Bad Fuel Economy

[Phatboy]

Hi guys, on my R34 GTR I usually get 280kms per 50L on BP98. I drive the car pretty conservatively eg., 60 km/h in FIFTH GEAR. Hardly ever over 4k rpm or on boost in city driving, which is 99% of the miles.

Mods include: GT2860-5s, PWR 700cc injectors, powerFC+boost controller, pod filters, cam gears, dump/split pipe, hipower catback, etc.

The turbo and bigger injectors only went 2 months ago. The fuel economy (or lack of) existed before the bigger injectors went in though, so it's not them. I also keep the engine well lubed with 300V filled to the line. The cars only done about 55000 kms.

I thought it must be the O2 sensors, but from reading my PFC hand controller both of the sensors seem to have fairly responsive voltages (though they read different voltages - is this normal?). I took it to my mechanic and he took a day and $200 to clean the sensors and re-dyno and tune to as lean as is safe an AFR, optimising for cruise speeds. He said the sensors are a bit off with their readings, but wouldn't recommend replacing them as they certainly aren't dead.

This however only increased my fuel economy from 280 to 300 km per 50L - still crap.

Dunno if it's related but the boost readout on the stock MFD occasionally shows 0 boost no matter how I drive, despite no change in the boost readout on the PFC hand controller. Guess it's a boost sensor thing.

My mechanic doesn't think this is related, but is also at a loss to explain the crap fuel economy. Any ideas from any here?

Thanks very much guys.

[gt-ahhh!]

are you having any problems with surging?

believe it or not, i suddenly got bad surging/economy with my r33, was getting 280km to 45L...

disconnected the O2 sensor and now i get 360km to 45L.

[Victory]

I cannot believe that you could do 60 kph in fifth without labouring the motor. My GTR loves to rev and I would very seldom get above third gear around town at 60 kph.

One of the keys to fuel economy is to not labour the motor and to keep it in the rev range where it has boost just starting to come on. I just got 580 km from my current tank with the fuel light just starting to come on and I more than keep up with traffic.

[blind_elk]

I cannot believe that you could do 60 kph in fifth without labouring the motor.

My thoughts exactly. 5th gear is for highway cruising, not puttering around town.

Stay in 4th at least, you should see an improvement.

[Wacky Dee]

I don't think it's his driving .. I have a GTT and the exact same fuel economy. My tuner suspects it's either the AFM or there's an intake leak giving the sensor incorrect readings.

Check your AFM and also have someone check for intake/vaccum leaks .. I do have a small vaccum leak somewhere 'after the butterfly' (whatever that is) because my EBC display doesn't show -99 vaccum on dile like it used to. But I've been told it'd be a costly excercise trying to track it down so I'm stuck with it as it is.

Edited March 27, 2009 by Delta Force

[toddlls]

My pick would be split intake/intercooler hose somewhere or blown exhaust gasket or hole in exhaust somewhere close to the block.

Another possible is split fuel hose but you would smell the fuel in or around car if this was the case.

Goodluck

[Phatboy]

I cannot believe that you could do 60 kph in fifth without labouring the motor. My GTR loves to rev and I would very seldom get above third gear around town at 60 kph.

One of the keys to fuel economy is to not labour the motor and to keep it in the rev range where it has boost just starting to come on. I just got 580 km from my current tank with the fuel light just starting to come on and I more than keep up with traffic.

My thoughts exactly. 5th gear is for highway cruising, not puttering around town.

Stay in 4th at least, you should see an improvement.

I never really thought about that. Always assumed that a higher gear = lower rpm, lower injector throughput, lower boost = better fuel economy, less wear on engine.

I "feel" that the engine may be more laboured, but don't know enough about engines to explain why??

[Phatboy]

are you having any problems with surging?

believe it or not, i suddenly got bad surging/economy with my r33, was getting 280km to 45L...

disconnected the O2 sensor and now i get 360km to 45L.

My tuner suspects it's either the AFM or there's an intake leak giving the sensor incorrect readings.

Check your AFM and also have someone check for intake/vaccum leaks .. I do have a small vaccum leak somewhere 'after the butterfly' (whatever that is) because my EBC display doesn't show -99 vaccum on dile like it used to. But I've been told it'd be a costly excercise trying to track it down so I'm stuck with it as it is.

My pick would be split intake/intercooler hose somewhere or blown exhaust gasket or hole in exhaust somewhere close to the block.

Another possible is split fuel hose but you would smell the fuel in or around car if this was the case.

Goodluck

Hmmm... any idea how I would find out if it's surging, or if there is a vacuum/intake leak without further mechanical tests? Would I see anything weird on the PFC readings?

[gt-ahhh!]

Hmmm... any idea how I would find out if it's surging, or if there is a vacuum/intake leak without further mechanical tests? Would I see anything weird on the PFC readings?

surging you should be able to feel with your ass in the seat lol. mine was very noticeable.

as for vacuum leaks, there is a way to check this if you have a compressor and airgun. firstly, remove the afm and block the hose with something like a can of baked beans, needs to be a good seal, and tighten the hose clamp onto the can.

once youve done this, you can remove a vacuum line around the intake manifold. ive got an aftermarket greddy manifold, and i found the easiest was the one for the fuel pressure regulator, at the front of the fuel rail. stick the airgun onto this tube, get a mate in the car to turn the ignition on and open the throttle, then you can pressurise the whole intake system, and your mate can check the "boost pressure" from inside the car to make sure you dont overboost it.

you should be able to hear any vacuum leaks fairly easily, because air will be fkn flying out of them lol.

alternatively, you can start the engine and spray water on all the joints etc. and if its leaking supposedly the revs will drop when you spray a leak. but i reckon the airgun method is much more effective.

[Phatboy]

surging you should be able to feel with your ass in the seat lol. mine was very noticeable.

as for vacuum leaks, there is a way to check this if you have a compressor and airgun. firstly, remove the afm and block the hose with something like a can of baked beans, needs to be a good seal, and tighten the hose clamp onto the can.

once youve done this, you can remove a vacuum line around the intake manifold. ive got an aftermarket greddy manifold, and i found the easiest was the one for the fuel pressure regulator, at the front of the fuel rail. stick the airgun onto this tube, get a mate in the car to turn the ignition on and open the throttle, then you can pressurise the whole intake system, and your mate can check the "boost pressure" from inside the car to make sure you dont overboost it.

you should be able to hear any vacuum leaks fairly easily, because air will be fkn flying out of them lol.

alternatively, you can start the engine and spray water on all the joints etc. and if its leaking supposedly the revs will drop when you spray a leak. but i reckon the airgun method is much more effective.

Hmmm ok cool thanks for all the great info. What does surging actually feel like when driving?

As for leaks, might try the spray water one first as I am not that technically minded and have neither a compressor nor airgun. Might have to end up at another mechanics later on sigh...

[Jamie88]

First thing I would check, is your engine temp. If it doesnt sit at halfway into warm once youve driven your car for 5 minutes, replace your thermostat. That will most definately fix your fuel economy if your engine is staying cold. Cold engine tells the computer to give more fuel.

[MrStabby]

o2 sensors; at cruise they should constantly flick back and forth, if they arent doing that, they're stuffed, or the ECU cant control the mixtures for some reason (dying fuel pump, air leak etc etc)

Tuning lean; when the o2 sensors are operational (in closed loop control, ie at cruise or low throttle) the AFR will be 14.7. The only place you can tune lean is when the ECU is in open loop. Of course you can turn off closed loop with PFC, but that would be going backwards.

Re; 5th gear - you're 6 speed so is 5th 1:1? What revs are you doing at 60 in 5th?

[Phatboy]

o2 sensors; at cruise they should constantly flick back and forth, if they arent doing that, they're stuffed, or the ECU cant control the mixtures for some reason (dying fuel pump, air leak etc etc)

Tuning lean; when the o2 sensors are operational (in closed loop control, ie at cruise or low throttle) the AFR will be 14.7. The only place you can tune lean is when the ECU is in open loop. Of course you can turn off closed loop with PFC, but that would be going backwards.

Re; 5th gear - you're 6 speed so is 5th 1:1? What revs are you doing at 60 in 5th?

Oh man, the tuning bit is getting a bit technical for me... but I will get someone to check the O2 sensors while driving.

I can answer the last Q - I am at just 2000rpm with 5th gear at 60kmh. 4th gear = 2.5 krpm.

What's the best/most economical place to sit? I come on boost at around 3.5k

Thanks.

[Phatboy]

First thing I would check, is your engine temp. If it doesnt sit at halfway into warm once youve driven your car for 5 minutes, replace your thermostat. That will most definately fix your fuel economy if your engine is staying cold. Cold engine tells the computer to give more fuel.

OK, thermostat seems to be working. Engine temp in mid-range after few mins driving.

o2 sensors; at cruise they should constantly flick back and forth, if they arent doing that, they're stuffed, or the ECU cant control the mixtures for some reason (dying fuel pump, air leak etc etc)

When I'm cruising at 60kmh (2.4krpm in 4th), not on boost, on the PFC HC both O2 sensors read 0.04v rock steadily. What does this mean? Thanks.

[MrStabby]

When I'm cruising at 60kmh (2.4krpm in 4th), not on boost, on the PFC HC both O2 sensors read 0.04v rock steadily. What does this mean? Thanks.

It probably explains the fuel economy problem. Factory o2 sensors are narrowband, which means they read either "lean" or "rich". When the ECU sees lean, it keeps richening up the mixture until it sees rich, then it keeps leaning out the mixture until is sees lean again, over and over. So what you should see in consult is the o2 sensor readout swinging backwards and forwards very quickly. I'm pretty sure R33/34 have zirconia sensors, and according to wikipedia "An output voltage of 0.2 V (200 mV) DC represents a lean mixture. <snip> A reading of 0.8 V (800 mV) DC represents a rich mixture".

The R32 sensors are titania, and they read 0 when they're dead. Perhaps 0.04v on your zirconia means the same thing??? Do the sensor outputs bounce around when you blip the throttle? Maybe get someone with a wideband (ie any dyno) to check the mixture at a 60km cruise. If its not 14.7:1 then closed loop control is not working, and that will stuff your economy.

I'm not an expert on any of this stuff - i could be wrong.

Re: 2K rpm in 5th or 2.4k rpm in 4th. Probably not that much difference. You could run a back to back test looking at injector duty with CONSULT if you really want to find out. Lower injector duty = better economy (at cruise at least)

[blind_elk]

What's the best/most economical place to sit? I come on boost at around 3.5k

Thanks.

Coming on boost is load sensitive. It might be possible to be doing 4500rpm and still not have any boost if the load on the engine is low enough (such as "free-revving" the engine, or perhaps running downhill).

The problem of running in too high a gear is not really about the revs, but more about how easy it is for the engine to adjust road speed to throttle inputs. If you need a bit of acceleration, the engine will need to work harder (less efficiently) in 5th than it might in 4th or even 3rd.

[Phatboy]

OK thanks guys.

Yeah, I can totally understand... acceleration in a higher gear does labour the engine.

Hmmm... I don't think the O2 sensors are dead. The O2 voltages do respond immediately if I push the throttle. They are certainly not "stuck". Neither do they swing though... Will have to ask my mechanic re this.

Guess it's time to go to another mechanic.

[ian]

And keep an eye on my post http://www.skylinesaustralia.com/forums/Sp...-F-t263596.html - and check your clutch fan!

I'll know in a few days if that was it...

Ian

[blind_elk]

OK thanks guys.

Yeah, I can totally understand... acceleration in a higher gear does labour the engine.

Hmmm... I don't think the O2 sensors are dead. The O2 voltages do respond immediately if I push the throttle. They are certainly not "stuck". Neither do they swing though... Will have to ask my mechanic re this.

Guess it's time to go to another mechanic.

Can you turn "Closed Loop" on / off? (I know in the Wolf 3D you can)

If the sensors are actually working, then the fuelling is way too rich at your "cruise" speed.

[Phatboy]

And keep an eye on my post http://www.skylinesaustralia.com/forums/Sp...-F-t263596.html - and check your clutch fan!

I'll know in a few days if that was it...

Ian

Hey thanks for the tip.

How did you find out that it was the clutch fan? Not sure what it's mean to sound like.