Poor Fuel Economy Question.

boyleia1 · 04-24-2008, 01:19 PM

Hey I have a 1997 OBS that i bought 9 months ago with 180K miles on it. when i first got the car i replaced the plugs, wires, pcv valve, fuel filter, air filter, and used "sea foam" to clean injectors. 9 months ago i was getting 22-26 mpg mixed city and hwy driving. Now, with no noticeable change in conditions i am lucky to get 18mpg and thats when i drive a lot of highway. only city i am in the 16 mpg range. The car has 192K on it now and 3 weeks ago i replaced the knock sensor (it was bad) I thought the knock sensor was the cause of the poor economy because it had been bad for about 6 weeks. After replacing it economy is still bad. ANY one have any ideas or suggestions as to what could be causing this problem. Thanks!!.

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jey · 04-24-2008, 01:41 PM

Do you have a CEL?
When was the last time the O2 sensors were replaced? An old/failing ignition coil pack could cause problems too.

rkramer · 04-24-2008, 03:55 PM

Reset the ECU after the knock sensor? How about compression test? Could also be a plugged catalytic converter.

boyleia1 · 04-24-2008, 05:06 PM

I am not sure i know what a CEL is. O2 and Ignition have not been tested sense i have owned the car. Is there any way i can test those myself?

boyleia1 · 04-24-2008, 05:10 PM

How do i go about resetting the ECU because i did not do that after i replaced the Knock sensor. my check engine light was on before and turned off after i replaced it so i thought it was good to go. I will be doing a compression test but have not yet. and i checked the catalytic converters the first one (closest to the engine) appeared to be fine. and the second one was not there. it looked like previous owner took out all the honey comb looking stuff.

boyleia1 · 04-29-2008, 02:40 PM

I Tested the compression today.
Cylinder 1 = 200psi
Cylinder 2 = 195psi
cylinder 3 = 205psi
cylinder 4 = 200psi

it seemed high to me but i am not actually sure what the compression is supposed to be. Anyone know what my compression is supposed to be and if 195-205 is too high and what might cause it.

bulwnkl · 04-29-2008, 09:31 PM

I don't know what your compression readings are supposed to be, but all 4 cylinders are within 5% in your measurements, which is very good. How did you do the test? Do you have the FSM? It'll specify the procedure, which used to be something like:

Let engine idle until radiator fans cycle on and off twice,
Remove all plugs and disconnect fuel harness,
Block throttle plate wide open,
check compression.

Don't know whether it still is. If you don't do it according to the manual, and you may not get the same numbers as you'd get if you do follow the procedure.

03 bug eye · 04-29-2008, 11:01 PM

well does the car smell like fuel at all, could have a leak in one cylinder and that prob wouldn't come up with a code, i had that problem cause one of my o-rings went out. as far as your check engine light going off, if the sensor goes through 3 trips without failing the light will go off, but the code will still be stored in the ECU until it has gone 40 trips without failing. check your fuel pressure and make sur there are no leaks, good luck

The_Prodigy · 04-29-2008, 11:24 PM

Fuel leaks from injector seal, filler neck, fuel lines and etc. are possible. One thing that alot people have missed on figuring the loss of MPG is "tire pressures" Check them if you have not already.

04-24-2008, 01:19 PM	#1
boyleia1 Scooby Newbie Member#: 141519 Join Date: Feb 2007 Location: Michigan Vehicle: 1997 OBS Colbolt Firemist Blue	Poor Fuel Economy Question. Hey I have a 1997 OBS that i bought 9 months ago with 180K miles on it. when i first got the car i replaced the plugs, wires, pcv valve, fuel filter, air filter, and used "sea foam" to clean injectors. 9 months ago i was getting 22-26 mpg mixed city and hwy driving. Now, with no noticeable change in conditions i am lucky to get 18mpg and thats when i drive a lot of highway. only city i am in the 16 mpg range. The car has 192K on it now and 3 weeks ago i replaced the knock sensor (it was bad) I thought the knock sensor was the cause of the poor economy because it had been bad for about 6 weeks. After replacing it economy is still bad. ANY one have any ideas or suggestions as to what could be causing this problem. Thanks!!. * Registered users of the site do not see these ads.

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04-24-2008, 01:41 PM	#2
jey Scooby Specialist Member#: 32503 Join Date: Feb 2003 Chapter/Region: TXIC Location: Johnson City, TX Vehicle: 2011 Tribeca 1995 Legacy Wagon	Do you have a CEL? When was the last time the O2 sensors were replaced? An old/failing ignition coil pack could cause problems too.

04-24-2008, 03:55 PM	#3
rkramer Scooby Guru Member#: 118209 Join Date: Jun 2006 Chapter/Region: MWSOC Location: North Dakota Vehicle: 2002 WRX Sedan	Reset the ECU after the knock sensor? How about compression test? Could also be a plugged catalytic converter.

04-24-2008, 05:06 PM	#4
boyleia1 Scooby Newbie Member#: 141519 Join Date: Feb 2007 Location: Michigan Vehicle: 1997 OBS Colbolt Firemist Blue	I am not sure i know what a CEL is. O2 and Ignition have not been tested sense i have owned the car. Is there any way i can test those myself?

04-24-2008, 05:10 PM	#5
boyleia1 Scooby Newbie Member#: 141519 Join Date: Feb 2007 Location: Michigan Vehicle: 1997 OBS Colbolt Firemist Blue	How do i go about resetting the ECU because i did not do that after i replaced the Knock sensor. my check engine light was on before and turned off after i replaced it so i thought it was good to go. I will be doing a compression test but have not yet. and i checked the catalytic converters the first one (closest to the engine) appeared to be fine. and the second one was not there. it looked like previous owner took out all the honey comb looking stuff.

04-29-2008, 02:40 PM	#6
boyleia1 Scooby Newbie Member#: 141519 Join Date: Feb 2007 Location: Michigan Vehicle: 1997 OBS Colbolt Firemist Blue	I Tested the compression today. Cylinder 1 = 200psi Cylinder 2 = 195psi cylinder 3 = 205psi cylinder 4 = 200psi it seemed high to me but i am not actually sure what the compression is supposed to be. Anyone know what my compression is supposed to be and if 195-205 is too high and what might cause it.

04-29-2008, 09:31 PM	#7
bulwnkl Scooby Specialist Member#: 41070 Join Date: Aug 2003 Location: Arizona Vehicle: 2005 Black Tiger (Black Turbo Baja)	I don't know what your compression readings are supposed to be, but all 4 cylinders are within 5% in your measurements, which is very good. How did you do the test? Do you have the FSM? It'll specify the procedure, which used to be something like: Let engine idle until radiator fans cycle on and off twice, Remove all plugs and disconnect fuel harness, Block throttle plate wide open, check compression. Don't know whether it still is. If you don't do it according to the manual, and you may not get the same numbers as you'd get if you do follow the procedure.

04-29-2008, 11:01 PM	#8
03 bug eye Scooby Newbie Member#: 140571 Join Date: Feb 2007 Location: denver CO Vehicle: 2003 wrx aspen white	well does the car smell like fuel at all, could have a leak in one cylinder and that prob wouldn't come up with a code, i had that problem cause one of my o-rings went out. as far as your check engine light going off, if the sensor goes through 3 trips without failing the light will go off, but the code will still be stored in the ECU until it has gone 40 trips without failing. check your fuel pressure and make sur there are no leaks, good luck

04-29-2008, 11:24 PM	#9
The_Prodigy Scooby Newbie Member#: 116467 Join Date: Jun 2006 Chapter/Region: NESIC Location: N.Andover, MA Vehicle: 2004 STi WRB	Fuel leaks from injector seal, filler neck, fuel lines and etc. are possible. One thing that alot people have missed on figuring the loss of MPG is "tire pressures" Check them if you have not already.