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

[Homem Bomba]

Hi guys,

Here is another DIY guide, this time on diagnosing and fixing a common problem with the Toyota 4A-GE Engine (mine is in the AE86). I'm talking about a floating idle, where the idle goes up and down in a cycle, in a periodic fashion.

Spotting the problem:

[Homem Bomba]

I asked for help at the Youtube channel and got a lot of useful comments. At first I thought it could be the cooling system, so I bought a new rad cap and bled the coolant system. This was not the issue, so I moved up in the difficulty scale, by looking at the throttle position sensor. I disconnected it and reconnected it again to see how the engine responded. This was the result:

[Homem Bomba]

Finally, yesterday I fixed it, with a little help from my metalworking skills and simple tools!

In this video I showed how I adapted the throttle position sensor from another 4A-GE bluetop, that had a different prong layout (vertical instead of horizontal). I had to craft a part that fitted in the throttle body, to activate the "new" throttle position sensor from a vertical position instead of the horizontal one used by my old TPS.

I tried fixing the vertical one to the horizontal plate without machining the new part... the only result was that there was not enough twisting margin left in the sensor, causing the throttle to be stuck like 5mm after pressing the pedal, which made the engine go much slower. Due to the lack of opening in the throttle, the PWR light on the dash never came on (it stayed always in the ECON mode).

Enjoy, and tell me if this helped you!

[kawo]

Will this happen on small port AE92 engine?

[pack]

Sure.... the problem is the TPS sensor that is faulty....

[max_ae86]

It is also good to replace the TB seals (on both sides of butterfly shaft). It helped me a lot having a rock solid idle, no shaft movements and false air. A highly recommended and very chip fix.

[kawo]

my floating idle problem do not occur all the time, is it still TPS related?

[Homem Bomba]

If it's intermittent clean the contacts, if it remains I would take out the throttle body and clean the passages that run coolant to and from it; they adjust the idle on cold start by pushing a spring with varying force by expanding a cell filled with a substance like the wax in a thermostat. If the channels become clogged with rust, the coolant wouldn't flow correctly and you would get an erratic idle.

The idle air valve may also be the culprit, but before moving to the hard solutions, try burping the cooling system. Open the radiator cap half way so that it stays on the rad but does not seal, run the engine until the coolant starts to leak out naturally, and pinch the radiator fluid hoses while you do it to remove any air.

[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