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Was there ever any verdict on the fuel economy issues?

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Old 11-04-2017, 01:59 PM
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Was there ever any verdict on the fuel economy issues?

Hey guys, long time lurker, first time poster. I recently got my hands on a 2003 G35 Sedan that I've been taking care of for a long time for a neighbor. It's been maintained perfectly (if I do say so myself), but the fuel economy has bee dropping over the past 60k or so and it's bad enough that he wanted to get rid of it, so I picked it up. It's an auto.

Now when I say bad, I mean abysmal...this past tank, the light came on and I'm at 200 miles (about 13 mpg). Yes, yes, I know that "these cars aren't meant to get good fuel economy" but I've read through many threads and a lot of people are complaining about the same really bad fuel economy. However, I didn't see anything about a solution.

I went to change the spark plugs the other day with the stock NGKs, and it had a set in there and they were all in spec, and looked good...no fouling or burned look. Nice and ashy, so I put them back. I should mention that my commute is 80/20 hwy/city.

I read a few posts saying they had improvements after a fuel injection cleaning service, which I'm willing to try next. Or maybe dump a gallon of lacquer thinner in the tank since I'm a couple gallons from empty. However, the improvements reported weren't up to what I'd expect...say, going from 13 to 14 mpg, I expect a car like this to get 17-20 in the city.

Changed the air filter, it was reasonable dirty but not that bad. Hah, these cars dirty the air filter faster than any other car I've ever had. I have to replace the power duct part, the foam is gone and it's not sealing well.

I'll check the fuel trims to try and get a bead on the MAF...when I pulled it out to clean, it was pristine, but I sprayed it down with MAF cleaner anyway. I've had several Lexuses in the past, and their fuel economy and responsiveness is heavily dependent on MAF condition; a simple cleaning often brings the car alive and drops incorrect fuel trim readings from 25 or so to within spec. Wondering if the G's are the same. Have not checked for vacuum leaks yet.

I realize that autos have worse potential fuel economy than the sticks, but I have a hard time believing that it should be this bad. Also, I've read a few reports of sticks getting similarly bad economy. Galls me, because with 50/50 hwy/city, I managed 23 combined in my 2001 DeVille 4.6L, which had significantly more horsepower and torque. I'm reasonably lead-footed.

Any insights/links I may have missed are appreciated. Thanks.
Old 11-04-2017, 03:41 PM
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Keep in mind that caddy also has selective displacement but as for the poor economy on the G I would definitely check the pcv valve as it's notorious for killing mpg. LTFT shouldn't be more than +10% if it is then you probably have a vacuum leak, spark plugs should be replaced to rule them out. And I would replace the air filter as well regardless of light transmission.

Perform a full reset of all systems in case it's simply picked up some bad habits.

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Old 11-04-2017, 06:29 PM
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Keep in mind that caddy also has selective displacement but as for the poor economy on the G I would definitely check the pcv valve as it's notorious for killing mpg. LTFT shouldn't be more than +10% if it is then you probably have a vacuum leak, spark plugs should be replaced to rule them out. And I would replace the air filter as well regardless of light transmission.

Perform a full reset of all systems in case it's simply picked up some bad habits.

Thanks man, I'll get a new OEM PCV and hoses (when I pulled the front right spark plug I did notice that the pcv hose was hard.

Air filter's new, I'll get a new "Power Duct" because the sealing foam is just about gone. To do a full reset, I can just disconnect the battery for an hour, yeah?

By "selective displacement" do you mean cylinder deactivation? If so, nah, it didn't - mine was an '01 with the L37 (high-output) 4.6L Northstar.
Old 11-04-2017, 07:37 PM
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The battery disconnect method will reset the ECU yes but not the others. Make sure to pump the brakes to release any capacitive stored energy.
Old 11-05-2017, 12:49 AM
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Any time you can use the cruise control, it's amazing what that will do for your mileage! No need to ever replace a PCV valve, clean it with carb cleaner and replace that hose since it's dried out. Another hint, don't listen to the sound our the exhaust...turn up the tunes!
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Old 11-05-2017, 02:14 PM
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The battery disconnect method will reset the ECU yes but not the others. Make sure to pump the brakes to release any capacitive stored energy.
Thanks guys. Lol, I installed a new midpipe a few months ago because the previous owner hated that the VQ sounds like a truck engine. I do keep my foot out of it unless I have to pass (not often).

So pumping the brakes will deplete the capacitors and reset the other computers, or is there something else I should do to reset those?
Old 11-05-2017, 02:27 PM
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It still takes time but after you remove the battery pump the brakes. I don't know exactly how long because I've always just done the pedal dance which is immediate.
Old 11-05-2017, 07:11 PM
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It still takes time but after you remove the battery pump the brakes. I don't know exactly how long because I've always just done the pedal dance which is immediate.
Pardon my ignorance, but what's the pedal dance?
Old 11-05-2017, 07:47 PM
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Nickname for the manual ECU reset procedure I linked earlier in post #2
Old 02-07-2018, 11:19 PM
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OK guys, back with an update after so long.

Here's what was changed out:

- upstream O2 sensors (with OEM NGKs). One sensor's "cage" was mutilated, and now fuel trims are reading more accurately
- new OEM fuel pressure dampers. Rumbling sound gone
- new fuel injector clamps and o-rings. Fuel smell gone
- new OEM intake gaskets (all of them)
- new valve covers, gaskets, and PCV stuff (OEM, all hoses as well)
- new OEM knock sensor and Z1 harness. Old one died bathed in oil from the VC leak
- new OEM brake booster hose (manifold to passenger-side nipple)
- new Z1 plenum spacer
- did all the reset procedures

Car runs great, feels peppier (I assume from the plenum spacer). All installations were double-checked, everything is hunky-dory.

But I am still getting 11-13 MPG! Still 50/50 city/hwy, I don't beat on the car, maybe 1 WOT a day.

Fuel trims at idle are around 10%. Pressing the brake pedal at idle sends that to 14-17. No codes.

Under moderate acceleration, fuel trims zero out, then go negative (-11 to -19).

I'll make a smoke machine and see if there's a vacuum leak I missed...only other hoses are EVAP hoses (which are all pliant and nice), and the brake booster hose right at the brake booster.

What do you guys think?
Old 02-07-2018, 11:22 PM
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Check the condition of your air filter, I know it sounds obvious but it's crazy just how much of an effect is has on MPG if the motor is having to suck air through a dirty restrictive filter.
Old 02-08-2018, 04:49 AM
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Also clean the MAF sensor. It's easy and cheap, get a can of CRC MAF cleaner and you'll keep that can for the life of a car. A little goes a long way.

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