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Hi there, I have been meaning to join you guys for ages and my GU has finally twisted my arm into doing it!

My GU 4.8 (12/2001) is idling at 1500 rpm in neutral, 1200 in Drive, and ocassionally revs up to 2700 rpm and back down to 1500, back up to 2700 etc etc. It does this sitting in neutral,idling. It is also chewing through 40-50 litres per 100k's on LPG and Premium ULP.

I checked the timing and it is retarded at idle (1500rpm) and when I start to advance it, it seems that the computer takes over and keeps on advancing it until the knock sensors cause it to be retarded, then the computer advances again, and so on. This coincides with the revs going up and down. The most I can advance it is to 6deg retarded before it starts playing up.

I have just got Datascan II, and am trying to work it out. All I've tried so far is advancing the timing and it did the same as turning the cam sensor.

I have recently changed the MAF sensor, o2 sensors, coolant temp sensor, had the injectors done, throttle body clean, checked CAT, checked for vacuum leaks.

It's been to two mechanics but I have come to the conclusion that I am going to have to work it out myself. Any suggestions will be greatly appreciated.

Please help??? Thanks in advance -Troy :headwall:

I should have kept my GQ with the 454 in it (legally registered in QLD!), just the kids were scared of it and would only go in it with helmets or ear muffs on!
 

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Let me guess, has exhaust+ extractors fitted and done maybe 200,000kms+?
The auto's throttlebody plate and springs can wear causing this issue and the exhaust/extractors magnify the issue as they let the engine breathe more air then standard because of less backpressure.

Auto has a combined Mechanical/Electronic throttlebody, its cable driven but its geared so that the full throttle cable movement only opens the throttle plate about 10%(which is why you can drive it in limp mode with a failed throttlebody). The ecu opens the rest of the throttle depending on load/throttle position/engine temp etc, its an easy way to limit power etc.
Instead of a Idle Air Valve the throttlebody has a normally closed position of about 3 millimeters and it has the ability to close or open the throttle about 1-2 mm from this point to control idle revs.

As the throttlebody wears it can't close off as much air as it used to and will idle slightly higher or if you have an exhaust it will idle alot higher.

Idling standard is supposed to be 650rpm, mine was around 1000 in gear and 1200rpm in nautral and the heat/fuel usage /transmission temp/run on when brakeing was quite worrying as the aftermarket extractors dont have a shield.

The manual states that if this happens just replace it, but not wanting to spend the cash and fairly technical i dismantled mine and found there is in fact an adjustment screw inside to set spring load.

Iam running A 2.5" speedy exhaust/extractors(ceramic coated now to keep temps down), made a heat shield out of gal sheeting from bunnings and mine now idles back at 650rpm.

The fuel usage was still a bit high so i took some logs from the Nissan Datascan II tool I have and noticed my O2 sensors were not swinging as fast as they should, unsure how old they are I just bought some universal 3 wire replacements, fitted them and noticed a immediate difference on the highway.
Iam back down to 15L/100km combined average again.

I had a unichip fitted at christmas which was mainly to correct driveability around town as it used to drink in the order of 25L/100, it hasn't really changed the highway consumption which is a pretty good 13-14l/100km as its mainly O2 sensors and constant revs that help this.
The biggest difference has been the low throttle low/med revs has leaned out consideribly and these engines are known to run pretty rich.

Pretty happy with it overall and makes 124kw at the wheels with 4.11 ratios and 33's. The torque can't really be used as a real value on an automatic, but its always in huge supply with these ratios. Prob get closer to 135-140kw with the standard 3.5 ratios and standard sized wheels. Sits on 2350rpm on the highway at 100km/h gps speed.

Hope this helps

Hi there, I have been meaning to join you guys for ages and my GU has finally twisted my arm into doing it!

My GU 4.8 (12/2001) is idling at 1500 rpm in neutral, 1200 in Drive, and ocassionally revs up to 2700 rpm and back down to 1500, back up to 2700 etc etc. It does this sitting in neutral,idling. It is also chewing through 40-50 litres per 100k's on LPG and Premium ULP.

I checked the timing and it is retarded at idle (1500rpm) and when I start to advance it, it seems that the computer takes over and keeps on advancing it until the knock sensors cause it to be retarded, then the computer advances again, and so on. This coincides with the revs going up and down. The most I can advance it is to 6deg retarded before it starts playing up.

I have just got Datascan II, and am trying to work it out. All I've tried so far is advancing the timing and it did the same as turning the cam sensor.

I have recently changed the MAF sensor, o2 sensors, coolant temp sensor, had the injectors done, throttle body clean, checked CAT, checked for vacuum leaks.

It's been to two mechanics but I have come to the conclusion that I am going to have to work it out myself. Any suggestions will be greatly appreciated.

Please help??? Thanks in advance -Troy :headwall:

I should have kept my GQ with the 454 in it (legally registered in QLD!), just the kids were scared of it and would only go in it with helmets or ear muffs on!
 
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