Idle Control Question — F9 DME vs Stock 121.DX DME on 1987 NA 944

[heliolps2]

[Idle Control Question — F9 DME vs Stock DX DME on 1987 NA 944]

Hey everyone, I’ve run into an interesting idle control issue and wanted to share my experience to see if anyone has dealt with something similar — or has thoughts on what's going on.

Setup:
1987 Porsche 944 NA (270k miles)

Refurbished head, new rod bearings, full reseal of oil pump and balance shafts

New injectors, vacuum lines, ICV, TPS (adjusted correctly)

Lindsey Racing MAF kit installed (with proper 28-pin conversion)

Innovate AFR monitor shows healthy readings and decel fuel cut-off (AFRs spike lean when coasting in gear, ~20.9)

DMEs I’ve Tested:
Stock DX DME (28-pin)

I can adjust idle smoothly to around 900 RPM warm with the idle screw.

No major issues—runs decent even with the MAF kit, especially with some FQS tweaking.

Focus 9 Technology Sport DME (F9 DME)

When using this unit, I had a high idle initially.

Installed the ICV bypass jumper to manually set idle, and even with the idle screw bottomed out, I can only get it down to ~1000 RPM warm.

AFRs are good, TPS is working, and there are no vacuum leaks.

What’s Strange:
When I swapped the Lindsey Racing MAF chip and daughterboard into my 951’s F9 DME, and switched the FQS to the 944 setting, the car ran great—no stumbling, solid idle control. I’ve since ordered a new Focus 9 DME specifically for the 944 to test that next.

My Theories:
The F9 DME may have different idle logic or base maps than the stock DX unit. It could be expecting more bypass air or a different throttle setting at idle.

There may be a difference in how the MAF signal is interpreted by the F9 DME compared to the stock DME—especially at idle.

Timing advance at idle may be handled differently by the F9, causing the idle to hang unless more air is bled in.

ICV control strategy might differ (even with the jumper installed, perhaps there's still some residual behavior affecting idle speed).

Questions for the Group:
Has anyone else noticed idle quirks with the F9 DME vs stock?

Is it common for aftermarket DMEs to require more bypass air or different idle screw settings?

Should I avoid the jumper and let the ICV run under DME control instead?

Open to all opinions or insights—really trying to get to the bottom of this.

Thanks in advance!
—Helio