REPAIRS PAGE 15
IDLE COMMENTS - on engine speed of course!
This an area where I get many queries, which can be very roughly be divided into cold or
hot idle problems - or both! The most common is a sustained idle rpm of
about 2k even when hot - "embarrassing at traffic lights" is the usual comment I get! The
solutions are usually not too difficult - but it helps to appreciate the
basic workings of the system first.
On cold start up the ECU creates a rich mixture - which is necessary to compensate for
the poor fuel atomization at low temperatures.
The ECU does this in 2 ways -
a) Energizing the cold start injector. This is a single point injector mounted on the front of the
TVIS plenum which continuously sprays fuel into the air stream. It's a bit crude
in that there’s no real control over just how much fuel gets to each cylinder -
but it works. The analogy is the choke as fitted on older cars.
b) By doubling the open duration of the cylinder injectors.
The combination of these two is to lift the cold idle revs - although ‘what
rpm you get is what rpm you get’ - as there is no user control over this
- for good or bad it’s in the hands of the ECU! This is usually about 2400rpm
when ‘stone cold’-- quite high but this is normal for a MK1.
As the engine starts to warm up the ECU progressively reduces the cylinder injector
duration, which starts to bring the rpm down. Then the cold start injector is
shut-off, bringing rpm down quite fast, then finally the cylinder injector
durations are brought down to the ‘hot’ timings to trim the rpm to the hot idle
speed - 800rpm. As mentioned above, this is out of your hands - it's the
ECU responding to the signals from the coolant temperature sensors. However to
emphasize - the ECU is not in direct control of the rpm - only indirectly
via the injectors. (This is a bit crude compared to current designs which do
have direct closed loop control by using idle speed control valves.)
Nevertheless it's not a bad system, but there can be minor variations between
engines as a results of differing conditions and settings.
Now as well as the above electronic controls there is also a purely mechanical device
which operates as the engine warms up - the Auxiliary Air Valve.
Thankfully its much simpler than it sounds -its just a waxstat
valve that controls a small air bleed around the throttle plate. When cold it is
open, so giving a lift in idle rpm. As it warms up (its fed by coolant), the wax
capsule expands and progressively closes the valve to stop the air bleed and
hence the rpm lift. It is physically located under the throttle body and is a
simple bolt on unit.
So, now the engine is hot, all the above idle rpm boosts should have reduced to zero and
the rpm should be around 800 rpm.
At least this is adjustable - via the setting screw on the
throttle body. This screw is in the pillar sticking up - easily
recognisable as it has a black plastic cap on top of it. This is just a simple
manual air bleed valve across the throttle plate, and is intended
solely as a trim to hot idle rpm.
It should not be used to attempt to influence cold idle rpm as it will have only the
smallest effect when cold but will send the hot idle speed haywire! It also has
minimal effect on mixture and hence should not be used to attempt to influence the mixture settings.
(Mixture can actually be tweaked by other means - but that subject will be in a
separate article coming soon.)
That's a brief description of the system, so coming back to the problem of a highish idle
speed even when the engine has warmed up -
First of all - and this is to purely to eliminate any ‘daft’ causes - check for air
leaks into the induction tract. Do this by inspecting the vacuum hoses coming
off it - check visually for splits and cracks and flex them to detect any
embrittlement. (If there is such an air leak it will lift the idle rpm. and also
make it roughish.) If the throttle body joint or intake manifold gasket are
leaking in there is usually there is usually also a bit of a whistling noise from
them at idle (ie high vacuum conditions.).
That done and admittedly it's a bit subjective, but it is worth
a quick check first to save any embarrassment at possibly missing the ‘obvious’
-on to the next steps -
If it is very minor - say 1000rpm, then the idle screw will most likely tweak it
down. If however it is say 1500rpm then it probably won’t - but this is
not a disaster - its just the cause is a bit deeper.
As you have read through the cold start-up control description above you might have
noticed that the only input that determines the control system reponse (ie
output) for idle rpm is coolant temperature. This is quite a logical and
time honoured system - just as long as the input-output chain works.
Taking the output side - this is the ECU and the injectors - the expensive
high-tech end! Fortunately these components very rarely go seriously
wrong - the problem is invariably (99.9%) the input end which is much
cheaper and easier to deal with!
The ECU actually gets its coolant temperature input from 2 sensors - one for the
cold start up injector system, and one for general ECU use including the
cylinder injectors. As to why have two - it’s most likely for electronic
reasons but I prefer to believe it is for reliability - if one sensor
fails the other one is still operating its ‘system’.
These sensors are resistors that vary with temperature, and although failure is rare,
they can ‘drift’ with age and indicate an inaccurate temperature, thus tricking
the ECU into an incorrect injector response.
More likely though is a much more mundane cause - which is that the sensors
themselves are getting tricked - as they are not sensing the real coolant
temperature. This is in turn due to air in the coolant - the MR2 is quite prone
to air locking - as mentioned in my articles in recent club magazines.
The cold start sensor (and just for the record the Toyota designation for
this is ‘cold start thermo-switch’) - is located on the thermostat
housing which is mounted above the gearbox. This is easily located - it has a
white nylon wing nut (the engine air bleed valve) on it with a hose leading up
to the coolant expansion tank. The sensor is close to it and has a brown plastic
connector (at least it is on my N/A).
The main engine coolant temp sensor is located at the nearside of the cylinder head
roughly below and between the cam covers. This has a green connector on it.
As the coolant system is prone to air locks (particularly after a coolant change) both
of these sensors can sometimes be fooled when air is present. (Air, being a poor
conductor reduces the thermal contact between the sensor and the water -
and the sensor reads low.) Hence the cold start system remains operational for longer, giving high
hot idle rpm.
If you do have this problem I would initially suggest one or two ‘bleed sessions’ -
as per my Coolant article in issue 16 of the club magazine.
If however high hot idle problems persist then the next item I would go for is the
auxiliary air valve (AAV). It could be the AAV waxstat itself - they do
eventually leak wax. When the wax does leak the valve stays open - which
keeps the hot idle speed too high. This can be checked in-situ - by using
the idle adjustment screw as a means of probing the AAV.
To do this have the engine idling hot, then progressively screw in the idle screw. The revs
should drop to around 600rpm where it will not be happy and probably cough to a
halt. This means the AAV is OK - it is closed when hot as it should. Naturally you will then have to reset the idle screw -
it's worth remembering how many turns it took to starve the engine. If
however there is no or little difference in idle rpm, then the AAV is staying
open and must be changed - they are sealed units. I don’t know the Toyota
price but I would try Fensport first to see if they can get BluePrint replacement units.
These are the actions most likely to produce results - if they don’t then it could
be the coolant sensors themselves, or some deeper problems (which are
fortunately rare). l’ll cover these in the next article.
If however anyone is currently really struggling with this problem - don’t wait for
the next issue - members can just ring me anytime on - see number in club
magazine. (I can’t guarantee 100% solutions - but I can promise sympathetic suggestions!)
[Footnote - the above article relates to the N/A - the S/C has a different
system (closed loop with ISCV). I wouldn’t actually call it better - it
just has a different set of problems - but that’s yet another story for later!]
IDLE COMMENTS - Part 2
The last dealt with hot idle
problems as they seem to be quite common - usually high rpm. This time it's cold idle - often it's also rather high
at 2400-ish rpm - in theory the control mechanism is different to the hot idle
system - but inevitably they are linked when control components 'drift'.
Feedback from members following the last article have been really
invaluable on this - and credits go here to John Boxall and Adrian at Fensport.
This feedback indicates that the Auxiliary Air Valve (AAV) is perhaps the
biggest culprit for both hot and cold idle. This makes sense - it is a waxstat
device - and the wax capsules do leak eventually - so the valve no longer opens
and shuts when it should. The principle is similar to the thermostat in the
cooling water system.. (The AAV is a bypass air channel around the throttle
plate meant to lift cold idle rpm and be shut when hot.)
Given the age and mileage of all our cars this is quite plausible - loss
of wax means it could be more open than it should be - hot or cold. This then
means that both hot and cold idle revs are affected - upwards.
If this is the case, then an elegant solution (from the above-mentioned)
is to fit a flow restrictor in the AAV air bleed circuit - thus effectively
compensating for its deficiency.
This might sound complicated - but it takes no more than a spare plastic
tyre valve cap and half an hour or so of easy labour.
The method makes use of the fact that the throttle body has a very
conveniently (tyre valve cap) sized air bleed recess to 'supply' the AAV -
restrict this and idle rpm will decrease.
So, to do this mod -
a) Remove plastic 'swan neck' immediately ahead of throttle body - 2 or 3 hose clips to slacken
off. (There may well be rust on the lowest one - as its in the rain-drip zone of
the engine cover - presoaking with penetrating oil is a good idea.)
The AAV supply recess is at the bottom right of the throttle body just upstream of
the throttle plate - its 10mm dia - little finger end size - thats where to fit the restrictor.
(There is another recess further up to the right - but its much smaller - and rectangular -
for the idle speed adjuster. No need to worry about
restricting the wrong one - the valve cap will only fit the AAV recess.)
Also the inside of the throttle body will be covered in a thin black oily
film - looks bad at first - but its normal. Its due to the PCV emission control
system where crankcase oil fumes are pressure vented to the throttle body -
hence the term PCV - pressure crankcase ventilation. The theory is that it is
then burnt with the fuel - but enough condenses to coat the intake system. Not
pretty - but just a nuisance.)
b) Find a plastic tyre valve cap and drill 5mm hole hole in the 'top'.
c) Push cap into recess - cap top first - and press in with medium finger pressure.
It won't go in 'flush' but not far off. The bit left protruding out is no
problem - the throttle plate can't hit it.
d) Refit plastic swan neck.
e) Test car idle rpm..
This modification should drop the cold idle rpm to a less busy level
- but it obviously depends on individual
engine characteristics. With a 4mm hole mine dropped to 1100rpm - but a tendency
to die on any throttle lift at start-up. Hence the suggestion to use 5mm
diameter. Tuning 'to taste' can obviously be done by altering the valve cap hole
size - bigger holes lift rpm and vice versa.
Also the hot idle speed might change slightly after this modification -
easy to deal with - adjust to suit on the throttle body adjuster (pillar with
rubber cap - if still fitted - at the boot side of the throttle body.) Turn
adjuster screw head clockwise to drop revs - and vice versa.
Overall a simple, cheap and elegant tweak - and there's no doubt that the
simplest are the best - does anyone else have similar 'gems' to offer?
ALAN JONES