I was reading up on the operation of the IAC system and I apologize for the incorrect info - as it turns out, any IAC count above 60 indicates the system is beyond its normal range. I’ve also found that closer to 30 counts is normal at operating temperature.

That said, is the 80 counts immediately upon start up? Because if so, it’s normal for the idle speed to be higher during cold start and the counts at temperature will be more helpful in determining what’s going on.

How would you describe the fluctuation? Is it changing quickly or slowly? What are the minimum/maximum RPM’s?

As for closed vs open loop - the ECM will only enter closed loop when the O2 sensor is at a certain temperature. Since you mounted the sensor further away from the manifold (stock location) it will take much longer to heat up enough to enter closed loop.

 

80 counts was at temp. It did go down when I fixed the vacuum leak, but I feel like there are still more leaks.
I did reset the IAC (a-b ALDL terminal reset process) and the counts lowered and the fluctuation is least often but still not perfect.
Since I reset the IAC I'm thinking I should check the timing to see if it's till at 0 at TDC with the wire unplugged...
It's a pretty fast fluctuation from usually 700-800, some times it holds at 800 and then the fluctuation comes back randomly.
When ever I try to set the idle or the timing the fluctuation is usually there which makes it really hard to set either accurately.
Would you recommend a 3 wire heated 02 sensor so it goes into closed loop? Right now it's not going into closed loop at all. I can do a road test at temp with my scanner and it shows open loop driving around town.

 

A 3 wire sensor isn’t a bad idea, but keep in mind that even with the sensor in the factory position, the engine most often reverts to open loop at idle because the sensor still cools down. The ECM is used to seeing open loop at idle, so a 3 wire sensor can be added after the cause of the fluctuation is resolved.

Does there seem to be any relationship between the fluctuation and using the brakes? When using the brakes, can you hear a whistle in the cabin? If yes to either, disconnect the brake booster hose and see if the fluctuation is gone - if so, the booster is leaking and will need replacement.

 

I don't hear a whistle but if I apply the brakes the idle increases, particularly if I pump them, the fluctuation will go up from 700-800 to 900-1000. Does that mean the booster is bad as well? If I disconnect the hose the fluctuation is still there.

 

It indicates the booster has a minor leak, but if the fluctuating is still present with the booster hose disconnected, then it’s not the source of the problem.

Are there any factory systems you deleted during the install? EVAP, EGR, etc?

What about any accessories you added?

 

AC was not transferred from the donor truck to the C10.
VSS is not being used (it is brand new though and still on the vehicle).
Stock headers were replaced with aftermarket ones, aftermarket air cleaner.
EGR is still there, brand new and brand new EGR solenoid.
Would it be helpful if I posted photos of the engine, sensors, etc?

 

Photos may be helpful - if you’re going to post any, please take at least one with the air cleaner removed and from the top, looking down on the TB so all sides of the manifold are visible.

Did you ever verify the timing as you stated in post #12? I don’t think the timing being off a few degrees would be an issue, but if it’s off quite a bit, that could be a contributing factor.

My next suggestion is to unplug and cap ALL vacuum ports to definitively rule out any vacuum dependent systems causing the problem. MAP sensor, EGR, EVAP, PCV, brake booster… any vac port you can find.

Also, next to the TPS, the input relied on most by the ECM is coolant temp. After the engine has cooled down overnight, turn the key on, engine off, and check the ECT reading on your scanner - it should be within 5* of ambient temperature.

Also check the MAP sensor while you’re looking at live data - it should be reading either zero or the same as barometric pressure (I can’t remember which).

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Cool, I'll take some photos.

I checked the timing and it was spot on at 0.

Thank you, I will try that and all of the other things you are saying to check. More soon as soon as I give this all a try/check!!!

 

I capped all of the vacuum ports and the idle fluctuation went away. I reconnected everything with it running, and the last thing I reconnected was the map, and the surge came back as soon as the map was connected. I just replaced the MAP though. Capping all the vacuum ports gave me a MAP code and #42 EST code on the scanner but these went away after everything was reconnected.

I didn't do the coolant temp test but my digital dash gauge reads engine temp from the second temp sensor and the the scanner reads the temp from the sensor by the water pump correct? These 2 numbers match so I'm assuming the coolant temp is being read correctly by sensors. I will double check in the morning.

Below I've attached all of the live data readings. Idle air is still 80ish. When I do the IAC reset, it won't really hold an idle to try to get the 550ish base idle. Could I try doing this with the MAP disconnected and maybe if I get the base idle lower the IAC counts would lower to the proper range?

 

First and foremost, never believe that a new part means it’s a good part. I’ve been burned by that line of reasoning more times than I care to admit.

If the MAP sensor is causing the fluctuation, but replacing it didn’t resolve the fluctuation, you first need to determine the MAP circuitry is functional.

The MAP works similarly to the TPS - in fact, they both share the same 5 volt reference circuit from the ECM.

At idle the MAP signal should be reading 1-1.5 volts and at WOT it should be approximately 4-4.5 volts. Look for the signal to be at or above 1.5 volts at idle, indicating to the ECM the engine is under load and requires more fuel.

If you have a handheld vacuum pump, use that to draw vacuum on the sensor and check for erratic readings.

Make sure the tube and fittings (or hose) that connect the MAP sensor to the intake aren’t leaking.

As for IAC counts, I’d wait and see if any problems are found with the MAP sensor/circuit. These ECM’s are basic but they do have some learning capabilities - and if the ECM determines the MAP signal is unreliable, it substitutes a fixed value based on TPS input. The ECM may be keeping the IAC counts high on purpose.