Can anyone help? I have a 1995 318 coupe. The car appears to have an
intermittent flat spot when driving, almost like a split second stall. I
can't pin it down to certain revs or speed as it happens randomly. I've had
BMW do a diagnostic check but nothing showed up. It's been OK for the past
few weeks but now its back. Every time I hit a flat spot the car gives out a
brief clunk or jolting sound (sorry thats the only way I can explain it) and
it's pissing me of now. Any help will be much appreciated.

TIA

UK Dave <anon (AT) anon (DOT) com> wrote

"UK Dave" <anon (AT) anon (DOT) com> wrote


It sounds like a faulty Throttle Position Sensor perhaps, or a bad fuel
pressure regulator.

The TPS tells the computer that your foot position has changed, and that
more gas, or less gas, is needed given the new position your foot has taken.
Slowing down is not liekly to result in the flat spot being noticed since we
are turning off the fuel flow, or reducing the fuel flow, to slow down. On
the other hand, speeding up sends a demand to the computer for more gas. The
TPS might not be sending the request properly, but your foot position is
adding more air while the fuel delivery is remaining constant. this can
cause a flat spot. The TPS should throw a code that the computer should pick
up and store. A quick trip through the diagnostic machine should tell you if
the TPS is acting up.

The fuel flow regulator can also cause a flat spot that will not be picked
up by a code. this little device is on the fuel rail and shunts extra fuel
that the fule pump provides back to the tank. Let's say the pump provides 60
psi, but the injectors only need 45 psi at idle, and only need the full 60
at full throttle. The regulator's job is to make sure the right amount of
fuel is available at the injectors all the time, and it varies what the
injectors can get because of changing vacuum. Under high vacuum (closed
throttle), the regulator sends most of the fuel back to the tank, under low
vacuum (open thottle) the regulator will close and keep most of the fuel at
the injectors. The regulator can cause a flat spot.