1999 Lesabre High Idle & Hard Start

My 1999 Lesabre has started to idle at something between 1500-3000 RPM with my foot off the accelerator. It does this at random times. For the past three days it's run fine. Now it cranks and is hard starting unless I slightly depress the gas pedal. Then it starts & runs but idles high at times, and doesn't seem to want to idle properly.

Here are the things I have checked so far.
1. IAT - Idle Air Temp Sensor (Thermistor) Service Manual states it is 100 K OHMS at -40 Degrees Celsius. I've downloaded the Temperature chart for a 101 K OHM Thermistor and at 25 Degrees C it should be 3000 OHMS. My IAT measures from 4K to 2111 at around 77 to 79 Degrees F. I suspect this part as the temperatures in Phoenix don't change enough during the day to make this large of resistance change. NAPA TS4026

2. TP Sensor. My Service manual states that the TP is just a POT. And it measures 4K OHM. I've checked the OHM value through thru the range and it is steady from the low end to the high end with no intermittent jumps.
I know the VDC should be from around ~.4 VDC to ~4.5 VDC at WOT.

3. IAC - Idle Air Control (Stepper Motor) I've removed it and everything looks nice and clean. No carbon or gummed up parts. The port in the TB looks shiny and clean. I've reset it according to the service manual.

4. MAF Sensor - Haven't looked at it yet.

I've yet to check the coolant Temperature or the DTC's because I'm missing my USB to Serial Dongle so I can't read the OBDII port.

I've searched with Google and searched this forum and found around 20 pages of postings with no valid answers, other than a vacuum leak, or dirty TB, or defective TP Sensor, or defective MAF, or replace the IAC.

I'm requesting information on case histories of this type problem, as my searches don't find a final fix for all the searched problems.

Thanks.

Larry

In thinking about how the IAC Stepper worked I decided to have my wife to cycle the ignition switch while I was touching the IAC and listening to it step the pintle. It sounded like it acted properly, and I could feel it ticking. But, I wanted 100% proof that it was moving. I removed it from the Buick, and had my wife cycle the Ignition switch. The pintle didn't move. In 10 attempts it only moved a short distance ONCE!......

NAPA Part Number 2-19203 IAC was purchased and installed. That fixed my problem.

I disassembled the old IAC and now I know EXACTLY what was going on.
The Magnet has a split down the length, and it was dragging on the other
metal part of the motor. That is why it worked at random.

So, for the folks out there that see a High Idle problem of 1500 to 3000 RPM and random hard start problems if you don't crack the gas pedal a bit, here is your test procedure.

REMOVE the IAC from the car TB mount, and cycle the Ignition, watching the pintle move. If it works properly it will unscrew itself out of the armature.

Just depress the spring towards the IAC Body, and screw the pintle back into the IAC, making sure the spring captures and holds the pintle so the armature can adjust the length when it rotates. The pintle should not be extended more than 1 1/16" in length when installing in the TB.

The attached pictures show why mine didn't work. Notice the split down the magnet causing it to drag when trying to turn.

Postings on this forum state that the IAC is good for 70K. What a surprise, my 1999 Buick Lesabre has 69752 when I installed the new IAC.

GREAT JOB GM! A JOB your MOM can be PROUD of!


Larry - 3rd Generation GM Buyer........But getting much smarter for any future purchases!