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Discussion Starter · #1 ·
EDIT - Typo, TQ = TC (Torque converter)

Sorry this is lengthy, but I wanted to provide a detailed description.

So the car has had this issue for a while now. Say I'm sitting at a light idling with the air on. After a bit, staring a a tree or a bird, I'll feel the car shutter. One good thump, like it missed. Looking at the RPM gauge I'll see the needle bouncing up and down 150rpm.

At first I thought it was bad plugs, wires, coil pack, or vacuum lines.

It's got new plugs, wires, coil pack was tested and passed (also tried another pack and the issue persisted, but it was a cheap china one so I went back to the original) and checked all my vacuum lines and it's got excellent vacuum.

Then I noticed that it only seemed to happen when the AC was on and it happened in time with the audible click of the compressor clutch engaging.

The AC works great and there's no noises from the compressor which makes me think the issue is when the sudden load pops onto the serp belt it causes the engine to bog. But I can't think of any good reasons why the AC load would cause the engine to start bouncing the RPM's?

My only two ideas as maybe low fuel pressure but I don't have a tester. This doesn't happen with the RPM's elevated with the throttle. Even sitting at a light with the foot lightly on the gas so I idle around 1000 RPM the issue goes away.
Or maybe something with the Torque Converter? But it's not giving me any issues aside from a weak lockup solenoid.

Ideas?

99 SE 3l OHV
 

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Would anyone else like to confirm that this might be one of those dreaded clogged IAC valves? I hit mine up with Brake cleaner or Carb Cleaner regularly.

Also it doesn't hurt to replace the PCV valve if its old.
 

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Sounds normal to me - if your saying that the compressor is kicking in and the RPMs drop a little and come back up to normal, the pcm is just compensating for the load.
 

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Discussion Starter · #5 ·
Sounds normal to me - if your saying that the compressor is kicking in and the RPMs drop a little and come back up to normal, the pcm is just compensating for the load.
Yes having the RPM's adjust for load is normal. What I'm experiencing though is the RPMs bouncing

Here's a breakdown of the events


  • Stable RPM at idle, a red light for example.
  • Feel car shutter, like maybe someone gave me a quick push on the bumper
  • Examine RPMs and note the needle bouncing up and down +- 150RPM
  • RPMs level out again after 5-10 seconds until it happens again
I've recorded it in action here (this is only a minute or two after starting it. It seems to happen a bit more pronounced after driving):
 

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Cake monster
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Yes having the RPM's adjust for load is normal. What I'm experiencing though is the RPMs bouncing

Here's a breakdown of the events


  • Stable RPM at idle, a red light for example.
  • Feel car shutter, like maybe someone gave me a quick push on the bumper
  • Examine RPMs and note the needle bouncing up and down +- 150RPM
  • RPMs level out again after 5-10 seconds until it happens again
I've recorded it in action here (this is only a minute or two after starting it. It seems to happen a bit more pronounced after driving): YouTube - video-2011-03-14-11-34-26
That doesn't look related to the A/C system at all.
 

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Discussion Starter · #8 ·
So I went and bought a new IAC at Kragen

The idle is different now. When the AC clicks on the RPM's jump (as they should) and when it clicks off they fall back down, however, when they fall back down the RPM's still bounce a bit.

I'd say the idle is better, but not fixed.
Here's a pic. I've got a new video but my wireless is acting up. I'll get the video up as soon as I can

 

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Discussion Starter · #11 ·
Geta new pcv while your at it. Will likely help if yours is old
The tube that runs from the front head or the back head? There's two.

Checking the rear PCV hose it seems to feel loose. Is this supposed to be a tight fit? It's very very easy for me to lift it out. The front hose is in good condition

 

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As far as it is not loose and without oil smearing in the immediate vicinity, it is okay. And it looks okay to me.
 

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Discussion Starter · #13 ·
So while the idle is more responsive to changes in load, it's still got the fluctuating RPM issue.

Tomorrow I'm going to disassemble and clean thoroughly the throttle body and check the TPS while I'm in there.

Any other ideas? Running out of them heh
 

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Discussion Starter · #14 ·
My last post here: It's fixed! The idle is rock solid now.

I pulled the throttle body and noticed that the flapper wasn't actually closing. It got close, but there was still probably 1/16th an inch of space for air to flow. So I cleaned out with carb cleaner and a brush, adjusted the throttle adjuster so it just barley caught the.. dunno what it's called. But the flapper shuts perfectly now.

Also there was thick thick carbon buildup on the UIM in the port that ran to the throttle body, chunks of carbon came out.

After all that, it idles perfectly now and I can't honestly ever remember the last time the throttle was so responsive!
 
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