Have you determined that the stumble or hesitation is more related to
throttle position than engine RPM? If you get hesitation when passing
through one certain throttle position, that suggests TPS problem. If
you only get the hesitation above 3,000 RPM, that suggests a coil
problem. Since you have a Marelli car, a coil problem of course would
only affect one bank, so it would not be as pronounced as with a Lucas
CEI car with a single coil.
You do NOT need to remove the TPS to test it. There is a 3-wire
connector beneath/behind the throttle capstan. You can backprobe that
connector by inserting the probe of your voltmeter into the socket,
with the connector still connecting the TPS to the harness. On mine,
the three wires were Red, Green & Yellow. The Green wire is a constant
5V input to the TPS and the Yellow wire was a constant .02V on mine,
(voltage to ground).
All tests are done with the key in the run position, engine OFF. The
Red wire voltage (red probe of your voltmeter inserted into red wire
connector and black probe to ground) should vary between .32V at idle
to 4.5V to 5.0V at WOT (Wide Open Throttle). With the probes connected
as described, slowly and smoothly turn the throttle capstan by hand
from the idle position to the WOT position. The voltmeter reading
should smoothly increase, without any spikes or gaps in the reading.
A common failure of the OEM TPS will result in a spike or gap in the
reading when the throttle is turned to ~ 20% open, or the position that
the throttle is most frequently in during cruising.
A vacuum leak almost always results in an elevated idle speed, so I
don’t think that is your problem. The kickdown switch ONLY operates at
the end of throttle pedal travel, when you push the pedal that last
~1/4" against the spring incorporated in the cable next to the switch,
just outboard of the throttle capstan. I assume you are describing a
hesitation that occurs much earlier in the throttle travel, so the
switch has no bearing on your problem. Even if the switch were
defective or improperly adjusted, it would NOT affect anything but
transmission operation, having NO effect upon how the engine runs.
Let us know what you find.
George Balthrop, Clifton, VA USA
85 & 89 XJ-S Coupes; 89 XJ40 VDP-----Original Message-----
History and symptoms:
- Daily driver…no significant previous problems.
- Gave her gas when getting on the freeway and she choked and
bogged down…let up and she smoothed out. Eased the accelerator
down and she climbed up to 70. Drove for another 100 miles but
each time I even slightly pushed the accelerator down she bogged
- Got her home, posted the problem here, next day car was fine???
- Started to return to work the next day…was hard to start (had
to give her full throttle to start). She started but did the whole
bog down thing again when I began driving.
- Starts okay when warm but won’t runs okay as long as I don’t
give her any gas… bogs down with the slightest acceleration
now…especially if I get a hill.
I guess I have two questions -
(1) What is the best way to check out the TPS? The books a little
confusing…I have a digital multimeter.
(2) Before I RR the TPS could this be a simple vacuum leak
somewhere or something with the kick-down switch from the tranny?
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