TBI Surging/Hunting for Idle When Warm in Park

[michael hurd]

Visually, I posted a link in another thread.

http://api.viglink.com/api/click?fo...9233/pit-stop/gm-tbi-no-pulse-while-cranking/http://api.viglink.com/api/click?fo...9233/pit-stop/gm-tbi-no-pulse-while-cranking/

 

[Bubb]

Tech,
Thanks again for the link to the device. After researching this some more I've gone ahead and ordered that ALDL bluetooth box as well as a cheap Android tablet. It's cheaper than buying a fancy code reader, and I'm sure my kids will love the tablet too.

I'll post up an update with what I find out once it all gets here.

 

[Bubb]

Michael,
Thanks for the link, I see what you're talking about now and I'll take a look when I have a chance.

Question though: if this were a mechanical problem such as a bad magnet why would the problem go away when I unplug the base timing connector?

 

[Tech]

The Seafoam is a cheap diagnostic. It only takes one tank of bad gas to push things out of tolerance.
I bought a tablet for the truck it lets me keep an eye on everything. I use it as a heads up display at night, Google maps and it is a GPS I also put a mileage log on it. If I ever find a suitable backup camera it will go on it as well.

The left rear port in the TB above the IAC module is the IAC inlet. A ball of blue towel stuffed in it will cause the IAC to run in a different operating band and may buffer it a little. not a fix but it may help with the diagnostics.
You know the game what did you touch last. Is the battery voltage stable. If it is surging it could cause the engine to surge as well Did a fitting get cracked or a vacuum hose get pulled off when the alternator was swapped.

 

[Bubb]

Howdy Tech,
I didn't swap the alternator, just the battery. The problem persists with no change over the last couple weeks, I just haven't had much time to spend on it.

I purchased that OBD1 bluetooth adapter and am working with the developer (huge help) to get it working with my tablet. I think we have a solution and will try it out in the next few days and post up the results here.

That's a good idea about restricting the flow through the IAC inlet. I may give that and seafoam a shot. But now that I'm so close to being able to capture the problem when it happens with the app I kind of want to wait to make any more changes.

 

[Moparmat2000]

Remove dist cap, and rotor and look for this. I had issues with a stumble at idle, and under steady rate of speed tach needle would randomly jump up and down slightly, and upshift light would flicker at same time.

Cracks in magnet will throw off ghost signals to the ICM resultingbin random firing. However from the hunting up and down it sounds like kinda like an IAC motor issue to me. Is the IAC bypass port in the TB clean? An obstruction there can cause it to hunt too

Did you check the TPS with an ohmmeter for range of ohms from idle to WOT. Check unplugged with engine shut off. A bad spot in the TPS may cause problems if computer doesnt know which position throttle is in.

Hope this helps
Matt

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[Bubb]

Matt,
You and Michael are thinking along the same lines as far as a bad distributor. I wonder though, why would this only happen in Park/Neutral?

Got some more data to share in the next post.

 

[Bubb]

All,
I was able to get the app and bluetooth adapter up and communicating this morning. Thanks for this great suggestion! Here's a pic of some data showing about 16 surges until it finds its groove around the 157sec mark.

It makes sense to me that the MAP follows the IAC position. There are a few things that seem odd. The o2 sensor data, not sure if that fluctuation is a result of the surging, and what's with the huge voltage jump at the end? It's interesting that the desired RPM isn't really changing the whole time. And it's interesting that just before the behavior stops the spark advance changes.

The malfunction codes file includes some flags. It seems like the "Reset IAC" flag never goes off, not sure whether this is normal. I've tried two different IAC reset procedures, one from the web and one from the shop manual, and the flag stays there. Don't know what this means, if anything.

Appreciate any insight from the community.

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[Bubb]

Here's some more data from the Malfunction Codes file. Anyone know what "Air Fuel Mode Word" means?

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[Bubb]

Update for anyone following:

I did find some cracks in the distributor magnet, as was pointed out by a couple people here (thanks guys!). So I swapped that and the coil while I was at it over the weekend. It does run smoother now than before, but still has the same problem.

Here's a pic of mine:

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Anyone think a defective battery could cause this behavior?

I took some more data using that awesome ALDL Scan app, but am struggling to see what may be causing it. The data looks similar to my earlier posts above.