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2013 STI idle starts at 15 AFR, just goes richer and richer

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I am trying to tune a 2013 STI - forged build EJ25, wants to run on speed density on 98 fuel.Essentially the problem is that it starts and runs, starting at 15 AFR, but then just gets richer and richer, before stepping down to 10:1 (which is where I have turned it off). ECU is commanding 1.79 ms of fuel consistently, as you can see in the log but it just keeps getting richer.Using MerpMod as customer wants to later use flex fuel (attached is the latest file with definitions so it can open in Ecuflash, and latest log).

Switched off codes for TGV delete, Air pump delete, rear o2 delete, Maf delete.

Changed IAT settings to provided settings

Changed MAP sensor settings to provided settings (its a 5 bar unit) - I can attach the gradient and offset calcs if it helps.

Has an EZ33 throttle, NA intake manifold.

Twin rail setup, fuel system has two pressure dampeners, and is providing consistent pressure less vacuum. Setup at 3 bar base pressure with the vacuum off.

Big cams - i.e. 272s.

Tried switching off low pulse width adders, rear o2 compensation, removed warm up enrichment, removed closed loop learning. I have confirmed the onboard o2 sensor is close with my tailpipe wideband.It wouldn't start until I upped the deadtimes to 1.5 times what was provided by Raceworks (inj223). They tested about about in between what was supplied by Raceworks and that 1.5 times figure. I got the flow test results also, they were very consistent, and they tested to 1430cc at 3 bar, although they are 1550cc. I've not updated the deadtimes to what they tested at (slightly different), or the actual flow (1530cc vs 1430cc) in the map yet - but I have my doubts that will actually resolve the issue as the ECU is commanding the same fuel but they are just getting richer.

Any thoughts on where to go next?

Attached Files

I'm not familiar with the fuel model on those but it sounds like the air and or water temperature compensation is not working and as both rise and the charge density reduces, your consistent fuel delivery becomes progressively more mismatched to the air mass flow.

Thanks for the suggestion Michael. IAT goes from 38 to 37 while this is occurring as shown in the log I provided - I doubt enough to cause a drop in the AFR from 15 to 10. The coolant temp was pretty constant also. This effect was occurring over many repeated runs.

Do you have any other ideas?

Since this is reflashing a stock ECU -- are you sure the ROM definition is correct? Do the values in the all the tables you can access look like they make sense? A mis-alignment of data location could mean you aren't changing what you think you are, and are intead corrupting some other table.

Did this ever work, or has it just starting this behavior?

Thanks for your reply David.

Yes, all the values in all of the tables make sense, the rom definition appears to be correct.

This is the first time I've run this car on MerpMod, and indeed the first time I've used MerpMod. I've mainly been using Dimemod and Carberry before on the subies. I am also suspicious it might be a MerpMod bug.

The car was running fine on a retuned stock flash (albeit with an altered MAF curve due to a larger COBB intake) prior to the customer making a whole bunch of mods at the same time - 1550cc injectors, upgraded IAT sensor, upgraded MAP sensor, larger throttle body.

My next thought is to take it back off SD mode and convert it back to MAF to see how it runs / if the same problem occurs. Do you have any other ideas for tests that might highlight where the issue is?

Are you monitoring the rail fuel pressure while this is happening?

A rather(!) long shot, but if the regulator and/or rest of the return side can't bleed off the pressure because of flow limitations, the pressure could be ramping up.

Hey Gord, thanks heaps for the input. This was something I suspected too, but forgot to list :)

Not monitoring as such, but visual on a digital fuel pressure gauge in cabin, and an old school analogue gauge on the turbosmart fpr under the bonnet. Both agree, and both are expected value/staying as they should (i.e. base fuel pressure less vacuum at idle).

Thanks all for your help so far.

So to give an update - I converted the same MerpMod tune to use the MAF rather than the Speed Density. Idle then stayed on target, and it ran and drove fine.

I still need to figure out where the SD settings or maybe even the IAT/MAP sensor/wiring is not working properly - but at least it narrows it down.