Unusual LTFT shortly after engine startup

[97AutoXVette]

Good afternoon. Over the past couple weeks, after finishing installing the ported top end and doing some tuning, I have an interesting fuel trim issue:

After I start the car, it idles fine and AFRs are in check right around 14.7/1. After about 30 seconds or so, sometimes a minute or 2 of driving, the LTFT go very high, often maxing out at +16.4%. What is interesting is I constantly monitor actual AFR with my wideband, and the AFR drops very low (11/1, sometimes even down to 10/1 which is as low as the gauge will show (so could be lower)). Then, after a few minutes of running pig rich, the LTFTs will drop back down to zero and often a bit negative, bringing the AFRs back around 14.7/1.

I cannot seem to figure out why the car wants to raise the LTFTs so much shortly after driving even when the AFRs are way too rich. Note that this is when the car is cold or warmed up, doesn’t matter. Has anyone else experienced anything like this?? Any help or recommendations for tuning this out would be helpful. Today on E85, the car almost wanted to stall cause the enrichment became so extreme shortly after starting it up. Then after a few minutes, it went back to normal.

My mods are in my signature as an FYI. This is on a 2002 GTP.

 

[Scottydoggs]

+16 is a vac leak.

 

[SgtMarshal]

I was going to do a turbo swap so I had an N/A intake manifold on. I decided not to go through with it so I put the supercharger on. I changed the tune, but for some reason, the tune I loaded wasn't quite right for the car. The IFR was a tad lean and the maf was out of whack. the difference for me was that my car on startup would read +16.4. I thought I had a vacuum, but it was just a tuning issue. I would look for a vacuum leak first, but if you don't find a leak like I couldn't, then maybe some of the cells in your maf need adjustment, and if you have changed injectors, maybe your IFR isn't exactly what it should be for the range when the car goes lean.