2004 GX Prado Erratic Idle Problem

You probably won't get a fault code unless the engine light comes on and your mechanic should know this. Is it as simple as an ECU reset? Do you have any other symptoms? Is power affected?

From the limited knowledge that I have, the MAF is the likely culprit considering what must have happened when all that super heated air passed across it's sensor wire.

Take it to another Toyota dealer if you have no luck. It's amazing how they can differ depending on the who the head mechanic is at the time. I've done this before once and it worked.

no disrespect to replies here but try www.pradopoint.com (maybe .au)

Lots of Prado specific stuff there.

I had a similar "problem "with a Fairmont several years ago.

I took it to a dealer who checked and reset the ECU to no avail.
You know why?

THEY ARE SUPPOSED TO DO THAT.

On nearly all injected cars the Stepper motor or IAC (inlet air control )idles up when it gets a positive signal from the VSS( vehicle Speed Sensor)

The reason is to smooth the change to 1st ratio in the auto transmission as the car comes to a halt and stop driveline clunks. Several manuals I have owned do the same thing, although ido not know why on a manual.

Now I am not certain taht a Prado does this but I am very confident.It was only when i started to study fuel injection systems that I learned about this feature.
So don't worry it is quite normal, But I was in the same position that it bugged me when I first noticed it when crawling slowly in Sydney traffic and I became obsessed with it .
Regards Philip A

As you can see , both of the Pradopoint entries are unhelpful.

Slow return to idle is a different "problem" and may be due to the base idle being out of adjustment. While the recent IAC valves are much more powerful than previouly, it may pay to check the base idle by blocking the inlet to the IAC and manifold, and seeing how fast the car idles. It should be about 500PM , Ther should be a small allen head srcew or something near the throttle butterfly to adjust it. But be careful , because you may have an air leak in a vacuum hose or elsewhere that is causing the rise in base idle.

This may be part of your problem, but even the VSS idle up will not account for your increase in fuel consumption. This is more likely to be O2 sensors that have died or are dying.
BTW low range runaway has more to do with convertor lock up than idle in autos. Most electronic transmissions lock the convertor in low range if VSS is positive and TPS is zero, ie going down hill.


Most Bosch designed ECUs, and I will bet the toyata is a clone, have several inputs to do things from the VSS .
1 injector cut off on overrun
2 idle up when moving
3 operation of the EEV (evarorative emission valve)
and probably some others such as camshaft advance retard.
Regfards Philip A

The IAC is supposed to control idle to a set target RPM at idle when stopped.

It may be that there is a vacuum leak and the BASE idle is way up near the IAC set idle speed. Maybe the first thing to check would be the base idle.

When you are moving the IAC will increase idle to try to attain a fast idle say 1100RPM, but depending on the ECU , if there is a vacuum leak the high base idle may add a bit. Some will raise by a set % and some by a closed rev feedback loop.
In addition the ECU will probably advance the timing from say 1000RPM -1200RPM onward which will increase the revs further.

I would start by blocking off any vacuum taps progressively , then drive it and see what happens. Remember if you block off the brake booster you will have very hard brakes although you probably have ABS. Maybe try some soapy water around the inlet manifold joins to the heads if you can see them. Check the oil mist vents.
Modern cars do not have as many vacuum connections as older ones, but I recall being very surprised at how "old fashioned" as AFAIR an 08 Prado was that I helped a bloke with in Kunnunurra, as his aircon belt was squealing.

I found to my amazement and delight that it still had a Vbelt that I could adjust for him, when most cars went to a multi groove belt 2 decades ago.

So it may have a relatively old design of IAC , which is not bad IMHO , in fact I think it is great.

Think about what items were touched by the mechanics when they changed the heads,answer just about the whole of the upper end of the engine and go through everything they touched.

Regards Philip A