High Idle and Running Lean (bogging)

[Turblown]

This is on a 1g NT and my profile is up to date. I'm not sure if the two are related, but here is the situation:

Problem #1: High Idle:
- Used to have high idle (2000+ rpm) on startup with the BISS all the way in.
- Blocked both FIAV and ISC with duct tape to test.
- Was able to set idle in the correct range (750+/-50), but fluctuaed with accessories on (no ISC).
- Blocked only the FIAV (JB Weld) and removed coolant lines = Idle at 1000 with the BISS all the way in.
- TPS Adjusted to proper voltage (.48v to .52v) = no change
- Datalogger shows ISC = 10.5% to 12.5% with no accessories on

From what I understand the engine should stall with the BISS all the way in, but on my car the idle won't go below 1000. Since the idle was good with the ISC blocked, it would seem that it could be faulty. I have tested the ISC and it seems to be within range.

- - - > Is this definitely an ISC problem or is there something else I can check?

Problem #2: Bogging during shifts
- Small popping noises (backfires?) and loss of power during upshifting or downshifting
- Datalogger shows FTRL = 110%, FTRM = 100%, FTRH = 100%
- No other testing done.

- - - > Does this sound like the spark plugs, oxygen sensor, fuel pump or what?

 

[ekanddsm]

The ISC can pass all bench tests and still not work when warm. Have you tried a different T/b all together? Have you looked for any other vacuum leaks?

 

[Turblown]

The ISC can pass all bench tests and still not work when warm. Have you tried a different T/b all together? Have you looked for any other vacuum leaks?

I haven't tried a different TB, but I may be able to get my hands on a different ISC, just to test it out. I'll definitely try that. All of my vacuum lines looked good, but I can check again.

 

[steve]

The ISC can pass all bench tests and still not work when warm.

And that your ISC can check out ok but not work because the drivers on the ECU are blown or the circuits damaged from capacitor leakage.

 

[Turblown]

And that your ISC can check out ok but not work because the drivers on the ECU are blown or the circuits damaged from capacitor leakage.

Well, I just replaced the ECU with a used 90 NT ECU and switched the pins. I checked it for capacitor leakage and it looks good. I guess it could be on it's way out also. I'll have to look into it a bit more.

 

[steve]

You'll have to look at the 4 ISC pins and check to make sure that the ECU can pull each low as needed to step the ISC.

 

[Turblown]

You'll have to look at the 4 ISC pins and check to make sure that the ECU can pull each low as needed to step the ISC.

Thanks Steve, for your help. I did the Hayne's test for battery voltage on one pin on the harness side and that is good. I found the pins on vfaq.com.

58 Coil "A1" for idle speed control motor
59 Coil "A2" for idle speed control motor
67 Coil "B1" for idle speed control motor
68 Coil "B2" for idle speed control motor

What do I physically do once I've located the pins on the ECU? Should I be checking for voltage between the pin and ground? Do I have to test on the ECU side or can I just test on the harness side (in the engine bay)?

 

[Turblown]

Well, I've yet to find a solution to my issues. I tested the ECU by swapping it out, but no change. The ECU I'm using has no cap leakage or fried drivers. I adjusted the accelerator cables to make sure they weren't pulling on the throttle at idle, no change. I pressure tested the intake and replaced some vacuum lines, no change. I still have a higher than normal idle (1000rpm) and I'm getting a low fuel trim of around 114% now. I know these issues aren't a big deal, but this car is my daily driver and it's basically stock, so I'd like to get it running right. If anyone has any input I would appreciate it. I'll keep at it and update when I get a solution.

 

[Turblown]

Problem #2: Bogging during shifts

I finally got to swap in a used oxygen sensor to see if it would make a difference. It seems to have fixed the bogging issue. I took another log and my fuel trims changed to FTRL = 100%, FTRM = 111%, FTRH = 95%. Not sure what the issue is with the Med and High fuel trims. This car is basically stock. The used oxygen sensor came from a GSX. Would that make a difference?