bad gas mileage before and after tune-up - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

kenversusryu · 03-30-2006, 06:26 PM

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 · 03-30-2006, 07:16 PM

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 · 03-30-2006, 10:41 PM

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 · 03-31-2006, 01:38 AM

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 · 03-31-2006, 05:12 AM

Quote:

Originally Posted by BigVinnie

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 · 03-31-2006, 05:13 AM

Quote:

Originally Posted by kenversusryu

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

.45-.55 volts at closed throttle IIRC.

**mrmephistopheles** · 03-31-2006, 05:39 AM

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 · 03-31-2006, 10:17 AM

Quote:

Originally Posted by mrmephistopheles

ry your stock intake again.

werd

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

kenversusryu · 03-31-2006, 05:35 PM

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

krustindumm · 03-31-2006, 06:10 PM

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 · 04-03-2006, 09:55 PM

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 · 04-03-2006, 11:12 PM

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 · 04-04-2006, 03:53 AM

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

03-30-2006, 06:26 PM	#1
kenversusryu Zilvia Member Join Date: Sep 2005 Location: cali Posts: 259 Trader Rating: (0) Feedback Score: 0 reviews	bad gas mileage before and after tune-up 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??

03-31-2006, 06:10 PM	#10
krustindumm Zilvia Junkie Join Date: Dec 2004 Location: Iowa Age: 40 Posts: 473 Trader Rating: (0) Feedback Score: 0 reviews	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. __________________ ASE Master Technician

04-03-2006, 11:12 PM	#12
krustindumm Zilvia Junkie Join Date: Dec 2004 Location: Iowa Age: 40 Posts: 473 Trader Rating: (0) Feedback Score: 0 reviews	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. __________________ ASE Master Technician

04-04-2006, 03:53 AM	#13
kenversusryu Zilvia Member Join Date: Sep 2005 Location: cali Posts: 259 Trader Rating: (0) Feedback Score: 0 reviews	it says on the fsm that it should be 4volts at 4k rpm. I don't have a consult. Last edited by kenversusryu; 04-04-2006 at 07:27 AM..

03-30-2006, 07:16 PM	#2
BigVinnie Zilvia FREAK! Join Date: Apr 2003 Location: Walnut Crizzle, Crackifornia Age: 46 Posts: 1,266 Trader Rating: (0) Rides Gallery: BigVinnie Feedback Score: 0 reviews	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.

03-30-2006, 10:41 PM	#3
Blues13 Zilvia Junkie Join Date: Jul 2003 Location: OC Posts: 435 Trader Rating: (2) Feedback Score: 2 reviews	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.

03-31-2006, 01:38 AM	#4
kenversusryu Zilvia Member Join Date: Sep 2005 Location: cali Posts: 259 Trader Rating: (0) Feedback Score: 0 reviews	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....

03-31-2006, 05:35 PM	#9
kenversusryu Zilvia Member Join Date: Sep 2005 Location: cali Posts: 259 Trader Rating: (0) Feedback Score: 0 reviews	i removed the ram air setup. if the maf signal is bad.. wouldn't it affect drivability? at idle ?

04-03-2006, 09:55 PM	#11
kenversusryu Zilvia Member Join Date: Sep 2005 Location: cali Posts: 259 Trader Rating: (0) Feedback Score: 0 reviews	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?