The famous stumble...fixed?

Gotcha, I had the battery disconnected when I did the install work. I'll reset the battery again, hoping to have it dynotuned in a week or so. Thanks again for responding.

At what power levels & mod levels are you seeing this stumble? I'm on a fairly stock 05 LGT and I don't have it at all. I have a stg 2 downpipe, but running 13psi max right now, stock fuel pump, and no open loop delay & per cylinder timing comp additions.

 

Car hits stock fuel richness at 10.1-11.1 AFR no problem either. Just wondering if this is an issue with a new Fuel pump, another mod, xyz boost, or when you go over certain amount of power.

i'm not sure anyone knows what exactly causes it, but my current assumptions are injector changes (fuel pumps usually get changed then too). so stage 3+.

 

From what I've seen and read, I think per cylinder PW compensation tables (or really their reason for existing) are what's causing the bulk of what we see. once we change injectors (and sometimes pumps) we "change" this table, and I don't think we know enough about the table to be able to adjust it back to where it needs to be (or some just zero it out).

 

Actually, i'd be interested to see a datalog from you of fuel correction vs. RPM on your current ROM (would be better if fully stock, but you're close), and depending on what that looks like, doing the same datalogging with your IPW compensation tables zero'd out. Steady state logging, across the full range of RPM (closed loop).

 

zero-ing the tables should only induce a 10-12% fuel correction where you have harmonics, and a 2-3% difference in cylinders/injectors. not a great thing to cause, but i'd be interested in seeing it. and you should be fine with "slow" driving, plenty of time . after all, we all seem to be running with incorrect or no injector tables ourselves.

Edited January 13, 2015 by Flinkly

Mine is pro tuned stage two.

What other values would you like to see in the data log Flinky?

 

So the stumble mainly happens in closed loop, not open loop right?

Nightmaresmk, there's other things that could do it too (i was being a little shortsighted in my earlier post): fuel rail, headers, intake, major maf changes (tune), deleting/changing your ipw per cylinder comp tables, etc. I mean, if it is why the per cylinder tables are there, then anything that changes fuel flow or major airflow thru the engine, would foul up the stock table.

 

I actually haven't yet, but i should zero mine tables and redo my datalog.

 

Covert: engine rpm, ipw, afr, correction, maf (just to remove non-steady state), learning (if you have it enabled) , and before that, send over your rom so i can see the comp tables. And yeah, stumble seems to be low ipw accross the ful rpm range. So closed loop, steady state or slow acceleration/hill climb. Need to keep it steady state enough for nearly constant afr.

 

The stumble is a two parter, from what i've seen. The actual "engine bumpiness" is due to two injectors needing to stay open as much as 6% longer than the other two, while the overall harmonics seem to drop fuel flow vs. expectations causing hesitation while its correcting. Both extremes happen at the same time. The regulator or hose fix is just to minimize the %'s listed above, and maybe move them.

 

When plotting correction vs. rpm, the overall harmonics are visible, and i'd guess that they correspond nicely to what was in your tables (assuming you don't already have a bit of stumble from mods already).

 

But i'm speculating from only my experience and what i've read. No real data.

 

Sent from my XT1028

Edited January 13, 2015 by Flinkly

I'm redoing my entire fuel system under the hood. new rails id1000's and new lines with a new fpr.

Flinkly, got a lot and a hex for you to look at. I wanted to attach it here but the file limits and sizes are tiny. Will PM you for your email.

Hey fella's. Well since my last dyno tune I've upgraded to the following:

DW 65c

ID 1000 inj

IPT top feed conversion with line kit

JDM TGVs

BNR TD-06 20g

FMIC

38mm EWG setup

Perrin Turbo inlet

Greddy BOV full atmo

 

There's a whole mess of other things I've done but are irrelevant. So after the installs, I brought it down to Dave at Cryotune. He did his smoke test and found some small leaks. Anyways when we attempted to do the road tune he couldn't figure out what was causing the stumbling/hesitation effects. He went through all of hi parameters and said the only thing he could find was that the fuel pressure was inconsistent. Not is exact words mind you.

So after trailering the car home, I let the car sit for awhile, and about a few weeks later I finally get the chance to do some more troubleshooting. That's when I read about the fuel line and FPR swap. So I bought the FPR kit, installed it and still have the stumbling effect except that it dropped to between 2000-2500 vs the 2500-3000rpm range initially.

I'm wondering if I should see if it gets tuned out after I retune it or to just go ahead and get longer fuel lines too beforehand.

i'm not sure anyone has really tuned it out (not to say they aren't awesome at tuning). I've heard you can mess with MAF and a few other tables to try to smooth it over, but that's just masking the problem. IF it is fuel harmonics, you'd need bungs in your header at each cylinder, and to log AFR at each, simultaneously. to be able to re-create the correct IPW vs. RPM comp tables.

 

short of that, you could zero your comp tables, log where and how big your stumbles are, then try to adjust your comp tables globally to correct. would probably be better than running OEM comp tables, but could be worse for the engine than running zero'd comp tables.

 

so overall, best bet would be to get a few feet of hose...

 

p.s. i'll admit i'm surprised no tuners have attempted a 4 AFR sensor dyno tune to look at this problem at the source. expensive, but if they could fix it, worth it I think. then again, Cobb is getting people to pay $250 for the STI regulator (and maybe they have figured out how to fix it from there and just don't share...).

very interesting

well, I've only had time to look at the base ROM data.

 

looks like, with zero'd comp tables, (an older) stock Subaru should have a big stumble at 2400-2800 RPM, at 8ms IPW and above ("high" load). there are two smaller spikes (occurring at "low" load, 2ms IPW+) at 4.4k and 6.4k.

 

single injector compensating up to +10%, with a range of up to 10% (at one point, it's 0%, 3%, 6% and 10% for each cylinder at the same point in the table. should see quite the stumble. but this only happens at 8ms IPW and 2.4k rpm, and a little less so at 2.8k rpm for the same IPW. it quickly drops off to a 4-6% range max for the same RPMs and higher IPW.

 

other than that, it only goes to about 3% compensation max (although it varies per cylinder by 6%). there are two of these occurrences.

 

there is also an odd +6% at 800 RPM and 2ms IPW (lowest IPW and RPM in the table). looks like there's some low flow oddities in the system too. makes me wonder if the older subarus have injector "knee" tables, or if this was their way of correcting for that.

 

So for the "high" load spike, with zero'd tables, i'd expect a +5% correction factor, with two cylinders being off by +/-5%, and two being off by +1% and -2%. enter stumble. Same sort of occurance at "low" load at 4.2k and 6.2k, but with fueling being off by +/-3%.

 

And of course, any fuel system changes would move these spikes, and might make them better or worse. could also expose spikes not currently seen. I suspect the STI regulator reduces spikes, making them easier to compensate for with learning and correction factors.

 

now to see what the logs have to say about the "high" load spike, since I assume that was the only one captured (due to how I wanted the log). hopefully there were some steep hills...

That log was past a big hill (only started recording once IAM was past .5). I could get some more hilly drives in for you if need be .

Hello...thanks for the post. Been looking for write-ups, reviews and pics on fueling upgrades. I am currently in the middle of a ID top feed conversion. I thank you for the link to the dorman connectors, that's the last bit I need to finish my install...

Brian Thanks so much for this wright up, i ordered the regulator and quick connect fittings from your part numbers. i skipped the gauge and got everything for under $120.

im doing this conversion because i am about to also install the dw65 and dw 805cc injectors. hoping to nip the stumble in the bud before it happens to me lol

Hello...thanks for the post. Been looking for write-ups, reviews and pics on fueling upgrades. I am currently in the middle of a ID top feed conversion. I thank you for the link to the dorman connectors, that's the last bit I need to finish my install...

 

Depending on how your running the top feed fuel lines you may actually be better off getting an aftermarket regulator. You can use the one I posted with no issues, but if using the AN ID fuel lines, something like the aeromotive regulator with AN fittings may be nice.

 

Just a thought. Glad people are still trying this fix out. The guy who now has my old car with this set up loves it as well.

Performed this mod over the weekend. It definitely smoothed over one type of stumble my car had (I'm thinking there are probably different types of stumbles). Mine happened in warm weather when the engine was fully warmed up, often after being stopped or at low speed, then when you ease into the throttle to move, especially in 1st & 2nd gear, there would be a sort of hesitation "blip" about 1/3 second long. It was kind of annoying, and would sometimes happen several times during a ride. After this mod, so far I can't duplicate the issue

 

Also, it cleans up that crazy nest of brackets/hoses/etc and leaves behind something a lot simpler looking.

 

Thanks Brian!

The "fix" has been known for several years. All it really takes is elongating the fuel lines to reduce the "water hammer" effect (what is causing the stumble). Relocating the FPR can help with the really stubborn ones. Subaru even elongated the lines themselves in the 15 STi. We have performed the mod successfully more times than I can recall. That is why we keep 25ft rolls of fuel hose on the shelf, Lol. Good to see you finally got around to doing it yourself.

sub'd for solid info.

fyi i did what the op suggested in post #1 - basically swapping my 05 LGT FPR with an 07 STi FPR. If anything, I now have less fuel hose

yeah, i'll end up with equal or less fuel hose, as i'm doing the same. can't copy the OP's methods though, Subaru felt a more difficult to remove FPR location was in order, and other than having to go to some ridiculous methods to remove one of the two screws holding it on (Phillips head? come on Subaru...) I "had" to make this to adapt the new STI to my fueling:

 

http://i1191.photobucket.com/albums/z471/flinkly/Subaru/Fuel%20Pressure%20Regulator/P1020177.jpg

Possibly on your car moving the FPR off the manifold/fuel pipe was enough to alleviate the water hammer effect. We normally simply replace the lines with a set approx 12" longer (new cars have the FPR integrated into the fuel pipe as shown above).

yeah, i'll end up with equal or less fuel hose, as i'm doing the same. can't copy the OP's methods though, Subaru felt a more difficult to remove FPR location was in order, and other than having to go to some ridiculous methods to remove one of the two screws holding it on (Phillips head? come on Subaru...) I "had" to make this to adapt the new STI to my fueling:

 

You made that? Nice work!

I need to do this.