mjdarg
Active member
Hey guys,
I have been having trouble getting a decent idle recently. I have been running a few months with the ISC unplugged while I try to figure out the problem. Stock 91 VR-4, using TMO to datalog. The ISC stays around 76 or 77 steps during normal idle, while the engine is around 1500 rpm. If I ground the connector on the firewall, the ISC steps drop down to 9 or 10, and the idle drops down to 800 where I set it with the BISS. As soon as I un-ground the connector, I can see the ISC steps increase in TMO first, and then the rpm and advance increase as an effect. I have also noticed that if the ISC is unplugged, grounded the connector makes no discernible difference to how the engine runs.
Is there something simple that I'm missing?
The O2 sensor cycles, but when it is idling around 800, the frequency seems to be about 0.5 Hz (once every 2 seconds). Around 1500, it cycles much faster, around 1.5-2 Hz. Does this seem normal or would it affect my problem in any way?
One thing I've wondered...how does the ECU determine the required number of ISC steps to maintain correct idle? Does it simply compare the current engine rpm and compare it to the setpoint? Maybe there is something wrong with my ECU... /ubbthreads/images/graemlins/dunno.gif About six months ago, my idle would increase while turning, which was the ISC correcting for the power steering, but I guess too much. I sent my ECU to ECMLink and they couldn't find any issues.
Lots of rambling...what do you guys think?
I have been having trouble getting a decent idle recently. I have been running a few months with the ISC unplugged while I try to figure out the problem. Stock 91 VR-4, using TMO to datalog. The ISC stays around 76 or 77 steps during normal idle, while the engine is around 1500 rpm. If I ground the connector on the firewall, the ISC steps drop down to 9 or 10, and the idle drops down to 800 where I set it with the BISS. As soon as I un-ground the connector, I can see the ISC steps increase in TMO first, and then the rpm and advance increase as an effect. I have also noticed that if the ISC is unplugged, grounded the connector makes no discernible difference to how the engine runs.
Is there something simple that I'm missing?
The O2 sensor cycles, but when it is idling around 800, the frequency seems to be about 0.5 Hz (once every 2 seconds). Around 1500, it cycles much faster, around 1.5-2 Hz. Does this seem normal or would it affect my problem in any way?
One thing I've wondered...how does the ECU determine the required number of ISC steps to maintain correct idle? Does it simply compare the current engine rpm and compare it to the setpoint? Maybe there is something wrong with my ECU... /ubbthreads/images/graemlins/dunno.gif About six months ago, my idle would increase while turning, which was the ISC correcting for the power steering, but I guess too much. I sent my ECU to ECMLink and they couldn't find any issues.
Lots of rambling...what do you guys think?