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Discussion Starter #1
Hi all,
I've been doing some thinking about my 4.8, i recently changed to 3:9 diffs in my auto and it drives much better. I also noticed my fuel consumption has dropped around 20%. A conversation I had last weekend made me realise something bout the 4.8 engine. It seems us auto owners think that our auto's are weak and that's why the TCU lets the torque converter slip a lot. However that does not seem to be the case. Below 2200rpm the engine has almost no power(according to the owner of a manual 4.8 that i was talking to), so in an attempt to use its power the TCU lets the auto slip to let the revs climb. So imo the aim of the game should be to keep the engine in a rev range which provides enough power so the ECU does not tell the TCU to disengage the torque converter.
(disclaimer: I'm not a mechanic, and these are my personal findings on a 2002 4.8 auto. Standard air intake+snorkel, PVA mod, cruise mod, 3:9 diffs, 33's, 2" lift, front and rear bar, roof rack and awning, and an idiot hoon driving it on petrol. Gas will be stripped off it in 2 weeks time).

So, i used to get 22-24L/100km. Best ever was 19.5L/100 while on the highway. That improved a little with the 3:9's but nothing drastic. So the following was a recent experiment I did that returned 17L/100 and me still refusing to drive like gramps.
Never drive in D, always tiptronic.
Keep rpm over 2200 at all times, ideally between 2500 and 2700.
Before putting the foot down use the tiptronic to down shift and give the auto 2sec to engage (like you would in a manual).
Have the attitude that ever time the torque converter is slipping fuel is being wasted.

From my reading, there seem to be 8 positions on the throttle body. If you go a little more gently with your right foot you can start to feel each of them. This plays into the next mod I did. the accelerator peddle seemed very 'heavy' so after a quick look I noticed a decent sized spring in the foot well. Disengaging it gave the accelerator peddle a much lighter feel, it still returned to 0 position quickly and by itself but didn't require a brick to push it down anymore. This did not throw any error codes, hasn't upset the cruise and the throttle body seems fine with it. Those with more knowledge about it than me may have something useful to say about this. ideally I'd like it to be even lighter still. The lighter peddle means it's easier to hold 2500rpm with your foot for a few hrs of driving.

Interestingly a few ppl have mentioned their cruise control 'surging' or jumping, I had this happening before I put the 3:9 diff centres in but hasn't happened since. Engine braking has improved dramatically both on and off road.

Hope this is some food for thought and any other good suggestions please add them. Cheers MP
 

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Interesting. Do your new fuel consumption figures take into account the change in diff ratio? 3.9/3.54=1.10 Your OD will be seeing 10% more km.

I drive my 4.8 fairly easily and revs are nearly always under 2300 but i return 15.7L/100km with a mix of city and town with 2 inch lift, 33's, steel bar and winch.

I do find the cruise control surge a bit especially if you set it under 80km/hr. We have a few long roads that are now 60km zones and setting the cruise is a waste of time. It just cannot stick to that speed without accelerating and decelerating.


I would like to swap to 3.9's but a little hesitant as i already have 83% reduction gears in the transfer. Will be very low.

Did you notice a change with the PVA mod? I couldn't feel a thing.


"From my reading, there seem to be 8 positions on the throttle body"

Where did you hear this?

I too find the throttle a bit firm. I might look into replacing the spring.
 

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Discussion Starter #3
yes speedo is accurate(marks adaptors speedo correction unit), reads 100km/hr while at 97km/hr on the gps. so that means the odometer is out by -5%. All my figures have been adjusted to compensate.

Found a slight positive change with the PVA mod. but mine often sees 4000rpm.

Think the bit about the throttle was from either the haynes manual or the online service manual. Let me know how u go with the throttle spring
 

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So are 3.9 gears worth the effort?

Have you adjusted your base timing? When i checked mine it was already at 10 degrees.

I just noticed you have a roof rack which might explain some extra fuel use. I'm still pretty happy with sub 16L/100km on freeway.

I'm not 100% convinced with the PVA mod but at the same time i did it when i bought the car so haven't run a couple of tanks through with it not done. With engine cover off i watched the actuator while cracking the throttle and it seems to close with minimal input even under no load.
 

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Discussion Starter #5
If i had the split ring i would reverse the PVA mod now to test and see. 3:9's are without a doubt the best mod I have done to it. I haven't looked at the timing, wouldn't have a clue how to do it. too much effort to take the roof bars and awning off :( Last tank was 18/100 and that was almost all of it city driving in really heavy stop start traffic.
 

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Same, I lost it while removing. Did you also unplug anything? I just left it as is so the actuator still moves but doesn't touch anything.
Just borrow or buy a timing light and hook it to the battery and the special lead around the front ignition coil. Mine took a little stuffing around but eventually it worked. With engine 100% warm shine the light on the crank pully and a line will appear. There are 3 markings 0, 5 and 10 degrees. If you rotate the cas it will advance of retard ignition.
 

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Discussion Starter #7
I"ll ask my mechanic to have a look at the timing when I drop it off next week to get the gas stripped out of it. Old vapour system that was never efficient but gas was cheap and since it was on it when i bought it I was happy to leave it, but now with fuel prices etc its time to strip it.
Just disconnected the actuator and blocked the vac hose. What was ur timing set on initially?
 
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