05-10-2017, 12:37 PM
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
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
