum leak (hose off etc), throttle not closing completely (cable
too tight, bypass screw wide open, throttle stop screw loose etc). If you
can't find anything obvious, then do this...

2. SECOND CHECK
Remove the intake ducting from the throttle body, and locate the idle air
intake opening (about 5 o'clock just in front of the throttle plate). With the
engine running, close the hole with your finger, and see if the idle stops
floating. Note that this is only possible with a MAP sensored engine.

With AFM engines you need to close the hole off temporarily with a small
soft rubber plug, a couple of layers of duct tape, or similar. Put the ducting
back on and start the engine. Blocking the hole on a fully warmed up
engine with a working thermostatic idle-up valve, should only drop the
engine speed by 100 rpm or thereabouts. If the idle drops by 1000 rpm,
then the valve is not working i.e. it is staying open when it should be
closed. If the valve stays open, you will have permanently high idle
and it will start floating once the engine is up to temperature.

Check that both hoses from the engine cooling system to the valve get
hot with the engine running. If one stays cool. the water is not circulating.
Remove the valve and clean it. Make sure there is a clear flow from
inlet to outlet. Drop the whole valve into a pan full of boiling water and
see if the valve moves about 8 mm. Check the hoses are not blocked.

After this, your idle should go high on cold start and then drop to 900 rpm
or so when the engine is fully warmed, and no floating idle


But if neither of these checks solves the problem, then you will need to
start investigating the TPS, engine timing, electrical idle-up valves and
other possible causes.

Cheers... jondee86