TBI Surging/Hunting for Idle When Warm in Park

[Tech]

With the "desired" idle speed bouncing it appears that the engine is just doing what its told.
The faulty TPS suggestion was good. With the motor off but key on slowly cycle the pedal looking for jumps in the numbers on the screen.

Just a guess here but it looks like it takes forever to get into closed loop. I think that is what happened around 185 on the second graph. The ECU could be blipping the throttle expecting to see a response from the O2 sensor and not getting it.
My BB goes closed loop a lot faster than that from a cold engine.

I may have to look at my distributor magnet I get the random but constant burble not quite as smooth as it could be

 

[Bubb]

Tech, thanks for following up. Here's a more recent pic showing the engine speed vs desired speed--it's definitely unable to get the speed where it seems to want to. Looking at it closely, the IAC movements lag the drops in rpm by a little bit, leading me to believe that something is trying to kill the engine, then the computer comes in to save the day, overcompensates, then the cycle repeats for a while.

I threw in the towel and took it to a shop last week. They weren't able to find anything obvious, which was both good and bad. They suggested I drive around with the EGR line plugged, but that didn't make a difference. I'm gonna take it back there later this week.

I don't know if it's related to the weather heating up, but the problem is getting worse. When it's warm enough it's now surging at idle in park at stoplights too.

Think it's possible a leaking intake manifold could cause this? I was spraying some carb cleaner around there and was able to get it to surge by spraying in a specific spot near the TB, but I don't know if that's because it was breathing in the carb cleaner fumes.

What is it that indicates closed loop? The O2 sensor range jumping up? What are the triggers that allow it to go into closed loop?

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[sntrym]

Check the third entry in this thread out. http://67-72chevytrucks.com/vboard/showthread.php?t=432837. This is very common with these trucks.

"On these distributors the housing/upper bushing area can get so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues."

 

[Bubb]

Thanks sntrym. I've replaced the distributor with no change, so I don't think this is the issue, unfortunately. I've had the truck in a local reputable shop for four days in a row now, but no diagnosis yet.

 

[sntrym]

Usually something like this is the CTS. You checked the one in the intake manifold and not the one in the driver's side head right? (There are 2)

 

[Bubb]

Sntrym, yes I have actually replaced both of these in the last couple years I've owned the truck. Looking at the graph in my earlier post, the CTS output is constant while this is going on. I don't see anything in the sensor input to the PCM that would indicate a problem there. I have also replaced the PCM and don't believe there's a problem there. It seems to me that something is trying to kill the engine, then the computer dumps in more air and fuel to save the day, overcompensates, and the cycle repeats. Just need to find that thing that's trying to kill it. In case you didn't see from my first post, the weird thing is that this all started right after a battery change a couple months ago.

 

[michael hurd]

When was the EGR new, it may be partially stuck. With two fingers and then engine not screaming hot, try pulling up on the EGR diaphragm on the bottom of the top 'hat'. It should want to stall.

 

[Tech]

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As installed

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One Happy Engine

Do you have both apps?
ALDLDroid has a closed loop button that you can add to the display and shows real time updates on screen that may help. Your O2 is not happy. A vacuum leak is certainly worth fixing.
If it was a Honda I would tell you to top off the coolant, they hunt when it gets low!