I have had weird idle surge issues with my 1999 1.8 FMII and Hydra from the first day I purchased the car. I stumbled across an old thread on M.net and found some idle settings that worked nearly perfectly for me. I still need to dial in the cranking fuel trim for the appropriate temps, but the car idles the best it ever has with these settings. The Hydra is notoriously tough to get the idle right with and I wanted to post this here as I searched for a long time before coming across this info. Hopefully some other people on here may benefit from this.

Source: https://forum.miata.net/vb/showpost.php?p=7534362&postcount=52

To quote that post...

Joining the conversation a bit late. I've had my Hydra 2.7 installed since November and had issues with both idle surge and droop. Following are summary of changes I've made (or tried and backed out) to deal with both:

• AC Clutch Idle Speed Target Trim from 200 across the range to 208 at -20 to 120 at 90 in 8RPM/10 degree increments (Tuning Maps, Idle Speed Control) - keeps idle closer to normal when AC kicks in and engine at or close to operating temperature
• Decel Fuel Cut Lower Limit UNCHANGED - tried ~200 RPM across full map (Tuning Maps, Engine Trimming) - keeps engine from fighting decel in gear before clutch in - had to return to default as lowering this setting triggers idle surge
• ISC AC Clutch Anticipate from 10 to 5 (Settings, Idle Speed Control Settings) - keeps idle closer to normal
• ISC Error Ignition Trim from zero across the range to 14 to -16 in 2.0 increments from -280 to +320 (Tuning Maps, Idle Speed Control)
• ISC AC Fan Anticipate from 8 to 6 (Settings, Idle Speed Control Settings) - keeps idle closer to normal
• ISC Return Anticipate from 8 to 3 (Settings, Idle Speed Control Settings) -
• ISC P Term from 150 to 100 - settles down idle surge
• ISC Thermofan Anticipate from 8 to 6 (Settings, Idle Speed Control Settings)
• Minimum ISC Valve Duty Cycle from:

14.9 14.9 14.9 14.9 14.9 14.9 14.9 14.9 14.9 14.9 14.9 14.9 14.9 14.9 14.9 14.9
to:
37.65; 37.25; 36.86; 36.47; 36.08; 35.69; 35.29; 34.9; 34.51; 33.73; 32.94; 32.55; 32.16; 32.16; 32.16; 32.16

• Moving Vehicle ISC Target Trim from 200 to 50 to allow idle to return to normal more quickly (Settings, Idle Speed Control Settings)
• Post-decel Cut Fuel Trim from 10 to 0 across the temperature range (Tuning Maps, Fuel Trimming)

Upping the Minimum ISC Valve Duty Cycle went a long way to reducing idle droop. Still have a bit sometimes, but where it used to be so bad the car would occasionally stall clutching in for a stop sign, now it just very occasionally droops a bit and recovers. Also, dealing with the droop took away the main trigger for the idle surge. Idle behavior is very close to stock now. I have a bit of RPM fluctuation visible on a digital gauge, but nothing I can detect on the tach or hear in engine note or feel.

Unrelated to idle, I have also made the following changes:

• CAN Protocol from CAN disabled to Basic OBDII ECU 1 (for ODBII ScanGaugeII gauges)
• Knock 1 Amplifier Enable from off to on
• Knock Threshold from:

5 0.49 0.53 0.57 0.63 0.67 0.71 0.75 0.8 0.84 0.84 0.84 0.84 0.84 0.84 0.84
to:
5 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20[/QUOTE]"

 

Much appreciated ! I've been fighting various idle and return to idle issues . I'll try these mods tomorrow and see if they improve things. Fredb

 

I used all of the settings but the ones mentioned at the very end and the AC settings (no AC). The car now idles like a stock Miata, but at a slightly higher rpm. I think I hover around 950 now and it is perfect!

 

Cats awarded , much improved over what I was running. I'd feel better if understood which changes did what and why , but definetly improved return to idle . Much thanks . Fredb

 

Cheers and yeah, not totally sure what the hell all that did but it seems to work very well.

 

Many thanks, Luke, for the find and posting here. Even on a 1.6, the ISCV has responded well to the new settings. Decel has improved as well.

 

I tried applying these settings to an 1.8 FMII hydra 2.5 setup as far as the settings are the same, but this left me with an idle speed of 1500. After some tinkering I got it to a point where it is sort of acceptable, I only get some idle dipping on startup and when idle control takes over from under ~1500 rpm. My settings: P=240, I=120, D=100, MIN PWM=16, MAX PWM=90, Valve steps=25.

What are your PID settings, or do these settings not exist in 2.7?

 

 

I'm at P=100, I = 20, on 2.7.

 

I purchased my hydra in 2017 and the idle has surged in my supercharged Miata the entire time. No issues, just embarrassing at stop lights for the most part. I could not get anyone to help.

I put in everything except the A/C stuff since no A/C, and it now idles awesome! It idles over 1050, but it idles. Now I have to get the idle down, but I’m so happy to find this.

 

Good stuff!

 

Did you get your idle lower? Like I said mine is high and I would like to get it down to more normal. If you got it better, what did you do.

 

There is a setting for the idle rpm somewhere in the hydra software. It's been a while. I could only get down to 950-975 rpm at idle before having issues, so you aren't that far off.

 

I need to give this a try... my idles issues have been mostly sorted compared to what I used to have to deal with, but I think it's time to boot up the old laptop with the serial port connection and check my settings against this post's suggestions. Thanks!