Idle Problem....

[Fletcher Blades]

Ok, sorry I havent managed to post before this, but I've been busy...

I have heard of low batteries causing all sorts of strange problems, and
theres been a lot of good info going back and forth, but no-ones mentioned
the correct way to adjust idle speed.


The ECU takes control of idle speed, and maintains it at about 750-800 RPM,
once it "realises" that you are idling.

Often it takes the ECU a second or two to realise when you are idling.

This is why its not un-common for your engine to nearly stall, or
occasionally stall completely, before the ECU says "Hey, we're idling, I'm
going to take control". This is because your "natural" or "base" idle
adjustment is off. Although less common, you can also have a too fast a
natural idle, and the car will come to rest at 1200 RPM or so for a few
seconds, before the ECU realises whats going on, and grabs it back to
750-800.

So to set your base idle, you have to over-ride the engine computer...

Warm the car up to normal operating temperature.

Pop the bonnet, and find the "diagnostic" connector. Its a small black
plastic box about the size of a box of matches, in the engine bay, near the
top mount of the left shock absorber.

Open the lid of the Diagnostic connector, and short together with a
paper-clip or small piece of wire, the two terminals labled TEN and GND.
(actually, theres a couple of GND's to choose from, doesnt matter which one
you use).

Now your car will be idling at its "natural" idle speed... (if it didnt
stall).

Now is the time to turn the adjustment screw on the throttle body, to get
the idle speed into the 850-950 RPM range.


If you've been adjusting this screw with out jumping TEN and GND, the ECU
has been in charge of the idle speed, and you wont have seen any change.
(it will have still changed the base idle speed, just you wont have known
how much and in what direction you were changing it).


This will take care of most idle speed problems, but there is another one
you might have...

You only need to try this one if you've already tried (and failed) with the
above, its much less common...

There is also a valve on the side of the intake plenum that adjusts how much
air bypasses the throttle plate, depending on coolant temperature. (possibly
called ISC valve? I forget). It can in some cases become stuck in one
position or another.

Its about an inch round and two inches long, and has two small water-hoses
going to it. (on the 1600 at least), and is held onto the plenum with four
8mm bolts.

At least in several cases I know of, simply checking its operation has been
enough to free it up, and fix the problem.

Heres what to do...

Remove the hose clamps, and water tubes and unbolt it from the intake
plenum.. (take care not to drop or lose the figure-8 rubber gasket).

Now you can look into the valve, at the plunger and spring, through the
holes that are normally against the plenum.

Note the position of the plunger.

Now, place the whole valve into a pot of water on the stove, and bring it to
a slow boil. Look and see if the plunger has moved.. (carefull now, its
hot!). If you've got more time, you can also cool it in the freezer to see
if the plunger moves back in the opposite direction. But just watching it
return to its mid position as it cools should be enough.

If the plunger hasnt moved, then its definitely faulty... If the plunger
does move, re-fit it to the car, because you've probably just freed it up.
Mine did this about four years ago, and has been fine since. I've also seen
it fix three other cars.

Good Luck,

Also, you coul check the tech section in on the web-page on how to make a
diagnostic code reader, to see if the ECU thinks theres a problem...

Fletch.

[Graham Aitken]

This explanation of the ECU taking over the idle speed sounds like a problem
I get.

I've got a 1998 MK2, with a Jackson Racing Supercharger. The idle speed has
been set at 950 RPM, but I guess not in the ECU. When I initially come to
rest, e.g. at traffic lights, the idle goes straight to 950, but then after
a couple of seconds drops to about 750, stays there for a few seconds, then
comes back up to 950. This sounds like what Fletch is saying about the ECU
taking control once it realises that the motor should be idling, but then
fortunately the idle does come back up to 950.

The problem is that if I touch the accelerator while the revs are at 750,
everything just coughs and splutters and threatens to stall. Invariably this
happens just as the lights turn green ...
Any comments anyone ?

Also, as the weather gets colder (I live in Taupo, and it's cold now), the
supercharged engine is taking longer to start when cold. When I first had
the supercharger installed (after last winter), it always took a couple of
goes to get started when cold, then it slowly comes to life, but it's
getting worse. I need to do something about it, so would appreciate any
comments.

Graham Aitken.

[Fletcher Blades]

Graham,

I dont know much about either the Supercharger or the new model, but I'd
expect you have a supercharger set-up problem (possibly fuel related?)
rather than an ECU problem.

There is a mailing list just for sebring/Jackson Racing superchargers, and
I'm sure they'll be able to help you.

To join the list, go to...

http://members.xoom.com/sscorweb


Good Luck,

Fletch.