Bad mileage without cats?
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From: Gothenburg, Sweden
Bad mileage without cats?
I removed the cats off my car and put an 2,5" x-pipe in its place. It seems that the fuelconsumption has increased since then. I havn´t done any measurements regarding mileage but it actually feels like mileage gone down. Is this possible?
What is your opinion?
/Tobias, Sweden
What is your opinion?
/Tobias, Sweden
Burning Brakes
Joined: May 2001
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Tobias,
Firstly we will require to know what MY car you are driving otherwise we will be "trouble shooting" in the dark.
In the S4 cars, MY89 onwards at least you have to tell the LH ECU what system you are using, cats or no cats and also whether you have auto or manual gear box. This is done by the coding connector that is fitted to the mounting plate of the LH & EZK control units.
For example, the 8 pin coding connector has different arrangement for the 8 pins interconnections.
We also have to know whether it is a ROW car, a USA car or what country is has specifically been delivered to (in some instances) and whether it is manual or auto.
In addition later cars not fitted with cats are also not fitted with an oxygen sensor nor are they fitted with exhaust temperature safety cut out circuits. My 1990 S4 is not fitted with cat, oxygen sensor and the associated circuity, so some circuity may possibly need to be changed as well.
Looking at the fuel system electronic control, LH - Diagnosis/Troubleshooting chart, if you leave the oxygen sensor in the circuit, you can probably experience an oxygen regulation stop - rich fault which can give you the high fuel consumption.
Please also note that the control plug also has to let the LH ECU know what grade fuel you are using.
So depending on what MY car you have the control circuity can be very different.
Tails 1990 928S4 Auto
Firstly we will require to know what MY car you are driving otherwise we will be "trouble shooting" in the dark.
In the S4 cars, MY89 onwards at least you have to tell the LH ECU what system you are using, cats or no cats and also whether you have auto or manual gear box. This is done by the coding connector that is fitted to the mounting plate of the LH & EZK control units.
For example, the 8 pin coding connector has different arrangement for the 8 pins interconnections.
We also have to know whether it is a ROW car, a USA car or what country is has specifically been delivered to (in some instances) and whether it is manual or auto.
In addition later cars not fitted with cats are also not fitted with an oxygen sensor nor are they fitted with exhaust temperature safety cut out circuits. My 1990 S4 is not fitted with cat, oxygen sensor and the associated circuity, so some circuity may possibly need to be changed as well.
Looking at the fuel system electronic control, LH - Diagnosis/Troubleshooting chart, if you leave the oxygen sensor in the circuit, you can probably experience an oxygen regulation stop - rich fault which can give you the high fuel consumption.
Please also note that the control plug also has to let the LH ECU know what grade fuel you are using.
So depending on what MY car you have the control circuity can be very different.
Tails 1990 928S4 Auto
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Tails
everything you said is valid apart from one thing. If you remove the cats but leave the O2 sensor and everything else connected and working properly you should see no difference (apart from worse emissions) . The O2 sensor senses before the cats.
My guess is the extra responsiveness due to the lack of cats and the X pipe (also the nicer sound) encourages more use of the right foot thus worsening fuel consumption.
everything you said is valid apart from one thing. If you remove the cats but leave the O2 sensor and everything else connected and working properly you should see no difference (apart from worse emissions) . The O2 sensor senses before the cats.
My guess is the extra responsiveness due to the lack of cats and the X pipe (also the nicer sound) encourages more use of the right foot thus worsening fuel consumption.
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From: Lillington, NC
I had no drop in fuel mileage with an x-pipe (no cats) on my '87 S4. I also ran Autothority chips.
When cruising on the highway, I felt like I barely had any pressure on the gas peddle to maintain 70 mph.
When cruising on the highway, I felt like I barely had any pressure on the gas peddle to maintain 70 mph.
Last edited by cold_beer839; Oct 30, 2008 at 03:38 PM.
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Drifting
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From: Davidsonville, MD
I removed the cats off my car and put an 2,5" x-pipe in its place. It seems that the fuelconsumption has increased since then. I havn´t done any measurements regarding mileage but it actually feels like mileage gone down. Is this possible?
What is your opinion?
/Tobias, Sweden
What is your opinion?
/Tobias, Sweden
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More horses = more oats. 
Tobias has a somewhat rare, ROW '86 32V.
I get a consistent 14 MPG (16.8 L/100KM) around town with my mods, and heavy foot. I have managed 17, but it drove me crazy to drive so slow.
I haven't gone on any trips far enough to get a good highway reading, but my L/100KM gauge shows <10 L/100KM (~23 MPG) at 70+MPH on flat ground.
Tobias has a somewhat rare, ROW '86 32V.
I get a consistent 14 MPG (16.8 L/100KM) around town with my mods, and heavy foot. I have managed 17, but it drove me crazy to drive so slow.
I haven't gone on any trips far enough to get a good highway reading, but my L/100KM gauge shows <10 L/100KM (~23 MPG) at 70+MPH on flat ground.
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From: Gone. On the Open Road
When you removed the cats, did you also remove the air pump? If so, did you by accident disconnect the vacuum line to the tank vent diverter valve (small black plastic) near the air pump diverter? I have seen a case where simply disconnecting the vacuum from the tank vent diverter caused fuel consumption to increase by 50%.
Former Vendor
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From: Danbury,CT
I have an 84S 5spd and after removing the cats and making a custom dual exhaust with 2.5" pipes and magnaflow mufflers with a crossover tube and o2 sensor in the crossover tube,I am getting between 23-26 MPG higway averaging 75mph! Before with stock single exhaust and cat con I was averaging 17-20 MPG!!I could not believe the difference in power also.I love my car!!!
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