IAC raising RPM's

Tiny
BIGDAN120
  • MEMBER
  • 2001 PONTIAC AZTEK
  • 3.4L
  • 6 CYL
  • AWD
  • AUTOMATIC
  • 173,000 MILES
I have had this issue for three years. Been diagnosing it since. When the cooling fans turn on at 223 like there supposed to if you keep your foot in it sitting still for another Minuit at 2500 rpm it will take over by itself and run idle up to about 3500 by itself. If you turn off and restart it is fine until it gets back up in temp. Had a GM tech look at it in thirty five years he has never seen one do this. And he sees no evidence of anything on scanner. Now he never had scanner plugged in while it did this. There are no codes, no air leaks, fuel pressure is where it should be with no fluctuation at time of it. Throttle body cleaned new IAC, new computer. Checked for updates for it and it is current. I think its electrical. Because when it did this once before the cooling fans relay started clicking fast like it was confused. Wiring to fans has been tested new cooling sensor and plug both cooling fans are fine.
Friday, September 22nd, 2017 AT 12:00 PM

4 Replies

Tiny
STEVE W.
  • MECHANIC
  • 15,233 POSTS
What do you have access to in regards to test equipment? From the description I would look at the alternator and its harness. It is possible that there is a lot of noise on the power from it and that couples into the IAC control when the fans start pulling a lot of power and draw the system down. An oscilloscope connected to the IAC and alternator might show you what the issue is.
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Friday, September 22nd, 2017 AT 11:44 PM
Tiny
BIGDAN120
  • MEMBER
  • 6 POSTS
I have a modis Scanner.
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Monday, September 25th, 2017 AT 6:34 PM
Tiny
BIGDAN120
  • MEMBER
  • 6 POSTS
I found that the Map sensor voltage is really high koeo 4.75.
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Monday, September 25th, 2017 AT 6:35 PM
Tiny
STEVE W.
  • MECHANIC
  • 15,233 POSTS
With the engine off the MAP reads sea level pressure. On a GM with the engine off that would be around 4.5 volts. 4.75 is a bit high, but for a used sensor it isn't that far out of line.
What you want to do is test it with the engine running at an idle, with the KOEO of 4.75 I would expect to see an idle voltage around 0.7-0.8 VDC
That voltage should rise as you open the throttle and manifold vacuum drops.
If the voltage stays steady or jumps as you gradually open the throttle then you have an issue.

Toss your scan tool on. Pull up the PIDs for IAC, TPS, MAP, short term fuel trim, fan status, engine temp. Start it and let it heat up until it will act up and watch for one of the sensors to start showing abnormal readings as soon as the engine rpms start to rise. You're looking for a sudden change in TPS, IAC numbers or voltage when things start to "go wrong"

You can also use the scope in that tool to look at the output from the alternator and see if it suddenly starts putting out a bunch of noise under high current draw. A failing diode could start dumping AC on the line under high draw and that can cause some strange things.
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Tuesday, September 26th, 2017 AT 2:03 AM

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