928 high fuel usage
08-31-2014, 04:43 AM
#1
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928 high fuel usage
Hi my 928 is running strange . High fuel usage very low tick over when up to temp and slight popping back fire in exhaust on over run in exhaust I.E. when foot of gas peddle coasting along.
08-31-2014, 04:59 AM
#2
Rennlist Member
What year is it?
If it has K-jet injection, your pressures are probably off and need to be adjusted using the correct CIS gauges or it has a vacuum leak.
As the only things controlling fuel consumption are the fuel control pressures and vacuum in the mechanical fuel injected 928's.
If it is L-jet or LH-jet (L-jet is unlikely in your location as it was North America only) it is the AFM (L-Jet 1980-1984 NA)
or MAF (LH-jet 1984+ such as the S2, S4 etc) (check resistances using the WSM (work shop manuals)),
temp II sensor (check resistances using WSM),
fuel pressure dampers and or regulators (remove vacuum lines and smell openings for fuel vapors),
or it could be a vacuum leak, although unlikely as that would cause a lean situation and not affect your fuel consumption as much as the others because your 928 probably doesn't have a O2 sensor, with it most likely being a ROW model.
So it can't adjust fuel for the correct stoichiometric mix of 14:1 air fuel ratio and the CO has to be adjusted manually on the MAF using an exhaust gas analyzer to check for the correct fuel mixture.
I hope this helps.
If it has K-jet injection, your pressures are probably off and need to be adjusted using the correct CIS gauges or it has a vacuum leak.
As the only things controlling fuel consumption are the fuel control pressures and vacuum in the mechanical fuel injected 928's.
If it is L-jet or LH-jet (L-jet is unlikely in your location as it was North America only) it is the AFM (L-Jet 1980-1984 NA)
or MAF (LH-jet 1984+ such as the S2, S4 etc) (check resistances using the WSM (work shop manuals)),
temp II sensor (check resistances using WSM),
fuel pressure dampers and or regulators (remove vacuum lines and smell openings for fuel vapors),
or it could be a vacuum leak, although unlikely as that would cause a lean situation and not affect your fuel consumption as much as the others because your 928 probably doesn't have a O2 sensor, with it most likely being a ROW model.
So it can't adjust fuel for the correct stoichiometric mix of 14:1 air fuel ratio and the CO has to be adjusted manually on the MAF using an exhaust gas analyzer to check for the correct fuel mixture.
I hope this helps.
08-31-2014, 11:31 AM
#3
Rennlist Member
It's either an '87 or 88 S4, he has one of each according to previous posts.
Potdog, assuming stock chips the fuel should be getting cut off on decel by the LH, over 1300 rpm. This apparently isn't happening, hence the popping, which means the idle switch isn't working. That may also explain the low idle-- the idle speed is controlled by the LH and the ISV, which is also triggered by the idle switch.
The idle switch lives on the left side of the throttle-body, under the intake. You can hear it actuate, a tiny "click" just as the throttle returns to the idle-stop. And you can check it with a multimeter, disconnect the 35-pin connectors from the EZK and LH (best to do this with the battery disconnected) and check the resistance between pins 2 and 5 on the LH 35-pin harness connector. With the throttle closed it should show a closed contact (< 10 ohms), and when the throttle is opened slightly it should show an open circuit.
This is described on page D24-11 of the WSM along with a helpful picture of the connector:
Potdog, assuming stock chips the fuel should be getting cut off on decel by the LH, over 1300 rpm. This apparently isn't happening, hence the popping, which means the idle switch isn't working. That may also explain the low idle-- the idle speed is controlled by the LH and the ISV, which is also triggered by the idle switch.
The idle switch lives on the left side of the throttle-body, under the intake. You can hear it actuate, a tiny "click" just as the throttle returns to the idle-stop. And you can check it with a multimeter, disconnect the 35-pin connectors from the EZK and LH (best to do this with the battery disconnected) and check the resistance between pins 2 and 5 on the LH 35-pin harness connector. With the throttle closed it should show a closed contact (< 10 ohms), and when the throttle is opened slightly it should show an open circuit.
This is described on page D24-11 of the WSM along with a helpful picture of the connector:
