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1996 X-90 Idle Speed Controller Problem
#1
Hello, I am a new member to the forum, but have been lurking for a month or so now, reading up on various repairs on my new to me X-90.

I have a 1996 X-90 with a stock 1.6L 16v engine with 115,000 miles. I have an unusual issue that I have not been able to find a post that sounds similar.

My engine runs beautifully until it gets really warmed up fully, then after a random amount of time at full temperature (sometimes a minute sometimes 30 mins or more) the ISC seems to cease operating and the idle goes down to 300-400rpm. Up until that happens, the ISC is working correctly and holds RPM at 800 or 1000 with A/C on.

Once it drops RPM when hot, the ISC will occasionally pop on and raise the RPM back to proper level for a few seconds, then it goes back down to 300-400 again.

Unplugging the ISC makes no difference when the RPM drops off. If I manually apply 12v to the ISC when it is idling at 300-400 the RPM shoots up as it should, so the ISC valve itself is fine (i have cleaned and tested it very well).

A little background, I have cleaned and tested the EGR (squeeky clean). There are zero air leaks in the system and the engine holds 19"+ of steady vacuum, no fluctuation or flutter. All vacuum hoses checked and routed properly.

No CEL light now. When I bought the car the CEL was on and showed a bad O2 upstream sensor, which I replaced and cleared the code, it has not come back on since. Compression is good and even, timing is correct, plugs are new and gapped correctly. I have tested all the little electric vacuum solenoid valves on the engine, and they all work properly.

TPS checked perfectly, it is properly calibrated, and the idle circuit is on at idle.

When the ISC is working, I was able to set the duty cycle to 50%. Then a few minutes later the idle dropped to 300-400 again. The Idle air screw works correctly.

Other than the low idle, the engine runs perfectly and pulls smoothly even from 300 rpm. No stumbles or hesitation, and revs freely to full RPM's.
Also, no one has fooled with the throttle plate screw.

So at this point im stumped as to the cause of the low RPM. Since the ISC electrical circuit essentially goes dead, when it warms up, I wonder if it could be a relay I am unaware of, or possibly a fault in the ECU?

I would be grateful for any thoughts or advice you may have as I am just out of ideas. I am a professional mechanic, but specialize in antique marine engines and pre WW2 cars, so I am not as familar with these ECU controlled engines, as I would like to be. Hopefully someone here has had similar symptoms and can offer some pointers.

Thanks very much,
Don
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#2
I forgot to mention that I have a new, correct, working thermostat in the cooling system. (I think it was a 195F theromostat, if I remember correctly.) Also the Catalytic converter is clear, no clogging or meltdown.
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#3
ahhh lost my post.....
is compression 185 cold or 195 hot, at sealeavle? not 100?
block egr hose, at main, golf Tee then end hose so cant suck air, idle ok now? acid test here.
the EGR is 100 off line at idle and if below 1MPH. and wideopen throttle.
no changes, still 300 ?
its not EGR for sure. if is lets fix it.

so is ISC,
measures 12ohms, DCR (dc resistance) at ISC coil pins, and wiggled, still, and not glitching to infinity? at ISC PINS?
ISC wires broken.
7vdc across 2 wires, running at 300? if not wires cut or ecu, bad. (0 to 14.7v possible, but 0 is closed, and 300-400 normal for 0v)
if 0v there, bad wires /ecu
if 7vdc there bad ISC if coil is 12ohms then ISC jammed closed. (1989/90 is 6 ohms for others reading this)

here is my 96 ECU, is yours same>?#
the transistor that fails is here. q017 is blown, easy fix. replace it. its like $1 part. or about it is a 25 watt transistor to handle that 12ohm DCR load.
the signal is 200 Hz square wave. any scope made can watch it.

2SC2562
G16b , usa almost all use this, or near close, min is stick shift version, btw.
http://www.fixkick.com/ECU/1996/96ECU.html
http://www.fixkick.com
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#4
also
is crank pulley 17mm bolt at 94/ft-lbs, or 10 or 0, this 94 is a new rule, 1996 TSB rule) retroactive to all G16 made.
with a/t trans many are not right, due to mech not knowing how to lock a crank.
http://www.fixkick.com
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#5
(05-11-2017, 02:48 AM)fixkick Wrote: also
is crank pulley 17mm bolt at 94/ft-lbs, or 10 or 0, this 94 is a new rule, 1996 TSB rule) retroactive to all G16 made.
with a/t trans many are not right, due to mech not knowing how to lock a crank.


Thank you! First off mine is a 2wd 5pd manual. Crank pulley is torqued fine...i just checked.

Compression is 190 to 195psi hot on all cylinders.

I tested the EGR per your instructions and it made no change at all, so it is fine.

I measured the ISC ohms carefully and the coil is good and no loose connections on the pins.

Double checked the TPS idle switch and it is on and working at idle 100%

I measured the voltage of the wires to the ISC and got 11.7v occasionally, when disconnected from ISC, but it would drop out to .3v for long periods of time.

When plugged in the idle would jump up to 800 for a moment at random , and then drop to 3-400rpm. So I am thinking you are correct about the wiring or the ECU transistor being bad. I will start checking in that area next. Thank you for the sage advice!!!
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#6
Ordered the replacement transistor. While waiting for it to arrive, I went for a long drive today and the ISC control circuit finally failed 100%. It no longer operates the ISC valve at all. I am hoping the new transistor will cure the problem.

Are their electrolytic capacitors associated with the Q107 circuit that I should check?

Thank You!!
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#7
sure, the caps love to fail 1989 to 1995
all do , in normal usa, and climates
cant speak of that one guy here at the north pole, (very near)
the transistor will fail if 1 wire shorts to ground
or the ISC coil is shorted
this old old electronic has no output protections. at all , only input.s.
id put in at least a new large corner cap , and check the other electrolyics with a sharp eye for leaks or damage.
http://www.fixkick.com
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#8
Ok I changed the Q107 transistor and no change in the ISC operation. It stills ceases to work after engine has warmed up.
It will work again while cold, after being not run over night, then drops out again. Could the caps cause this malfuction? I inspected the
caps and they show no leakage, bulging or damage. Anything else I should look at?

IF i need to buy a replacement ECM (ECU) which other Suzuki's use the same one as the X-90. I know they share alot with the Sidekick,
so I imagine some of the ECU's from the Sidekick's will work too?
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#9
the caps do love to fail, they are life span rated at 25 years, in tx or AZ or NM, its hot, and theyf fail sooner.
its just a $1 cap, put in 105c spec caps. is best in all cases, (on old cars) its not just suzuki caps failing, its on all cars this old i see.
The ISC fails for..
1: isc bad.
2: q107 sure.
3: caps bad, the cap bad cause the ECU to roboot over and over, endlessly, and doing so how can it control idle , doing that, answer, it cant. hopeless this, and more things too cant work right.
4: wires to it. sure.
5: idle switch not closed.
6: P/S overload switch stuck and rpm at 1000 regulated, not dead ISC
7: same for A/C
that is about it, unless the ISC goes nuts due to air leaks (vacuum leaks or EGR stuck open)
http://www.fixkick.com
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#10
Ive seen car cabs reach well close to 180F, parked in the hot sun, ever touch the steering wheel, near death valley, CA. you cant touch it at all its roasting hot.
this kills caps dead. (the caps called electrolytic are filled with a boric acid mix, and it dries out. and dies)
http://www.fixkick.com
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