Idle trim varying with IAT

'05 LGT 5MT, 82K miles. Catted Perrin DP, STi UP w/ resistor mod, bulletproofed stock TMIC, stock airbox & OEM filter, SPT catback, APv2, Cobb stage 2. Located in Denver.

On and off for the past year, I've been working as time permits on getting everything just exactly perfect to prepare for a cryotune. When I started, my fuel trims were maxed out positive. Tons of smoke testing and boost leak testing ensued. The aforementioned bulletproofed TMIC resulted, as did a new inlet, PCV valve, BPV, and a variety of vacuum hoses, few or none of which were leaking but many of which were on their way out. Still high fuel trims. New plugs, upper O2 sensor, and MAF/IAT made little or no difference. Eventually caved in and got professional help from a local Subie shop who quickly spotted an under-threshold misfire and an up-pipe leak. An injector fixed the misfire, and the aforementioned STi UP replaced the POS Invidia. Now the numbers were much better, especially in the high ranges, but they weren't stable. A new set of grounds fixed the instability, and finally I had some numbers I could take to dinner with the parents.

EXCEPT:

The LTFT at idle changes with the ambient temperature. When the weather is cool, say 40-60 F, the idle trims are between 3% and 5%.

 

But when the temperature gets above 60 F, the LTFT at idle ramps up to 8%.

 

When the temperature gets into the 70s and 80s, and the car sits turned off for half an hour after running, and the IAT gets heat soaked up to 120 F, the idle trim easily gets to 10%. I've seen it at 14%, and it took 10 minutes of driving to get back down to 8%.

 

I've made sure the new MAF/IAT is still clean as a whistle.

 

When the trim is under 5%, turning on the A/C compressor will take it up to around 9%. When the trim is 8% or higher, turning on the compressor will only increase it about 1%.

 

Regardless of these conditions, the trims in the upper ranges stay rock solid where they are.

 

Anyone seen these sorts of shennanigans before? Any ideas what's going on? Thanks!

The problem depends not on the car's temperature, just on the ambient temperature. Today I first fired up the car in the afternoon, when the temperature was about 80 F. Immediately after starting, I pulled this gem:

 

I drove around for several hours with no change. Once the sun went down and the weather started cooling, the idle trim started dropping.

 

I then ran my umpteenth smoke test, just in case. Still tight as a drum.

 

I also repeated another test - I uninstalled the Cobb tune and idled the car in the driveway using the stock tune. Made no difference, the idle trim quickly climbed up to exactly the same on the stock tune.

 

 

Any suggestions or explanations, no matter how outrageous, would be immensely welcome. Thanks guys.

Im no pro, im just learning (no pun) this as i go

 

I got so tired of asking my buddy to borrow the shops smoke machine that i made my own smoke test machine ( paint can/ cigar )

 

 

my 2 cents is when your engine bay is heat-soaked, leaks appear. when it gets cold, it seals up?

 

 

try smoking it when shes heatsoaked?

 

 

 

im actually about to leave my place to smoke my motor again lol ()

Thanks. For that specific reason, I was careful to do yesterday's smoke test when the car was hot. Tested immediately after running it for hours. And as mentioned in my previous post, the trim numbers happen even when the car is just started up for the first time, if the ambient temperature is warm. The idle trim varies directly with the IAT.

First snowstorm of the season this morning. Outside temp 33 F, idle trims 3.5-4.5%. 6% with the compressor on for defogging. What I found most interesting is that the upper ranges moved for the first time since I replaced the grounds. This thing really likes the cold.

I was chasing this same "issue" last year and into the winter. Had multiple shops look at the vehicle, nobody could ever find a leak. I had it smoke tested, no leaks.

 

My advice would be get a smoke test for peace of mind and then don't worry about it.

 

Mine is a little different... it was tuned on a 95 degree day so on warm days it usually corrects minus a few percent. But in the winter on a 15 degree day it can idle as much as +12%.

 

Considering the vast differences in temperature + perfect operation of the vehicle, besides being confirmed multiple times I have no leaks... I now chalk this up as completely normal... at least for my car and my mods.

 

The upside of this whole experience is I finally have more confidence in my vehicle / normal operation.

Thanks! I came to much the same conclusion; I have asked cryo to go ahead with e-tuning, and hopefully if there is a problem it'll shake out during that process.

 

Incidentally, did you replace (OEM or upgrade) your injectors anywhere in your process? I can't shake the feeling a fresh set might be the answer, just hate to fire the parts cannon without clear evidence.

Nope I'm at 130k miles original injectors.

 

At one point I thought it must have been my turbo inlet hose (with no evidence), so I replaced that with a Mishimoto. Can't say I notice any difference from that, but I guess it's peace of mind in the future.

 

I'm probably rebuilding or shortblocking this car at some point and extend it's life. At some point I'll need another tune -- next tune I'm going to try to get closer to 50-55 degrees. Wouldn't it be funny if that actually evened it out over the course of summer/winter.