bng4bk

Well, I'm officially at the end of my rope. My 99SE auto runs great, shows no codes or CEL...its just thirsty. I'm still dying to know what it could be that makes my car get 40-60% worse mileage than others, even though it runs fine. I just filled my tank and recorded a whopping 14.8 mpg! WOOHOO! On the same commute, driving equally conservatively, my 5spd got 21 mpg.

For starters, I think the auto costs at least 2 mpg, but I'm still well shy of 21 mpg. I'm currently driving on a KS substitute to verify KS is OK, and there was no change, so I'll put that back.

It seems if the 99 coil packs were bad it would run crappy, though these are known to cause problems. Expensive to test via replacement.

I was hoping for a stuck thermostat, but the car warms up normally, so I don't think that is it.

The idle seems to run just a bit high at times, tho it is smooth. But it is an auto, so I don't suspect a cleaning of the IACV is really necessary. Besides the car only has 60k.

We are on oxygenated fuel til March, so I'd like to think that's worth at least 2 mpg, since my mileage varied noticeably since early November.

I also noticed that mileage doesn't seem to be any different when I'm putting freeway miles on either. That should be at least 3-5 miles better, I'd think.

So far I've systematically:
Replaced air filter
Replaced plugs
Replaced front O2 sensors
Replaced fuel filter
Cleaned TB/MAF
Ran Techron through to clean system twice
Ohmed KS, ran a 490Kohm resistor (no apparent change)
Installed MaxTuning stage 2 VB/ cooler
Installed Budget Y pipe

How about it? Any of you GURUs have any other ideas on the cause?

To all others: Please don't clog up the thread with the "I get 30 mpg in the city with mine" posts. I know how to calculate mileage, apparently some don't.

Defacto

ummm...SAME!!!! 99Se w/14 or WORSE MPG!!! I average around 200-210 miles to almost a tank (right at the line, intermittent light)

NmexMAX

iTrader: (22)

Gas leak? .

nismos14

iTrader: (56)

the ks may be bad and to top that off the coil packs may be intermittently going out. there is an easy way to test the coil packs. as the car is running remove the plug to one of the coils, if it causes no change in the way the car runs that pack is bad, if it does then plug it back in, unplug another one and observe the same thign again. hope that helps

Defacto

good idea...but my car has 26k on it...i mean SERIOUSLY....

bng4bk

I'm bypassing my KS right now, so it's not that. Just speculating here, but if any one of the coil packs were bad or weak, the car would probably run poorly or at least have an intermittent stumble. It runs like a scolded dog, tho. I will try that test, tho..thanks. I've read elsewhere on the org that many 99 coil packs were defective, but as i said, I think my performance, or a thrown code would indicate that.

bng4bk

Hey Defacto, we should start up the brotherhood of those who get unreasonably bad gas mileage from a 3 ltr V6, and don't know why. We could exclude anyone who gets over, say, 250 miles to a tank.. of course, we'll never hold a meeting cuz none of us can afford to drive to it!

ejj

iTrader: (5)

Yeah, if your coilpacks went bad you'd feel a drastic loss in performance.

How do the spark plugs look? Perhaps you have a fuel injector that's "stuck open", or just letting more fuel out than it should be?

Do you smell gas at all?

emoore924

Take it to a shop and have them put it on an emissions sniffer. If it is running really rich, there could be something wrong in the emissions stuff that it richening the mixture excessively. Should be an easy diag for most shops.

Defacto

I'm throwing a CEL as well, but since we're 99s, doing diagnostics are a pain in the ****. I'm gonna go to autozone and have it checked out, but I'm also going to try the fuel filter. I don't think i have a fuel leak, since whenever I park the car, i never see puddles beneath it. If there's junk in the gas tank, would that make a diff? by junk I mean the little metal flap that normally covers the hole where one sticks the gas nozzle...it's conspicuously missing from mine....has been since I can remember. I'm trying to think of whatever I can, i'm getting sick of having to refill at least twice a week, esp. when the lowest price gas I can find right now is 1.91 for 93 octane...

G20Flyer

Have you been running 91+ octane?

Hmmm id check that VB setup...

Defacto

93 octane...but i don't think a VB mod will fix it...

DAVE Sz

iTrader: (2)

fuel injectors. A bad KS won't even be half as bad for gas mileage as 1 partially working fuel injector. Check them with a screw driver or something. Mine are pretty bad cause I can FEEL the gasoline running under my feet, it's like it's clogged up or something.

bng4bk

Man, you lost me with that one. What are you checking with a screw driver? Gas under you feet?

dpassino

I actually wanna try this too...

3 questions..

1) is this safe for the car?
2) after unplugging one of the coil packs, do I need to drive the car to see if there is a difference, or am I looking for noises/idle changes?
3) if indeed bad, how much are coilpacks roughly?

bng4bk

Just spoke with a local Nissan mechanic. He agreed that 14.8 mpg was not normal. Gee, I feel validated. I asked him about sniffing the emissions, but it passed DEQ inspection recently, so he dismissed that. His best suggestion was that the MAF could be bad, but still within parameters, hence no codes. Of course the only way to check is to replace it to the tune of between $160 and $500 depending on vin.

On an upbeat note, he did say that oxygenated fuel can account for 20% loss in mpg, which could account for as much as 4 mpg! I did record 18 mpg before November, and right now I'd kill for 18 at 1.75 a gallon for premo.