Is This A Glitch With FAST?

[ZBass28]

It hasn't happened very often but a few times I have started the car (cold) and had an issue where the car starts at my elevated target warmup idle and will not come down as it is supposed to. I can try and manually change settings to bring the idle down and it just won't budge.

The only cure is to shut the car off and restart it, in which case it finds the correct idle and runs absolutely fine. What could be causing this?

 

[bobc455]

Is the IAC target changing or staying still?

If the IAC target changes, is the IAC motor actually doing what it is supposed to?

-Bob Cunningham

 

[ZBass28]

The IAC had changed, though I don't know how much. I only let the car run for about a minute in this condition before shutting it off and to be honest I was checking other things like throttle linkage to see if there was a sticking problem because everything seemed to be operational with FAST on the laptop, I just wasn't getting any response. The car started at 1400 rpms which is where I have it set and it just stayed there instead of working itself down to 950 rpms like it usually does in about 5-10 seconds.

Restart and everything was perfect. This happened once last year as well and I just passed it off as some anomaly.

 

[bobc455]

I had a throttle body problem once where the throttle blades were held too far open, just from friction. If I stabbed the gas and let them snap shut, the idle would go back down. The throttle shafts were plain ol' worn out (piece of crap @#&^).

When you say the IAC changed, do you mean the setpoint or actual motor position?

-Bob Cunningham

 

[ZBass28]

Oddly enough when it was doing this I was stabbing at the throttle thinking it would 'come down' like setting an old fashioned choke on a carbed car.

I am pretty sure the actual motor position on the IAC was changing. If I recall correctly, the indicated position was 90 when I shut the car down. I need to go in there and see what the start position at the start-up temp was compared to the 110 degrees I was at when I shut it down.


I wish this was a problem I could get to re-occur, unfortunately it caught me a little off guard and forced me to shut the engine off prematurely without taking the time to diagnose the cause.

 

[ZBass28]

Originally posted by bobc455
Is the IAC target changing or staying still?

If the IAC target changes, is the IAC motor actually doing what it is supposed to?

-Bob Cunningham

Well...I had the problem happen again today. Car hadn't been started for about 3 weeks and did not want to turn over on the first few attempts (it's been cold here). When it finally did start the idle jumped to 1500 rpms and just stayed there. This time I was able to watch some stuff and noticed that the IAC Target was not moving but just staying at 10 on the main dashboard. When I looked at the settings, it should've been at 62 and decreasing at that point. The graph was physically moving however.

Anyway, shutting off and restarting cured everything and the IAC target picked up where it should've been at around 40.

Any ideas?

 

[10SV6]

Physically check the IAC valve. It sounds like the FAST is trying to bring the RPMs down yet they're staying up. Sounds like the valve is sticking. When you turn off the car the IAC parks, "unsticking" itself.