4.8 auto power loss, lazy and heavy on fuel....

[2strokespirit]

Hi guys.... I did my intro and need to post away.....
I bought a 4.8 a few months ago, only has about 100 000km on the clock, it recently started to loose power intermittently, but only when I cruise at highway speeds (not using speed control).... At full throttle the acceleration seems to be good, but at speeds of 120km/h or less it is gutless... The autobox will kick back to third gear, even if you just want to accelerate slowly... It cannot maintain speed at 120km/h in 5th gear, its in 4th gear all the time.... It seems as if there is only power at 90% tps and more.... She uses more then double the amount of gasoline that she did in the past and I do not know what to do...
I do understand motors and I do understand what is going on in a FI motor.... I have tried the following....
Replaced the fuel pump as I was not happy with the fuel pressure...
I replaced both O2 sensors...
I swapped out the MAf sensor with my friend's patrol...
I removed the exhaust to check if maybe a muffler have collapsed as the exhaust is quit rusty...
I checked the vacuum lines..
Replaced spark plugs..
Chacked the intake manifold gaskets...
I hooked it up on the Nissan scanner and check the voltages on the tps sensor, MAf and accelerator pedal position sensor...
The throttle body actuator/motor seems to work fine, when I disconnect the MAF I can remove the intake pipe at the throttle body and rev the motor to a certain rpm, it moves freely and steady....
I am stumped, the dealer did replace my dashboard just before this all started, but They only unplugged the cluster and dash wires, everything there seems to work fine....
Another thing I noticed is that it seems to pull very good from 3500rpms, but under that, there is NO POWER... Recently it started to run much hotter with the AC on and in summer then I would like to see.... I would think this has something to do with the fact that the motor is so gutless, it is "working" to hard to maintain speed.... But even on idle and AC on, the needle will start to move to 3/4s+ on the guage, but there is no air in the system, all the water its pushing to the overflow bottle get sucked back in.... The cap is still good and the lines is very hard at operating temps....
The petrol filter has been replace, the lines has been checked.... I get no engine lights/codes, there is no coolant in the oil or any oil in the radiator... The engine oil is super clean.... I checked the air filter and snorkel to make sure all is open....
I checked all sensors and terminals.... Does the patrol has a crank sensor??
At the moment the intermittent happening is gone and it has no power all the time.... This might make it easier to find it, but i have no more ammo to throw at this thing....
Does anybody had something like this or any idea what can cause this....

I suspect that the ecu might have something to do with this, its the only thing I can think off....
Blown head gasket would showed up on the compression test which gives around 170 to 175psi on all 6 cylinders and the oil is super clean....
where can I download a workshop manual?

I will appreciate any feedback or help, I am very frustrated at the moment and the dealers are lost at this....

Thank You!

[Bob]

If you havnt done it already disconnect the Battery for say 30 minutes (to reset the ECU)

[threedogs]

Another member with a 4.5 is having similar problems, well mainly woeful fuel consumption.
He may have a result as to why very shortly as he is going away soon and needs decent fuel figures
Might not be related but GU's are famous for poor earths, maybe run a few extra from the battery to motor and to chassis and body
as well as find and clean any old ones

[Bulbous]

Between the MAF and the actual intake manifold is a sort of corrugated bit of plastic tubing. Check it very carefully for any holes.
As they get older, this plastic gets brittle and can get a hole. If it does get a hole then it's letting in air after the MAF has measured the air volume.

Happened to a friend of mine. Nissan wanted 16,000 AED to replace the head. It was a pinhole in the intake.

[threedogs]

@ Bulbous does it need to be there or can it be removed

[4bye4]

Had a diesel that suddenly started to get 100km less to the tank and was gutless. Turned out to be injector pump timing. Don't know the nitty gritty as was fixed by another shop, but you seem to know your way around a motor so thought I would suggest it. Mine was not a patrol motor by the way

[2strokespirit]

thank you all..... all earths has been check and loosen and retigthen.....

[Bulbous]

@ Bulbous does it need to be there or can it be removed

Well, I removed mine when I put a full K&N intake system. Not sure if you can just remove it though.
If you look in this picture and follow back from the Air Filter, past the MAF then it's the ribbed Plastic/rubber tubing at about 2 o'clock from the centre of the engine cover.
You need to stretch it and see if there is any perished bits inside the ribs.

54504

[Bulbous]

Here's a better picture courtesy of google - The black bit at the 3 o'clock position that feeds the throttle body

54505