Car info: I have a 05 STi and my mods are as follows: Cobb Intake, Perrin Turbo Inlet, Cobb TBE, Borla Headers, Helix 860cc Injectors, walbro 255lph fuel pump, NGK 1 step colder plugs, GM Boost Control Solenoid and a Cobb AP with a recent Protune. I have about 5000 miles on the clock. I started modding the car at 2000 miles when I went to Cobb Stage 2. At 4000 miles I installed the rest of the above mentioned parts and got the car ProTuned.

The problem: In the past week I noticed that my car has some kind of strange idle hunting issue. At full operating temp the RPM's slowly go up and down at idle. This hunting takes place between ~550 and ~720ish RPMs (data pulled using ECU explorer). The tach hardly moves but you can hear the motor changing speeds. I also have an oil pressure gauge and you can clearly see the oil pressure needle slowly rising and falling about .2 bar per sweep. Otherwise the car seems to run fine on all other accounts. No CELs, the power seems fine (pulls like a beast), etc. This issue is highly annoying to listen to and it just plain feels strange while sitting in the car so I really want to fix it.

At first I thought I may have over filled my oil and I have checked it three times now with the car at full operating temp and it seems spot on at the full mark. I used ECU explorer to investigate my idle AFRs and they seem fine (readings float a little but remain +/- .2 from 14.7). I also checked all the vac/crank case hoses that I touched while installing parts a few weeks back but I cant find anything. This seems to only happen with the car is at full operating temp because it is only present when I let it idle more than 15 minute or once the car has been driven and then let idle. Any ideas what it my be or what else I should check?

Your anal that's the problem

yea i think its normal, mine seems to idle funny 2 but its just the way the ej motors are.

This is Subaru number four for me and none of the previous ones did this. I know the boxers dont idle like inline motors, this is something else. I just started doing this about a week ago. Before then the idle was fine. It almost sounds like an aquarium water pump thats surging because the water is getting to low or something.

With the combination of the intake, header, and injectors, I would expect some rough idling. This may best be solved by bumping the idle up about 100 rpm or asking the tuner to work on the injectors a bit more.

put an earthing kit on it and ground it better, that may help too...

With the combination of the intake, header, and injectors, I would expect some rough idling. This may best be solved by bumping the idle up about 100 rpm or asking the tuner to work on the injectors a bit more.

Well, I bumped the RPM up 100 and everything is the same. Just for the heack of it I bumped it up a second 100 (200 total) and it was still exactly the same only 200 RPMs higher.

So here's the question. Should I just ignore this or is it a sign that anything may be wrong?

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Tom - for as often as you drive the car just live with it, lol.

my car idles fine..

have you seen this stickie?

http://forums.nasioc.com/forums/showthread.php?t=886454

I'm beginning to believe that this issue (Ive had it quite a bit) is the result of tuning for larger injectors using the MAF curve.

Under full throttle, fueling is fine because it works through both the injector size and the MAF transfer. But off of WOT, there are many functions, such as idle oscillation, that likely only take into consideration the injector size (and not the compensating MAF transfer). When the ECU thinks its making a small sweep using a small additional or small reduction of dutycycle, that actually makes for much more of a swing in fuel than the ecu expected, creating an oscillation.

Find out if your tuning involved the *actual* injector flow being put in the injector flow variable.

The ECU has its own ideas as far as how to damper idle instability. Tuners have no access to that algorithm, and as a result, changing other variables are likely to futz it up if they arent independently accurate.

-chris

I'm beginning to believe that this issue (Ive had it quite a bit) is the result of tuning for larger injectors using the MAF curve.

Under full throttle, fueling is fine because it works through both the injector size and the MAF transfer. But off of WOT, there are many functions, such as idle oscillation, that likely only take into consideration the injector size (and not the compensating MAF transfer). When the ECU thinks its making a small sweep using a small additional or small reduction of dutycycle, that actually makes for much more of a swing in fuel than the ecu expected, creating an oscillation.

Find out if your tuning involved the *actual* injector flow being put in the injector flow variable.

The ECU has its own ideas as far as how to damper idle instability. Tuners have no access to that algorithm, and as a result, changing other variables are likely to futz it up if they arent independently accurate.

-chris
Now there's a topic which many have written on, and which you can search.

Now there's a topic which many have written on, and which you can search.

And find writing about the subject... or UNDERSTANDING of the subject? ;)

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I feel you're pain as this is the exact same symptons I 've with my FXT. idles rough when at operating temp and worse as temps rise. Mine started after GT Spec Headers (wrapped) went on. If yours stutters during part throttle driving also it may be a O2 Sensor. I replaced mine and its the same. Runs pefectly fine at cold starts and during WOT pulls. But part throttle and idle are erractic as hell. Someone suggested I adjust Idle Speed Maps as this has known to fix the issue.

I really hope you've sorted this out?