Cold starts perfect, hot re-starts very lean question.

[Mike Hemingway]

Hey all,
Got cold starts (less than 60 deg. F coolant and intake air) perfect but after the engine hot soaks for 30 minutes or so the hot re-start is very lean causing surging. The engine compartment is very tight with a big V-8 and I see intake air temps, as measured at the air cleaner element about 80 deg. F higher than the outside air temp. The hot cranking and after-start adjustments seem good as the engine fires and idles fine for about 15 seconds, until the after-start is finished tapering off. At that point the AFR goes to about 14.5 which is lean for this engine (needs 12.5 for a good idle). I have been playing with the intake air temp. enrichments (none at 100 deg. F and below and 20% at 150 deg. F and above) which is helping. However, I am concerned that on a hot summer day with A/C on, sitting in traffic, etc., I will be running too rich as the intake air will be approx. 150 deg. F., maybe 180 deg. F after a hot soak.
Does anyone have any suggestions or experience with this situation and how did you handle it?
Thanks much,
Mike

[Matt Cramer]

Could you post a data log of the issue and a copy of your MSQ?

Where is the IAT sensor located?

[Mike Hemingway]

For the hot start surging issue I found the voltage reg. was surging (+/- 1V) and the pulse width comp. was too sensitive. That issue is fixed now. I still have a lot of IAT correction but the curve (-10% at 0F to +15% at 175F) is working fairly well from 40 deg. F to 120 deg. F.
The GM open element IAT reading does not seem as responsive as it should be. Attached is a picture of the engine compartment. The IAT sensor is screwed into the bottom of the air duct at the elbow near the coolant reservoir. From your experience is this a good location and does slow response seem correct?
The other issue I am working is X-tau settings. I have the accel. in the ballpark but am seeing major fuel cuts on decel. I have disabled all fuel cuts but I must be missing something. The attached excel file shows AFR as lean as 20:1 at time 1175 seconds which results in a stumble. Can you determine what I am doing wrong? (Note, the O2 sensor time lag is about 0.25 seconds at 3K RPM.) I am running V2.88.
Thanks for your help.
Mike

[Matt Cramer]

I don't see the picture, but the location sounds OK.

X-tau is going to need a very well tuned fuel table - how is the VE when you're not using X-tau?

[Mike Hemingway]

Thanks Matt.
The VEs are pretty good and I can run with only accel. pump simulation smoothly except I see the benefit of using X-Tau in the EGO data. I am trying to go lean when coasting above 1200RPM/MAP below 20 as much as possible. When I snap off the throttle (between shifts) the MAP goes into the lean region which is causing a lot of fuel cut. If I do not use X-Tau there is no hesitation/stumble but when X-Tau is active it cuts even more fuel so I get the hesitation.
Is there any way to use X-Tau enrichments on increasing throttle but disable all X-Tau enleanments on decreasing throttle with MS-II/V2.88? (I have been playing with the stim. and a hand vacuum pump and cannot find a combination of inputs).
Thanks, Mike

[Mike Hemingway]

Matt, an update.

V2.88 was not allowing me to enter 0's in the X-Tau decel tables, it gave me an error message to use minimum values. (Note: the megamanual says 2.850+ allows Tau=0). I went into the .ini file and changed the minimum limits to 0 for both X and T on decel. only (was 2 and 5). Using the stim. and a vacuum pump I see the PWs are not being affected by increasing vacuum now (the X-Tau gauge is still very active). So, it looks like I have dis-abled the fuel cut on decel. I will try it on the car when the snow clears from the roads.
Attached is the picture of the IAT location I tried to attach to an earlier post.
Mike

Installed Engine (800x533) - Copy - Copy.jpg