bad gas mileage before and after tune-up

[kenversusryu]

so ever since i installed a ram-air setup on my 95 240sx i went from 27mpg city/highway to 21.

I removed it and changed my spark plugs, fuel filter, o2 sensor, coolant change/flush, oil change, injector cleaner and I'm still getting bad gas mileage.

I guess i'll replace the distributor cap, rotor and maybe the coolant temperature sensor but what else could be causing this really bad gas mileage? should i check timing? The car runs good and no noticable problems at all. I ran a self diagnostic as well and came up 55 (no problems)

I am desperate for help. What else do i have to replace or check??

[BigVinnie]

Could be anything. I would check the TPS and make shure that it hasn't moved from it's stock settings. If timing is advanced or retarded to far it could also decrease gas mileage since the cams aren't properlly alighned.

[Blues13]

check your tire pressure and also your fuel lines. There is also a rubber hose for the fuel line by the gas tank in the back.. make sure it didnt develope a small leak.

[kenversusryu]

tire pressure is checked regularly and i doubt theres a leak in the fuel line. otherwise i would've smelled gas eminating from my garage everytime i worked on my car.

how does a throttle position sensor move from its setting? What's the normal setting?

Thanks for the suggestions....

[Nizmo240_22]

If timing is advanced or retarded to far it could also decrease gas mileage since the cams aren't properlly alighned.
True this could be the case, but ignition timing is independent to cam timing.

[Nizmo240_22]

how does a throttle position sensor move from its setting? What's the normal setting?

.45-.55 volts at closed throttle IIRC.

[mrmephistopheles]

I've found that the biggest enemy of fuel economy is a spirited right foot.
Double check everything. If that fails, try your stock intake again.

[krustindumm]

ry your stock intake again.
werd

got pictures of this ram air setup, maybe it is messing up the MAFs signal.

[kenversusryu]

i removed the ram air setup. if the maf signal is bad.. wouldn't it affect drivability? at idle ?

[krustindumm]

might only effect it when your moving, only way to tell is a DSO and maybe a wideband, or a Consult or similar graphing scan tool.

[kenversusryu]

okay here's an update.

Checked timing.. it was perfect at 20degrees

then i checked voltage on maf. Here are the results.

.5 volts at ignition on which is normal.
1.27 volts at idle which is low but still normal
2.5 volts at 4k rpm which is NOT normal... should be 4 volts.

could a weak maf signal at 4k be the cause of all the bad mileage? That would make sense since the ram air could indeed have messed up the maf somehow. The car itself runs just fine.. no bogging.. no rough idle or anything.. but the 2.5 volts at 4k is the weird part. Any suggestions or advice?

[krustindumm]

Isn't it a 5V reference signal? In that case 4V at 4krpm unloaded would be way too high. I think you should be seeing 2.1-2.2 at 2500rpm.

I always use the consult-II and watch g/sec though.

[kenversusryu]

it says on the fsm that it should be 4volts at 4k rpm. I don't have a consult.