140
Erratic idle on an RV8
Mike Simmonds (Woodcote Green 1504) from Berkshire posted a
message on the bulletin board at www.v8register.net seeking
help with an erratic idle on his RV8. Roger Parker (Green 4092)
the well-known MGBV8 guru posted a reply. (Jun 02)
Mike's
message said "I have an RV8 with an annoying problem. With
the engine warm, tick over is fine at 800rpm, if a little high
I feel. But when slowing down after reasonably fast driving,
the tick over is very reluctant to settle down, slowly passing
1,500rpm to settle back at 800rpm after 10 seconds or so. Effectively
this gives the engine no braking at all. I have checked as far
as I can the speed sensor in the speedometer cable, the advance/retard
vacuum and the throttle stepper motor but all appears to be
fine. I intend changing the stepper motor to try and rectify
the fault, but at £70 I would like to be sure before I
do this. Any clues anybody? Any help would be greatly appreciated".
Roger
Parker's reply posted on the bulletin board - "The Lucas
Hot Wire fuel injection system is in today's terms crude. It
is only a fuel control system with engine speed input arriving
through the ignition pulse which is exactly the same way as
the tachometer works. The crudity does offer some advantages
in terms of the scope for the able owner to rectify some faults.
The Hot Wire system first appeared fitted on the 2 litre 02
series engine fitted to the MG Montego in 1984. It was later
fitted to the 1.6 litre series engine of the Rover 216 EFi and
the 2 litre M16 engine of the Rover 820 and the early 220 and
420 Rovers.
One
issue that was a common fault was failure to achieve correct
"idle speed control" through a combination of error
conditions. The most common fault condition is where the initial
start sequence involves turning the ignition switch straight
through from off to cranking. These systems need a period of
approximately 2 seconds to compile a full system status check
and assimilate values for the main engine functions. This issue
was addressed with the 1994 Lucas GEMS8 engine management systems
where a built in delay was fitted so that even if you did switch
straight through from off to cranking the system would see a
2 second delay before allowing the starter to crank the engine.
With
the four and eight cylinder Hot Wire equipped cars it is quite
possible to create a fault condition whereby the system is then
operating with an incorrect value which then impacts directly
on the "idle speed control".
The
most influential value for the idle speed control is the coolant
temperature and this is also a sensor which has a record of
being less than 100% reliable. Remember that on the V8 there
is a completely separate coolant temperature sensor for the
temperature gauge so you do not know if you are seeing the correct
values from the coolant sensor for the injection system. Coolant
sensor resistances vary depending on the temperature applied
to the sensor, but as a guide at 20oC the sensor should have
a resistance of approximately 2,500 ohms, and |
at
80oC approximately 350 ohms. This gives some reference to check
the sensor as any higher figures than 1,000 ohms when the engine
is genuinely at working temperature means a faulty reading that
will imitate a partially warmed engine which sees extra fuelling
and indexing of the stepper motor to increase idle speed.
The
next most common problem is one of poor connection. Bear in
mind that all the values that are seen by the ECU require changing
resistances. By definition, bad connections create their own
resistances which in individual cases are an addition to the
sensor resistances and result in incorrect voltages seen at
the ECU.
Slow decline from high idle speeds is also a characteristic
of these systems. The original Lucas information indicated that
if the system took up to between 10 and 15 seconds for the idle
speed to settle after the vehicle comes to rest, then this was
acceptable. In reality 5 seconds is a far more realistic figure.
The road speed sensor is a gain a common part with some of those
Rovers I mentioned. It generates four pulses per cable revolution
and is actually effective from approximately 5 mph. Below this
speed and the cable speed is too slow. This sensor is present
only to ensure that idle speed control is disengaged when the
vehicle is moving, otherwise having the engine speed controlled
outside the driver input when moving at slow speed in a traffic
queue would (does) often present difficulties.
Lastly
I would mention the lambda sensors (oxygen sensors) as if one
or both have developed Faults then the resulting signal to the
ECU will cause a range of problems with poor running. It is
possible to tap into the signal wires from these and read voltage
fluctuations which are fluctuating quickly once the sensor is
hot. Slow voltage changes when cold are normal, with marked
increase in the rate of fluctuation occurring after a few seconds.
However if the changes are less than an average of one per second
with a fully warmed up engine, then the sensor may be faulty.
There
are other conditions that cause running problems, but these
are often affecting maximum power or simply do not allow the
engine to run. This does not mean that there are not other factors
that cannot influence the running of the engine, and for this
reason and MG Rover dealer or a Land Rover dealer or a specialist
with Testbook or the newer T4 has to be the simplest route to
see a complete diagnostic check of the Hot Wire system.
Editor:
it is worth reminding readers that there are a number of RV8
Workshop Notes on the ECU and other diagnostic matters which
should be read in conjunction with this very useful note. Underneath
that classic car exterior, the RV8 is a very modern and sophisticated
car in the engine and electronics department. See RV8 Note 130
on hunting down a misfire where Peter James found that a well
equipped specialist with the diagnostic equipment was able to
identify several problems and solve them rapidly. |
