[Supras] ISC non-operation

[berniek at technicaldevelop.com]

Skip and group:

    Resistance of the ISC on the '91 turbo checks fine at about 40 ohms 
per coil (each is center tapped).  I have another one here as a spare 
which I took apart and found relatively clean inside, and very well made 
with sealed ball bearings supporting each side of the permanent magnet 
armature.  I, too, doubt if the problem is the ISC proper  Nonetheless, 
on my '90 turbo, it reduced the idle from about 1200 RPM cold to about 
700 hot, and also compensated for use of lights, A/C, and such.  So the 
turbo apparently differs from the N/A in that respect. 

    However, all of the excess airflow comes through the ISC, determined 
with a piece of small bore Tygon tubing in series with the line from the 
accordion hose to the ISC.  Next week the salt mine is closed, so I want 
to put in a metal HG, 550cc injectors, Lexus AFM, 57 trim turbo, HKS 
exhaust etc.  The Walbro will follow shortly.  All of this means I had 
better get the car inspected here in NJ on Monday morning, before 
starting work.  So I used a short piece of Tygon tubing and a clamp 
arrangement to get the idle speed down.

    Right now it looks as if the simplest way to get the cylinder head 
off is to unbolt the upper intake, wire it back to the left inner 
fender, disconnect the precat at the elbow, turbo oil and water lines, 
and remove the head, exhaust manifold and turbo as a unit (other details 
omitted).  I have another head ready to go from an unfinished JDM 
rebuild, and have a 10" round lapping plate I'm adapting to a low speed 
DC gearhead motor for block lapping. 

    Access to the ISC will be easy when the intake manifold is pulled 
back and/or the head is off.  I have the feeling, though, that something 
else may be amiss as you had stated.  No codes are set, but I understand 
from Jon (HackerJ) that none are expected to be set.  Seem strange, 
though that idle RPM grossly in error is not flagged by the ECU.    
After reassembly, out will come the oscilloscope with a sharp insulation 
piercing probe.  Luckily I have a couple of spare ECUs.  At one time I 
was going to replacing the injector drivers (8 amp NEC TO-220 
Darlingtons) with logic level FETS for lower voltage drop, so a spare 
ECU or two was in order.  The objective was to try running the stock 
injectors at 58 PSI to match what 550cc parts would do regarding flow, 
but I bought as set of 550 ND injectors from Chris Hommes instead. 

       Any and all comments, especially about head removal as stated 
above would be appreciated.  Thanks.

          BernieK
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Skip Christian wrote:

Bernie/All,

	I disconnected the ISC valve on mine - no difference.  Even
tried opening & closing it all the way to see if the computer would
immediately return it to where it should be for idle - it didn't.
Finally replaced it - no difference.  If disconnecting the ISC doesn't
help, its not the ISC (idle speed control valve).  I do not think the
valve can respond quickly enough or far enough to cause the problem you
have.  Computer seems to be able only to open or close it one or two
steps, which only changes RPM by about 200-300 (if that much).  Its
there to fine tune the idle speed, at least on the N/As.  The valve, on
N/As does not step up the idle when the engine is cold.  On N/As, this
is handled by an increase in ignition advance of about 10 degrees.
	Really recommend you remove/disconnect any fuel related mods
that are functional at idle speed.  After disconnecting each one, cut
power to the computer to clear the memory, then start car and take it
out for, at least, a 15 minute drive to build up some data in the
computer.  Then check the idle.
	Does your's do the same thing (or similar thing) when the engine
is cold ??  Mine did.
	What mods do you have ???  This is the key (or there would be
more cases of this problem).  Dealer didn't have a clue as to the cause
- I asked the lead (most senior) service person.
Good Luck, Skip
P.S.  I only have E-Mail at work, so I'll be gone from around 2:30 today
until 7:00 Mon.  Sorry, for the rambling, I'm an engineer.........