2004 GX Prado Erratic Idle Problem

Submitted: Tuesday, Sep 11, 2012 at 17:17
ThreadID: 97965 Views:7316 Replies:6 FollowUps:9
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About 12 months ago I had the heads rebuilt on my '04 GX 4.0 V6 Petrol Prado (thanks to an inattentive apprentice leaving the radiator cap loose) and ever since the engine idles at 1200-1500rpm in the peak hour crawl. As soon as the car stops moving, idle returns to 800-900rpm. Costing about $300/qtr in poor fuel consumption.

The mechanic that did the head repair has checked MAF, Throttle body, EFI fault codes = nothing.

Took it back to Toyota. Same thing. Diagnostics (computer) shows no fault codes, but they can see the idle issue when the car is moving and it disappears when movement stops. Clueless.

Some threads talk about O2 sensors, others about coolant sensors, others about vacuum hoses. The sensors are expensive "trial & error" solutions. Anyone seen the same issue and found a resolution?

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Reply By: Holden4th - Tuesday, Sep 11, 2012 at 19:24

Tuesday, Sep 11, 2012 at 19:24
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.
AnswerID: 494721

Follow Up By: Darren D1 - Tuesday, Sep 11, 2012 at 20:05

Tuesday, Sep 11, 2012 at 20:05
Have reset the ECU several times with no success. No other symptoms that can be seen. I presume you are referring to the Mass Air Flow sensor?
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FollowupID: 770361

Follow Up By: Holden4th - Wednesday, Sep 12, 2012 at 20:24

Wednesday, Sep 12, 2012 at 20:24
Have reset the ECU several times with no success. No other symptoms that can be seen. I presume you are referring to the Mass Air Flow sensor?

Yes. The MAF determines fuel mixture, gear changes (AUTO) and idling speed and it is all based on the amount and temperature of the air passing across the sensor. If it's not reading it correctly then it won't make the correct air/fuel mix.
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FollowupID: 770431

Reply By: TerraFirma - Tuesday, Sep 11, 2012 at 19:51

Tuesday, Sep 11, 2012 at 19:51
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.
AnswerID: 494722

Follow Up By: Darren D1 - Tuesday, Sep 11, 2012 at 20:06

Tuesday, Sep 11, 2012 at 20:06
At $350 a "test" I'm reluctant to keep irish mine-sweeping.
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Follow Up By: TerraFirma - Tuesday, Sep 11, 2012 at 20:36

Tuesday, Sep 11, 2012 at 20:36
Well Darren not the Toyo dealer I use, they have diagnosed and even done a pre purchase vehicle inspection for me without charge. Off course I have built a relatiobship with them and have them service all our Toyo vehicles. Lets hope most Toyo dealers are a little more sophisticated than Irish mine sweeping. Who repaired your heads and who was responsible for the damage to begin with, Irish mine sweepers?
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FollowupID: 770365

Reply By: Steve M1 (NSW) - Tuesday, Sep 11, 2012 at 20:39

Tuesday, Sep 11, 2012 at 20:39
no disrespect to replies here but try www.pradopoint.com (maybe .au)

Lots of Prado specific stuff there.
AnswerID: 494728

Reply By: Rangiephil - Tuesday, Sep 11, 2012 at 20:47

Tuesday, Sep 11, 2012 at 20:47
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
AnswerID: 494729

Follow Up By: Kimba10 - Tuesday, Sep 11, 2012 at 23:10

Tuesday, Sep 11, 2012 at 23:10
Sorry but its definately not normal for a 120 petrol prado to be idling that high whilst moving without touching throttle, idle is normally around the 800mark. The idle may rise if running the aircon but even then only to about 900 then drop back down again when the compressor cuts out. I do agree with your theory as I have also noticed it when driving different vehicles, one vehicle we had (cant remember which one it was) but you would just touch the throttle and it would jump to 1100rpm but then take a few seconds for it to idle back down was a pain in the arse but the prados dont normally do that and you certainly wouldnt want it doing it in low range !!. Jump over to pradopoint.com, not saying replys on here are not worthy by any means, but pradopoint is purely for prados from the very first of them to the latest 150 models, sure some one on there may have had this issue before, or the other option is to do a google search eg ""prado idling high pradopoint"" more chance of some thing showing up...................
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FollowupID: 770372

Follow Up By: Darren D1 - Wednesday, Sep 12, 2012 at 07:08

Wednesday, Sep 12, 2012 at 07:08
Thanks all for replies. I've had the Prado for nearly 5.5 years and it never did this before. Further, the fuel efficiency has blown out from 15-16L/100km to 20L/100km so I know it's not normal.

I'll post on Pradopoint.com and see what turns up. Thanks again.
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Follow Up By: Kimba10 - Wednesday, Sep 12, 2012 at 16:32

Wednesday, Sep 12, 2012 at 16:32
http://www.pradopoint.com/showthread.php?23065-High-Idle-petrol-120
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FollowupID: 770410

Follow Up By: Kimba10 - Wednesday, Sep 12, 2012 at 16:36

Reply By: Rangiephil - Friday, Sep 14, 2012 at 10:26

Friday, Sep 14, 2012 at 10:26
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

AnswerID: 494850

Follow Up By: Darren D1 - Friday, Sep 14, 2012 at 11:08

Friday, Sep 14, 2012 at 11:08
Thanks Phil,
Yes, posts on Pradopoint are unhelpful as no resolution is confirmed and login registration to post is painfully slow.

Just to clarify, my Prado is manual not auto. And the "slow return to idle" is probably kind in its description. It almost seems like Driver Assist is forcing it to idle at 1500rpm. It doesn't seem to matter whether it is 2nd, 3rd or higher gears, if i remove my foot from the accelerator, whether in gear or not it returns to 1500rpm. Scary on a hill decline!

The mechanic has confirmed cleaning the MAF, and throttle body.

How would I test a vacuum leak when it needs to be moving for the problem to occur?
D.
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FollowupID: 770530

Reply By: Rangiephil - Friday, Sep 14, 2012 at 17:10

Friday, Sep 14, 2012 at 17:10
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
AnswerID: 494883

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