Surging Idle, Long Starts, Erratic RPM Behavior, Bucking at low RPM.

[Thumper28]

I'm running a freshly rebuilt LS1 with a T-56; all nicely housed in a 2006 Lexus IS250 (Link). The engine is mostly stock, with the only significant power adder being a mild cam (.560/.560 Lift, 216° in/220° ex duration at .050" lift, 114 LSA) with valve springs to match. I have an inlet temperature sensor but no MAF. I'm controlling it all with a Holley HP EFI.

Since day 1 I have had the same annoying issues:

- Surging idle (sometimes to the point of stalling out)
- Long starts (about 2 long seconds of crank time and a good amount of revolutions)
- Bucking while cruising at low rpm
- Erratic rpm behavior when I rev the engine (very, and I mean very, light tap on the throttle causes the rpm to rise significantly and then falls back below the target idle before stabilizing.)

I took the car to a local tuner who installs a lot of Holley EFI units and seems like a very knowledgeable and capable tuner. He mainly focused on daily driveability and I saw a major improvement from his work. He added timing at low rpm to reduce bucking and made several changes including lowering the rpm decel rate and dialing in the A/C kick settings to improve idle stability. Now the idle is very stable 90% of the time but it still surges a little during the first few minutes driving the car and also bounces around the target idle rpm before stabilizing when I let off the throttle.

At first I was happy with the results because it was an improvement over what I had, but I still have way too many issues (although not as severe) for this to be normal. I also know with my setup there is no reason I shouldn't have rock solid driveability. I can't help but think that all of these problems are related and that there is some magical setting that will fix everything.

Please help, I'll try anything.

[Danny Cabral]

I have an inlet temperature sensor but no MAF.

Of course you don't have a MAF sensor...this is a speed density EFI system. Which MAP sensor do you have?

- Surging idle (sometimes to the point of stalling out)

https://forums.holley.com/showthread...e-Tuning-Notes (EFI Idle Tuning Notes)

- Long starts (about 2 long seconds of crank time and a good amount of revolutions)

http://forums.holley.com/showthread....6254#post36254 (Good Starting Advice Thread)

- Bucking while cruising at low rpm

It's most likely still too lean and may require more timing advance (not WOT).
Read pages 4-10 of this Edelbrock EFI tuning manual, it's very good:
https://edelbrock-files-v1.s3.amazon...ers-manual.pdf

- Erratic rpm behavior when I rev the engine (very, and I mean very, light tap on the throttle causes the rpm to rise significantly and then falls back below the target idle before stabilizing.)

Adjust the IAC Hold Position & Ramp Decay Time. Also, temporarily disable the Decel Fuel Cutoff to ensure it's not the problem.

Now the idle is very stable 90% of the time but it still surges a little during the first few minutes driving the car...

Cold start & run quality is greatly influenced by tuning the Coolant Temperature Enrichment, A/F Ratio Offset & Target Idle Speed (RPM) Tables.

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We can definitely discuss any of these issues in further detail. I just wanted to provide you with some preliminary information.

[Thumper28]

Thanks for responding. I'm running a new Delphi PS10002 MAP sensor. I'll start working down the list and I'll post as I make progress.

[Thumper28]

Adjust the IAC Hold Position & Ramp Decay Time. Also, temporarily disable the Decel Fuel Cutoff to ensure it's not the problem.

I have been playing around with information you provided and have come a long way. The bucking issue was positively influenced by both timing and fuel enrichment, however, a timing advance based on coolant temperature had the biggest impact and I have managed to reduce my bucking by at least 75%.

Moving on, I needed to figure out the hanging RPM issue. IAC Hold Position and Ramp Decay time did not seem to effect the hang, neither did the decel fuel cutoff. However, when I unplug my IAC valve, the hanging completely goes away. Downside, my car won't start without feathering the throttle, and idles very poorly until the car warms up. Once the car warms up, it's been running so well that I can't even tell that the IAC valve is unplugged. How can I get the best of both worlds, IAC help during start and warmup and then parked once warm?

[Danny Cabral]

...however, a timing advance based on coolant temperature had the biggest impact and I have managed to reduce my bucking by at least 75%.

This leads me to believe you're not using enough timing advance in the Base Timing Table. The engine should run OK, without the additional "Timing vs Coolant Temperature". This is only to further optimize the warmup period, not to be an absolute necessity. I see most people don't even tune this table.

Read pages 4-10 of this Edelbrock EFI tuning manual, it's very good:
https://edelbrock-files-v1.s3.amazon...ers-manual.pdf

Once the car warms up, it's been running so well that I can't even tell that the IAC valve is unplugged.

Good, this indicates you have the throttle blades properly adjusted.

Moving on, I needed to figure out the hanging RPM issue.

IAC hanging issue upon starting? If so, adjust the Startup IAC Position "Hold Time" & "Decay Time".

How can I get the best of both worlds, IAC help during start and warmup and then parked once warm?

At startup, adjust the IAC Parked Position (%).
During warmup, adjust the Target Idle Speed (RPM) at various RPM.
Once hot, enter the Target Idle Speed (RPM) the same as your mechanical idle RPM speed.

EFI Software Help Information/Instructions:
‒ On the top Toolbar, click "Help" & "Contents". This opens all Help topics.
‒ When navigating the software, click "Help ?", drag it to any parameter and click again.
..This automatically opens the definitions for that specific parameter.
‒ Tuning information can be read by clicking the F1 key, when you're viewing any screen.

[Thumper28]

Danny, I have come a long way since my last post. I realized that my TPS was acting erratic, and after replacing it, I have a much more stable idle. I also got rid of the timing modifiers and adjusted my Base Timing Table so I can cruise at low speed without bucking.

I still have not worked out the IAC issues. I get consistent RPM hanging, and it has gotten even worse to where my RPM will completely stick at 2000 until I re-start my engine. I'm starting to think that I have a bad IAC, but in the meantime the car has been running great with it unplugged. I'm inclined to run without the IAC, but if I turn on my A/C without the IAC active, my engine will stall out 9 out of 10 times. Not immediately, but it will bounce around the target rpm and get worse, until it can't recover.

Does the A/C IAC Kick input only control the IAC, or does it affect other parameters as well? I was trying to figure out how I can up my target RPM, based on my A/C IAC Kick signal, but I could not find that feature in the software. What could I try next?

[Danny Cabral]

Does the A/C IAC Kick input only control the IAC, or does it affect other parameters as well?

It's just for the IAC motor. It has a momentary "IAC Kick" function, so you don't feel the A/C compressor turn on.

I still have not worked out the IAC issues. I get consistent RPM hanging, and it has gotten even worse to where my RPM will completely stick at 2000 until I re-start my engine.

Sounds like the TPS Position isn't always returning to 0%, and activating the IAC Hold Position.
The TPS Position must be at 0% anytime the engine is at idle. This must be corrected as soon as possible.
You may have to slightly back-off the idle speed screw (after a TPS AutoSet), so it's not on the verge of 1%.
The TPS isn't always returning to 0% (it's between 0% & 1%), and activating the IAC Hold Position.
Ensure your throttle linkage is returning freely, and use a stronger throttle return spring.

Verify the IAC motor harness connector pin-outs from this IAC tech page:
http://forums.holley.com/showthread....neral-IAC-Info (IAC Information)

32.4.1 IAC Ramp Down The Idle Air Control (IAC) motor is a stepper motor located in the throttle body that controls the idle speed of the engine by metering air. It also operates during engine cranking and when the engine returns back to idle. The settings here can adjust how that functions. The IAC moves from a position of 0% (fully closed, no air added) to 100% (fully open, maximum air flow). NOTE: If you change these settings, you must comprehend the function they perform. Contact Holley Tech if you have any questions. 32.4.1A IAC Hold (IAC Hold Position) This is the position the IAC motor will “hold” or “freeze” at when the TPS moves above idle (when TPS becomes greater than 0%). If it's too high, the engine RPM will “hang” and not return to idle. 32.4.1B Ramp Decay (Ramp Decay Time) This is the time (in seconds) it takes for the IAC to decay from the “IAC Hold Position” back to a “0%” position (Fig 76). It is a linear decay. 32.4.1C RPM Start (RPM Above Idle to Start Ramp) This value is the RPM added to the target idle speed that the IAC will automatically start to ramp back down to idle (Fig 76). If this is too low, the engine RPM will “hang” and not return to idle. 32.4.2 IAC Speed (Advanced Idle Control) This drop-down is used to select the type of IAC motor application that is being used (Fig 77). This selection drives the background parameters that control the IAC motor. These parameters have been fine tuned for each of these applications, eliminating the need for the user to perform further modifications. 32.4.3 Idle Spark Idle spark is a feature active only when the ECU is controlling timing (only when using a small cap computer controlled GM HEI with the TERMINATOR™ systems). If enabled/checked, the ECU rapidly changes the timing at idle to help maintain a more stable idle (Fig 78). 32.4.4 Startup IAC Position These parameters control the position of the IAC when the engine is cranking and immediately after it starts. 32.4.4A IAC Parked (IAC Parked Position) This is the position the IAC motor will be at during cranking and immediately after the engine starts. If it is too high, the engine will be at too high of an RPM once it starts. Too low and poor starting will result. Note that this is a temperature based table. The value changed in the handheld offsets this entire curve (Fig 79). 32.4.4B Hold Time (Startup Hold Time) This is the amount of time that the IAC will remain at the “IAC Parked Position %” (Fig 79). 32.4.4C Decay Time (Startup Decay Time) This is the amount of time for the IAC to decay from the “IAC Hold Position” back to a “0%” position. It is a linear decay (Fig 79).

[malikith]

If it's hard to start after it's warm, then you need to adjust the Startup Enrichment parameters.

[Danny Cabral]

If it's hard to start after it's warm, then you need to adjust the Startup Enrichment parameters.

And possibly the IAC Parked Position % too.
Startup Enrichment parameters can only be tuned after the Learn Table has finished self-tuning.

I added some important IAC related information to post #7.

[Thumper28]

I replaced my IAC hoping that this was all caused by a bad unit, and unfortunately, that was not the case. I played around with the settings highlighted above and still no luck.
RPM above idle seem to be working well. It ramps up for a few seconds after start, and then drops back down to my target rpm as programmed. At target idle, my IAC is at 15%. It doesn't hang until I start driving. Once it decides to stick it won't drop back down, until I restart the car. I'll be at a dead stop and the rpm will be around 1500-2000, TPS is at 0%, and IAC is at 50%. That's what I'm confused about. Why would the IAC stick at 50% when I'm clearly at 0% TPS and well above the target RPM?

[Danny Cabral]

That's what I'm confused about. Why would the IAC stick at 50% when I'm clearly at 0% TPS and well above the target RPM?

What is your "IAC Hold Position", "Ramp Decay Time" and "RPM Above Idle to Start Ramp" set at?
Also, if you manually pull up/return on the throttle pedal/linkage, does the RPM hang stop?
You may have to slightly back-off the idle speed screw (after a TPS AutoSet), so it's not on the verge of 1%.

[Thumper28]

What is your "IAC Hold Position", "Ramp Decay Time" and "RPM Above Idle to Start Ramp" set at?
Also, if you manually pull up/return on the throttle pedal/linkage, does the RPM hang stop?
You may have to slightly back-off the idle speed screw (after a TPS AutoSet), so it's not on the verge of 1%.

IAC Hold Position - 50%
Ramp Decay Time - 2 sec
RPM Above Idle to Start Ramp - 100 RPM

I like the backing off the idle speed screw idea. I'm out of town, but I will give it a shot on Thursday when I get home.

[Danny Cabral]

IAC Hold Position - 50%

Well, there's your problem. It's also set too high; try 20%.
The TPS isn't always returning to 0% (it's between 0% & 1%), and activating the IAC Hold Position.
Ensure your throttle linkage is returning freely, and use a stronger throttle return spring.

RPM Above Idle to Start Ramp - 100 RPM

100 or 1000 RPM? Should be 1000-1200 RPM.

I like the backing off the idle speed screw idea.

Use this method as a last resort (for finicky throttle body linkages only).

[Thumper28]

Danny, the hanging RPM issue has been completely resolved and IAC is working. Thanks for all your help. I still get a little overshoot and fluctuation when I first turn on my A/C, but it levels itself out. Now that I have a working baseline, I can tweak some of the parameters to see if I can minimize the fluctuations. Thanks, and I'll keep you posted on the progress.

[Danny Cabral]

Now that I have a working baseline, I can tweak some of the parameters to see if I can minimize the fluctuations.

Many of us have found that the Slow - "Advanced Idle Control" promotes the best idle quality.