I have a S5 turbo 2 engine in a 91 vert and I have some issues with it and trying to get better mpg. The engine mods are a blow off valve (I think it's a HKS knockoff), emissions stuff has been removed and I think it might have some type of fuel pump but not sure. When I got the car it was running extremely rich (burned your eyes and you could smell it on your clothes afterwards) and got around 8-10 mpg. The check engine light was on so I read the codes and it gave me 12,13,18,25,30,31,32,33,34,38,and 42? 12 and 18 are the tps, 13 is the Pressure sensor, 25 is the solenoid pressure regulator control, 30-33 are emssions valves, 34 is the BAC, and 38 is the AWS. So I adjsuted the Tps, noticed that the pressure sensor was a N350 and changed it out with a N370. After that the check engine light still came on. i took a look at the pressure senor again and noticed that it had some black box connected into it through the wiring harness. (Like a ebay special $20 performance chip or something) So i removed it and it tunred the check light off and that gave me better gas mileage (12-14, this is light driving with no boost). The car runs decently now, but it has a stumble with light throttle and low load. I have to push the throttle further down almost into boost to get it to go away. The car idles good when warm but when cold or when the clutch pedal is depressed, or when a load is applied (lights, e-fan etc.) the car aquires a hunting/bouncy idle. I'm thinking this is from the removal of emissions? When I do go into boost the car builds it quickly and goes to 6psi then drops to 5 as the wastegate opens. I would just like to fix the stumble and get better mpg (shouldn't is be more like 16?) I would appreciate any help, and if not thanks for taking the time to read my novel

 

The little box attached to the pressure sensor is a fuel cut defensor leave it on, if your running stick boost u can remove. but if u have an aftermarket exhaust u should leave it on...

 

as for the stumbling look for vaccum leaks, my s4 t2 had a couple leaks that would cause my car to stumble and i had to actually step on it for it to move!!! lol

 

I would check the coolant temp sensor. Also the wire for the o2 sensor gets hard and brittle. I've seen the wire short to the ground shield internally and this could cause the ecu to add too much fuel.

 

Well, the check engine light came back on yesterday displaying all the same codes...

 

sounds like a bad ecu to me if its displaying everything you already fixed

 

Since the emissions were deleted those codes will always show up. If it's showing codes to items that you believe you properly fixed then disconnect the battery for a few minutes or so and then reconnect the battery to reset the ECU as an S5 stores codes.

 

Yeah i kinda figured those emissions codes would keep showing up, but the tps and map sensor codes showed back up too... I disconnected the battery to reset the ecu and they haven't showed back up yet, haven't driven the car a whole lot yet though. I'll let ya know what happens. Could a vacuum leak cause this? and would the car still be able to hold 5psi? Also is the coolant temp sensor that the ecu reads diferent than the one that is displayed in the gauge cluster? because my gauge works correctly

 

My car is doing the same exact thing i have the emmisions(air pump) I dont have a check engine light though. I am working on replacing all the vaccum hoses but i went to mazdatrix and looked and they said i need 13 feet of 3.5mm and 6 feet of 6mm. Do i need anymore? My car is stumbling and the idle is at like 1100 and when i press the brake it lowers then raises and when i let off it settles.. I checked the grounds, the tps, airflow sensor, I even tried adjusting the idle per the FSM but the BAC valve does nothing. I checked the resistance at the BAC and it showed 13 but it is supposed to be 11.5k ohms if i remember correctly. If i removed the BAC valve would that fix the problem? Please help us. I did the idle adjustment including the bridge of the initial coupler and the Variable Resistor. The variable resister's resistance checked out right and it adjusts the idle some. But the BAC does nothing. Please help.