Hesitation 2400-2800 Fix

[Tuning Alliance]

Well I have worked out this issue some time ago and wanted to share my findings. You should have a super smooth ride like many of my customers.

 

This fix includes a combination of fuel and timing, however the primary issue is fueling. This sensitive region exists generally between 2400-2800 rpm, and between 0.8 to 1.4 load (depending on the setup). This also lies on the threshold of closed loop to open loop transition and when the engine crosses from vacuum to boost. As you may expect the engine wants more fuel as it begins to see small amounts of pressure. The oem tune CL targets 14.7-14.6 AFR up to 1.2 load. 14.0 at 1.4-1.6 load. Where the car is actually in boost 14.0 is not what the engine wants, thus is fumbles and hesitates.

 

The engine in fact wants more fuel. 13.9 @ 1.0 load, 13.6 1.2 load, 13.4 1.4 load. This is a pretty good starting point for the DIY crowd. Now you'll also need to match you primary fuel table to this table, or you should expect to see strange learned fuel trims. Additionally, refinement of the AVCS table, and maf curve should occur to optimize accurate fueling across the OL to CL transition.

[cryo]

I have noticed that this change works in some cars and not in others. Have you experienced this as well.

 

It seems some cars want the extended fuel line, some want closed loop comp table changes and some are happy with the fpr fix.

 

Then some just won't seem to get rid of the darn hesitation regardless thus the thousands of threads in a Google search.

 

Ironically on the 07+ ecu's we get a few more tables to play with over the 05/06 guys.

 

Funny that subaru implemented a much longer fuel line in the new fa20 motor and you can watch it vibrate at idle....

 

Dave

 

Dave

[Tuning Alliance]

Some seem to want a bit more fuel, less avcs, and less timing.

 

I've noticed little change with the added fuel line.

 

You mean the pulsation damper fix?

[Tuning Alliance]

If fuel pressure oscillation was a problem at a particular rpm and load in these cars, then I'd expect every car to have some range where it hesitates. Which I believe most cars drive fine.

 

Not sure why it would matter in the FA motor, as its only feeding the high pressure pump. As far as I'm aware everyone fires the fuel pump asynchronously to injector pulses, to avoid issues.

[cryo]

Well people have found a waterhammer effect which is why the extended line helps some *read very few* people but in my experience the extended line does not make a difference on the majority.

 

I was referring to the line kit supplied from cobb for the 08sti as well as some other manufactures due to them finding it was a pulsation/dampening mechanical issue as well. Some say the 08+ wrx was not effected but I have had people state the exact opposite which shares design with the lgt obviously. I truly feel that the ones that are effected are due to wear in another area of the system that makes the factory settings that work well for 95% of the cars causes the others to get this annoying stumble.

 

I honestly have found more often than not a sticky ocv,vac leak, bad plug,weak coil etc causing what most people attribute to the overall "subaru studder" which really creates an issue because everyone lumps a ton of different problems with the same symptom of a "studder". I have also had customers solve thier studder by accident while replacing turbos, intercooler etc which makes no sense unless they fixed a leak or something while in there and attribute it to something else.

 

I thought the same odd thing about the FA20 due to the mechanical really driving actual pressure but if you take a look at it you will see what I mean. It was obviously designed with pulsation in mind as it has two very odd curves for NO clearance reason. Very odd indeed.

 

Dave

 

 

 

Dave

[Tuning Alliance]

Yes, certainly many have actual mechanical issues as you mention, which magnifies the issue greatly (and causes others). In general in a good running car it will shutter lightly in this range with the stock settings.

 

My 11 wrx did it on the stock tune that lasted 2 days =), but it now runs super smooth with no mechanical change to the fuel system.

[covertrussian]

Here's the CL Target comp table from a 2004 FXT. When I owned that car it didn't hesitate at all, but it did have the FPR on the fuel rail. I checked the 2006 FXT tune and it's running a very similar CL Target comp table as the 04, while sporting the same motor as the Legacy. Just food for thought.

 

 

Also FYI, zeroing out the CL Fueling Target Compensation table will put the ECU into open loop mode in those ranges. Thus it's not a good idea to zero out that table, instead run a very small negative value, unless your trying to disable O2 sensor feedback for open loop tuning reasons.

 

See this thread at RomRaider. This post specifically:

 

Just FYI, NSFW - there's another trick on the GR's (not sure if it's a quirk, or also on other models). If you set the Closed Loop compensation tables to zero, you run in open loop permanently (that's why if you intend on not using these, you need to put in 0.0001 or something similar).

 

Another post about zeroing out CL zeroing and interesting results.

[Tuning Alliance]

Yes, I believe for LGT's that 05-06 are okay with the 0s, but canbus (2007+) do not like the zero's.

[Flinkly]

damn.

 

i've got a case of the stumbles, but this isn't my stumble. i've got something else entirely.

 

[Tuning Alliance]

damn.

 

i've got a case of the stumbles, but this isn't my stumble. i've got something else entirely.

 

 

Send me logs sir. What conditions is it occurring in?

[Flinkly]

Send me logs sir. What conditions is it occurring in?

 

At the moment i've got it stuck in open loop (to "simplify" logging/analysis) and with zero'd AVCS tables (to see if that was it). and it still exists. (i've tried the extra 2 feet of fuel line, STI FPR, and a few other tune things to fix it)

 

i plan on a limited run with per injector comp tables zero'd as well, as i suspect one of "these" tables (load comp suspected as well).

 

At the moment my mods are cobb catted DP, 850cc DW injectors, and DW65c pump. i've also logged fuel pressure, and while it's not absolutely constant, it is around 43psi.

 

after i unbrick my ECU, i'll get some logs. parameters to log?

 

oh, and it appears to correlate to RPM. but i'm obviously not sure what's ACTUALLY driving it.

 

EDIT: graphs showing percentage off of fueling target vs. things. Above one is lean and vice versa.

 

EDIT 2: I just need to pay you monies for a tune already...