Leaning out under boost

[j_ryan]

(History of the car: 2005 LGT 5MT, SPT intake, perrin turbo inlet, catless up & down. I got this car a couple of months ago from a friend. He bought it with an exhaust valve problem at 224k miles. He pulled the engine and fixed several other small problems. He did a few mods (which I plan to reverse like the exhaust), but it has not been tuned yet. Right now it's on Cobb's stage 1 OTS map. I've been reading a lot and trying to learn as much as I can until I have time to take it to the tuner. I've put a couple of hundred easy miles on it and fixed a few driveline issues. It has developed the first real problem that I haven't managed to solve yet. I'm in the middle of closing on a house and I don't have the time and money to take it to someone.)

 

The car had been driving as well as I would expect for an OTS map - not great, but I'd been taking it on my 10 mile commute a few times per week. It started overboosting up to 17-19lbs, but AFRs were generally good from 11-15 depending. That stopped, and boost seemed to peak around 12.5. Now, as soon as the car hits 2+lbs, it leans out. I've seen it hit 17:1 on the accessport's gauge (don't have a wideband yet). Obviously running lean under boost is a huge problem.

 

My first thought was fuel issues - I have a new Walbro 255 that I can install but haven't yet. But after some googling it looks like a vacuum leak could be the culprit? I plan to remove the intercooler today and check the blue T fitting. All other vacuum lines visibly look ok, but it hasn't been pressure tested yet. How likely would it be that a vacuum leak could let the car idle great and run okay while cruising, but lean it out under boost? I've noticed that AF learning is a lot higher than it used to be - around +10 or so now while cruising.

 

 

Now, with all that being said - I know the car is kind of a mess right now. I suspect the catless DP with a crap tune is causing all kinds of weird things. I won't have time to tackle any major issues until the fall, but I'd like to keep the car driveable in some capacity until I've got the money to buy a new short block. Thanks for any advice you guys may have.

 

EDIT: Forgot to mention - no CELs aside from a P0420 but that should be the lack of cats, right?

[whitrzac]

FPR vacuum line probably came off.

[serx7]

although there might be other issues, if you have a DP you are considered to be at Stage 2 and should probably at least be on the Stage 2 OTS, not Stage 1

[Tuning Alliance]

It could be a number of things unfortunately. Serx7 is right if you have a DP you should atleast be on the cobb stage 2 ots map.

 

If you want a custom etune, shoot us a PM we have some steps that can help you identify leaks also.

 

-Brian

[j_ryan]

Is the FPR vacuum line the one with the T fitting right under the intercooler? I've tried the OTS stage 2 tune, and the car didn't seem to like it. I'm not shooting for max power - I just got the car for a good deal and I'd like to daily it for a while.

[serx7]

The FPR vac line runs from a nipple on the driver side intake manifold - the nipple points the vac hose off to the side toward the fender and then makes a quick turn backwards toward the firewall.

 

I took the photo of the intake manifold attachment point linked below, after zip-tying it on the side of the highway the 1st time it blew off during a datalogging pull. AFR blew past 14.4 as measured by the stock O2 sensor in the 1 sec it took me to realize the 'psst!' i thought I heard under the hood might've been the FPR hose. Yes, stupid of me to have not zip tied it way sooner than I did. Unfortunately triple count FLKC registered a fraction of a sec before I let off (3900 rpm @ 19.6 psi), but fortunately the (built) motor was fine.

 

https://www.dropbox.com/s/lqol15ksdg0e2y3/File%20Aug%2001%2C%208%2014%2025%20PM.jpeg?dl=0

 

Also, I just realized your OP indicated your car has an SPT intake. Assuming that's still installed, you really should be getting a tune (after determining if you have other leaks that might be contributing to fueling issues). Even the Cobb Stage 2 OTS doesn't take into account MAF scaling effects that arise from having an aftermarket intake installed and I don't expect that running the Cobb SF OTS is an appropriate stop gap. The SPT intake is probably flowing more air than the stock airbox and the non-SF OTS MAF scaling (which is probably a bit rich in the 1st place), is probably still not sufficient to report the increased airflow, which is likely contributing to running lean under boost. You really should stay out of boost until you get a tune. The prior suggestion/s to run as Stage 2 @ this point is less about maximum power at this point, and more about protecting your motor from lean AFR damage.

[fahr_side]

The SPT intake is horrible. It doesn't flow any more than stock and it creates turbulence at certain rpm and air flow points that make a good MAF reading impossible. Ditch that and put the stock intake back on ASAP.

 

Check the FPR reference line is attached and not leaking as mentioned above, and change out the fuel pump to eliminate the question on whether your stock fuel pump is old and tired.

[j_ryan]

It turned out to be a faulty MAF. Swapped it out with one from my buddy's WRX, and instantly it was running perfect. Boosting straight to 15psi with AF scaling perfectly. Went ahead and did the new fuel pump and checked for vacuum leaks while we had it in the garage.

 

Next project will probably be replacing the turbo. I don't have a service record for when this one was installed, but I have to assume it's not the original VF40 at 226k miles.

[serx7]

Good to hear. Are you still running the intake, DP and exhaust on the Stage 1 OTS? Still don't recommend that. Fuel trims are for CL operation and w/o a wideband I wouldn't be boosting at all w/o seeing what your AFR looks like in OL.

[j_ryan]

No, I changed it to stage 2 OTS. With the new MAF it's actually running properly and making great power. That MAF must have been failing for a while because this car has never ran this well before. I've data logged several different conditions and all the numbers that I've seen (obviously not an experienced tuner) are spot on. No sign of knock anywhere and A/F is scaling perfectly. I don't have a wideband yet, but looking at A/F on the accessport and the numbers are great. I know a wideband is more accurate and I do plan on installing one.

[serx7]

Oh ok, Stg 2 OTS is def at least where you want to be w/ the DP on.

 

Note fahr_side's comment above on the intake you have.

 

Note also that the stock O2 sensor can't really be used for measuring AFR under boost, esp above WG boost. On my car, the stock O2 bottoms out at its 11.14 reading when my WBO2 reads 11.8. It does seem to agree w/ my WBO2 until ~12.3 and diverges as AFR drops below that.

 

During a 3rd gear WOT pull from 2500 rpm, my stock O2 bottoms out at 11.14 by 2800 RPM, under ~6.5 psi boost. This happens < 0.5 sec after I mat the pedal.

 

During a 3rd gear WOT pull from 3000 rpm, the stock O2 bottoms out at 11.14 by 3100 rpm, under 7 psi boost. This happens < 0.3 sec after I mat the pedal.

 

So basically, I couldn't use the stock O2 reading more than ~0.5 sec after I hit the gas even if I wanted to.

[j_ryan]

Oh ok, Stg 2 OTS is def at least where you want to be w/ the DP on.

 

Note fahr_side's comment above on the intake you have.

 

Note also that the stock O2 sensor can't really be used for measuring AFR under boost, esp above WG boost. On my car, the stock O2 bottoms out at its 11.14 reading when my WBO2 reads 11.8. It does seem to agree w/ my WBO2 until ~12.3 and diverges as AFR drops below that.

 

During a 3rd gear WOT pull from 2500 rpm, my stock O2 bottoms out at 11.14 by 2800 RPM, under ~6.5 psi boost. This happens < 0.5 sec after I mat the pedal.

 

During a 3rd gear WOT pull from 3000 rpm, the stock O2 bottoms out at 11.14 by 3100 rpm, under 7 psi boost. This happens < 0.3 sec after I mat the pedal.

 

So basically, I couldn't use the stock O2 reading more than ~0.5 sec after I hit the gas even if I wanted to.

 

I have the stock airbox, but haven't taken the time to replace the SPT intake yet. The SPT sounds amazing and I wish I didn't have to replace it :/

 

Yeah I noticed the AF reading bottoms out at 11.03 under boost. The accessport gauge should be accurate enough to at least know if its leaning out though, right? I'm still kind of babying the car right now until I get moved and have more money to throw at it in a couple of months.

[serx7]

The Grimmspeed CAI sounds great (I have it too), and its performance is pretty well documented.

 

Swapping the stock intake/filter back on ASAP would probably be a wise thing to do. I would think Cobb OTS maps are conservative enough that you wouldn't -need- to have a WBO2 to monitor things as-is. Until you get the stock intake back on however, your MAF readings may be incorrect as fahr_side mentioned earlier.

 

Re: your ? about using the AP gauge to monitor for lean AFR:

 

The AP gauge reads the stock O2 sensor. I don't know how your stock O2 correlates w/ a WBO2 shortly before it hits its 11.03 floor reading, but like I mentioned above, my stock O2 reads its 11.14 min value when my WBO2 reads 11.6. My Commanded Fuel Final is richer at 10.7 at higher loads and from midrange RPM on up. The way I have my timing tables set up, it'll knock intermittently if WBO2 measured AFR gets into and stays in the 10.95-11.1 range for more than a couple hundred RPM up top. Since the stock O2/AP gauge doesn't start to move above its 11.14 reading until the WBO2 reads 11.6 (or leaner), I'd probably either have a potentially toasted motor or at least a noticeably dropped IAM before the AP gauge tells me anything useful about an overly lean AFR.

[Tuning Alliance]

The Grimmspeed CAI sounds great (I have it too), and its performance is pretty well documented.

 

Swapping the stock intake/filter back on ASAP would probably be a wise thing to do. I would think Cobb OTS maps are conservative enough that you wouldn't -need- to have a WBO2 to monitor things as-is. Until you get the stock intake back on however, your MAF readings may be incorrect as fahr_side mentioned earlier.

 

Re: your ? about using the AP gauge to monitor for lean AFR:

 

The AP gauge reads the stock O2 sensor. I don't know how your stock O2 correlates w/ a WBO2 shortly before it hits its 11.03 floor reading, but like I mentioned above, my stock O2 reads its 11.14 min value when my WBO2 reads 11.6. My Commanded Fuel Final is richer at 10.7 at higher loads and from midrange RPM on up. The way I have my timing tables set up, it'll knock intermittently if WBO2 measured AFR gets into and stays in the 10.95-11.1 range for more than a couple hundred RPM up top. Since the stock O2/AP gauge doesn't start to move above its 11.14 reading until the WBO2 reads 11.6 (or leaner), I'd probably either have a potentially toasted motor or at least a noticeably dropped IAM before the AP gauge tells me anything useful about an overly lean AFR.

 

Sensor is too close to measure properly. Not really accurate under full boost as you can tell it doesnt move much in logs. The new FA20's have sensors farther away so less heat and less air flow change so it can fully read under boosted conditions.

 

-Brian

[fahr_side]

It's not a question of how close the sensor is, it's being before the turbo that's the problem. The sensor readings are skewed rich by the pressure in the header, which in boost can be 1.5 to 2x boost pressure. The higher the boost pressure the further rich the sensor will read.

Someone determined not to buy, borrow or install a WBO2 could relocate the stock primary O2 sensor to the downpipe for fuel tuning but note it only reads to about 11.2 AFR and drivability will suffer a bit in that location due to the time lag between the injectors spraying and the sensor seeing the results. Recommend moving it back to stock location once MAF scaling and fuel tuning is complete.

[Mnabeih]

i have had a similar issue like that,

it was the afr sensor need to be replaced :-)

it was operating good in cruising and as soon as the boost increase and exhaust gas flow increase it start to lean and reach 20 in my case, without any CEL

i changed the afr sensor and this case is gone and under boost it reach 11:1

[serx7]

i have had a similar issue like that,

it was the afr sensor need to be replaced :-)

it was operating good in cruising and as soon as the boost increase and exhaust gas flow increase it start to lean and reach 20 in my case, without any CEL

i changed the afr sensor and this case is gone and under boost it reach 11:1

 

I would think that since the car goes into Open Loop shortly after you get into any notable amt of boost, that it wouldn't even use the stock O2 and so the response of the O2 sensor wouldn't have an effect. Unless your old O2 was misbehaving to the point that your AF Learning 1D trim (presumably enabled in your tune) was pretty out of spec and causing OL fueling issues (since the D trim is applied to OL unless specifically disabled in the tune) ?

 

Anyway the car runs better, which is always a good thing.