brianrheffron

After extensive recommissioning work my 1989 digital dash S4 is back on the road after being laid up for 11 years.
Prior to being laid up when I lifted off at speed the instantaneous mpg figure went straight to 99.9 which I think was
what should happen and this was in general regarded as a good thing. Now when I lift off at speed the figure changes a
lot more slowly and doesn’t go above around 75 mpg. Apart from that the motor is running well, seems to be as
powerful as I remember and the idle is very steady.
Can anyone shed any light on this behaviour.

merchauser

possible throttle position switch?

dr bob

^^^^^ What Paul said. The LH box stops fuel flow when engine RPM is above something like 1200 .and. the throttle position switch detects that the throttle is closed. In that state, the digital display sees the speed of the car via the speedometer sender plus the zero fuel flow, and indicates infinite MPG. The display doesn't go that high, so we get the max it can show.

You can test for the throttle switch function with your DMM, from the LH controller plug at the controller in the passenger footwell. Remove the carpeted cover over the controllers against the right side there, and remove the large connector from the right side (against the car) controller. Use your DMM to measure continuity, and test between pin 3 in the connector and chassis ground. With the switch working correctly, you should see continuity with throttle closed, changing to open (infinite) as soon as the throttle is no longer closed. If you fail to see continuity with the throttle closed, the most common cause is a loose connector on the switch itself, followed by corrosion and sometimes oil in the switch itself.

Battery disconnected while testing, of course.

FredR

When you roll off the system detects the throttle closing and shuts off fuel completely until such time as the throttle position stabilises or the idle contact switch is made. In essence the computer is trying to determine an indeterminate number so it defaults to the 99.9 mpg number as the max it can show but it is nonetheless meaningless in MPG terms.

If your throttle cable is snagging a little as it closes in to the fully closed position that might just explain what you see as a low level of constant air flow is detected thus giving a number that can be calculated - that is something of an intuitive guess.

bwarren97

I had a similar thing with my '83 L-jet until several months ago. The switch itself had gone loose and was positioned in a spot where it wasn't being triggered at all. Of course, once I corrected it, the idle started bouncing up and down because it was trying to idle around 1900 so I corrected that, too.
I suspect a PO had made these changes to correct a stumbling idle or low alternator output, both of which I had mostly corrected beforehand. I didn't like high idle, however.