Thread: Bad gas mileage? Why!?

Oh wow the light comes on at 12.5 gallons?! That's just silly never mind that light then. If it worked it like the NX'x it should be a more consistent marker than the gauge.

Since I bought my car I have only ever filled the whole tank less than a dozen times. It usually only gets 4-5 gallons at a time. Never had any problems, I think there is just a lot of hype about running to little fuel in the tank.

There are baffles in tank I believe, and unless you are pulling massive G's or running up and down some really steep mountains or something I doubt starvation being an issue.

The charcoal canister is supposed to suck the fuel vapor at idle and unless you top the tank off (at least on some Nissans) I can't see any problems occurring.

Extra fuel means dead weight and old fuel in my eyes. Here in Akron there are pumps every few blocks.

Vadim what other issues are you concerned with?

Problem with fuel turning into gas is, it's not as usable then by the injectors. Sure your carbon canister tries to turn it back into fuel but that bypasses the injectors and sends it directly to the intake manifold. Aka causes a rich condition essentially.

Then on my old car I had evap issues, where the gas cap would never his when you open it. On the highway I would get 33mpg, but in the city it would get 17 because I wouldn't fill up as often while city driving.

But seriously sr20fan, your car sounds like it's in decent working order. Especially since your highway gas mileage is pretty good. Even though you said 350 miles to the tank which means absolutely nothing. But on a W10 you should expect about 27-29mpg on a fatty p11.

Yea, the car runs perfect, I was just concerned about my gas mileage and wanted input about whats up with it before I go and spend more money on things that don't need to be changed out. But yea, thanks for all the information though guys.

I spoke to a friend of mine just about a hour ago,I forgot that he completed a fully built ka24de+t on a s13 about 3 weeks ago. He has 740cc injectors @4bar and a gt35 turbo boosting 18psi daily. He told me he literally gets only 12-16mpgs in the city driving completely normal and 23-27 on the highway. He did tell me about the evap thing and how he capped his off. And when he did, he noticed a drop in mpgs in the city, like what you said Vadim.

I'm just going to live off that and put it to my head that nothing is wrong with my car and I need to stop tripping about it. I just don't want to have an issue that I would think there is something wrong with my car.

How did he cap his off? I put a bolt in mine, something that didn't let air through or out, and it was fine for a few weeks. Then when driving for 12 hours in hot weather I heard my fuel pump being very loud. I uncapped it as soon as possible.

Problem is the valve that was there before is ECU controlled. Having an older ECU means it doesn't know what to do with it .

I didn't ask how. I'm pretty sure he put a bolt cause I remember seeing a lot of 240s with bolts and such at some vacuum hoses and hard lines when they removed their emissions. And you know what, I just looked at my motor and you know how there is that evap valve thingy that says 1psi and looks like a tire valve stem with the green cap off, and then a vacuum hose that goes to a valve that has like 2 ports where vacuum hoses are suppose to go to the intake manifold,solenoid and such then on the bottom of it goes to the metal line? That black thing with 2 vacuum ports were capped with vacuum caps, and there is a vacuum hose that goes to the hard line next to the 2 other hard lines for fuel feed and return.

When I did the swap I had 2 jobs and never had time to finish it. All i had left to do was connect everything, but before I could finish everything my second job changed my schedule which didn't give me any days off which sucked because my p11 was my daily driver and I had to fetch rides to work for 3 weeks. .But another friend who did and knows a lot about fwd swaps helped me finish it. I didn't bother to look at everything cause the motor started up and was running great.Even when I did tune ups and suck. I'm using a b14 ecu that was tuned by JWT and reading a comparison chart from g20.net, b14 ecus don't have evap controlled by the ecu not like the 99 g20 does. Then I called my friend about and he told me the reason why he capped it is because of the ecu not controlling the solenoid and he also explained to me that "yea, having your evap working will save just a little mpgs, but your turbo now and will kill mpgs regardless."

Vadim your point about the injectors doesn't make sense to me. The fuel pump only sucks liquid from the bottom of the fuel tank by design. It shouldn't be sucking vapor at any time.

Capping the charcoal can isn't a good idea. The pressure or lack there of can cause issues with the fuel pump under heavy fuel consumption and makes seals leak prematurely. Vadim you might find a pneumatic charcoal can so the ECU doesn't need to be involved.

I know there is a problem with quite a few Nissans and the charcoal cans (not sure about the P series) There is a hose that goes to the can in some cars that when the car is topped off, gets filled with fuel and kills the charcoal can. The canister can't get rid of that much fuel all at once. That was the case on my NX and might be what your problem was.

I know you know this but there are other factors that are making your highway miles hit 33 and city 17 (like idling at stop lights and driving style). It's not the charcoal can unless something was wrong with it.

The canister only sucks the vapor at idle or on deceleration, which isn't going to hurt anything. And it is only a very small amount of vapor.

sr20fan your friend might have some other issues too. Larger injectors might be a big factor, but it's still hard to say without going over everything.

How loud did it get? Cause I know walbro 255 fuel pumps are loud when turning the key to on position, but I don't really hear it when driving and my exhaust is pretty quiet considering that I'm using crush bent 2.5 piping from downpipe to muffler. I just ordered a 3in downpipe and bout to order a mandrel bent catback from mandrelbentexhaustsystems.com

I don't think he would have issues and he doesn't complain about it at all. He is an OG with 240s. He had his 91 240sx since the first day it came out brand new(Hes 36yrs old just in case people were wondering how did he buy a 91 240sx brand new) and he was the first person in Hawaii to have a shop turbo charge a KA back in 1993. He still is using the original motor that came with the car, he just changes setups all the time. Only like 1/3 of his work is done by his hands. The rest is done professionally at a local shop. Talk about ballin'

Here is that chart I was talking about from g20.net. It's the real proper way to go from 98-99 sentra and p11 to 94-97 b14 ecu. Also it shows that there is suppose to be an extra connectition for a second camshaft position sensor which explains on the link. How to correctly use a 94-97 B14 ECU in a 99 P11 - G20.net - Forums
You will see that the evap is no longer used.

This is from UNISA JECS:

Anyways, about the gas mileage, I swear this has been happening even when I was N/A. Not as bad, but the only mods I had on my N/A were bc cams and the 2.5 crush bent exhaust. When I first bought the car, I seriously got 400 to a tank city/hwy driving. Then after the mods, I got 380 to a tank. Then one day, I decided to up my timing a little and after that I've been having fluctuating idling issues, hella bad gas mileage(like 250miles after filling up 14 gallons of gas) after spending more than 300 on new OEM parts(except the IACV) and issues with getting the idle down pass 1k to adjust timing after disconnecting the tps. So my only target now is getting a IACV and see if that does anything at all, even if it helps getting my idle down to do the timing.

P.S. those problems occurred on my N/A motor and seems like the turbo motor carried it on but worse since its turbo. That's the bottom line of what my problem is. I will try getting another IACV though and report what happens. I did transfer my IACV to my turbo motor due to the JDM IACV had a crack on the outside of it. Also 3 people I know locally told me IACV can cause terrible gas mileage, but don't some people block it off and use that 8mm bolt on the throttle body to adjust idle???????