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Discussion Starter · #1 ·
Two weekends ago I was tuned to 11.5:1 AFR in 80 degree weather. This last weekend it was about 60 degrees and my AFR went to 12:1 using the exact same settings.

Keep and eye out when the temps change a lot! This refutes the "tune it and leave it " philosophy.

Anyone else notice how the cooler weather creates a leaner AFR?

· Registered
3,131 Posts

I've seen this happen due to colder temps, but I've ALSO seen something more alarming happen on 4 out of 4 big single cars (factory ECU) that I've been close to.

Here's what happened on ALL 4 cars. Keep in mind that these 4 cars have 4 different turbos, two different headers, and 3 different fuel systems, 3 different fuel controllers, and VPC temp sensors located in various places on the intake tract. The common thread is that they are all on VPC, fuel controller, and factory ECU with raised rev limiters.

The cars were carefully tuned to about 11.5:1 (under boost) on a wideband. For the rest of the day, and perhaps for a few days afterward, everything stayed the same -- AFR in the 11.5 range. Then, a few days later with NO ADJUSTMENTS whatsoever on any of the electronics, and NO WEATHER CHANGE, the cars went anywhere from about 1 full point to 2+ full points LEANER. That's 13.5:1, up from 11.5:1. The AFR was then re-tuned and the cars held that tune indefinitely, perhaps with minor tweaking as is often needed to keep AFRs within 2 or 3 tenths of target from day to day or week to week.

So, for anyone who thinks they can go to the dyno or borow a wideband, tune the AFR, then just forget it, they better think again.

More detailed info on this can be found in the FAQ of

Steve Hayes

Discussion Starter · #3 · (Edited)
Hey Steve, thanks for the info. Do you recall if the cars were OBDI or OBDII?

I'm wondering how OBDII "readiness codes" play into the equation (or if they do at all). I recently dropped my van off to have the AC recharged and the customer service rep was explaining to a customer that she needed to drive her car for a certain amount time before they could inspect it. They just plug the OBDII analyzer into the diag port and take some readings, no visual inspection or sniffer!. I'm just wondering if the ECU learns driving conditions and adjusts the fuel maps (and maybe timing maps) accordingly, but differently on OBDI and OBDII. I have noticed that after I reset my ECU it takes a while for the AFR to come off stoich and fall into place. I think you have to drive the car for a while, shut it off, and drive it some more or some shit. Does this also apply to OBDI?

But yeah, I've had similar occurrences of varying AFRs, and it seems to be after I reset the ECU. After I have tuned, driven the car, and made some fine adjustments, it does seem to be stable (if the weather doesn't vary a lot). I had heard cooler weather made for a leaner AFR, but never quite noticed the proportions.

That FJO was a great investment into the car and I'm glad you suggested I get one a couple years ago. It could make all the difference in the engine lasting 200 miles or 200k miles.

· Never enough
1,347 Posts
This makes me glad I've invested in my AEM/FJO setup :D Almost always my IAT correction table will keep me within my targeted 11.5-12.0 in all different temperatures without any adjustments:D :D

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