elaw wrote:Just to toss out my two cents, and I think I've said a lot of this here before, but I've seen the same thing as the original posted. Funny thing is, I've experienced it both with speed-density and MAF. With my setup it seems most pronounced at idle - at mid and high load the tune seems pretty stable. But at idle it seems like I have to retune seasonally.
One thing that could be at least a partial explanation in my case is I'm using the factory PCV setup on my engine. So at idle, there is some "stuff" (air, combustion products, whatever other gases are in the crankcase) coming into the engine via a path that bypasses the MAF sensor. I'm also using evaporative canister purge, but that's not active at idle. Canister purge is one place I could easily see a seasonal variation, as there will be much more evaporation from the fuel tank in warmer weather.
My variations are also most pronounced in the low-flow regions of operation as well. Mainly idle like you mentioned and also during cruise where the revs are higher but the throttles are almost closed. Under mid/high load, it appears very stable across the temperature range. Just a thought, can air temperature effect the turbulence of the air entering the MAF? I know turbulence in the intake will have a more pronounced effect in the lower-flow operating regions.
I will be the first to state that MAF is not perfect but I can say that I have run SD, AN, blended, and ITB modes on this engine and it has never run better than it is now with MAF. MAF has solved most of my problems caused by ITBs and a high-overlap / high-duration cam.
I do not believe for a minute that the MAF sensor can correctly compensate for all temperature related factors across all types of installations without a little help. The MAF sensor can only correct for what it can see. Other temperature related effects outside the "view" of the MAF sensor still need to be accounted for. I'm not sure what the best solution is but was thinking that enabling the manual MAT correction curve was as good a place to start as any.
1977 BMW 320i with MAF, COP, IAC, Sequential fuel and spark, MS3 knock sensing
MS3 / MS3X / V3 mainboard, MS3 V1.5 a4