140
Erratic idle on an RV8
Mike Simmonds (Woodcote Green
1504) from Berkshire posted a message on the bulletin board seeking help with
an erratic idle on his RV8. Roger Parker (Green 4092) the well-known MGBV8 guru
posted a reply. (Jun 02)
Mike
Simmonds 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
replied 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 again 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 RV8NOTE130 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. |
