While out tuning today, I noticed that under certain conditions the vacuum shown in Tuner Studio does not match the vacuum on the boost gauge.

If I lift off the throttle, the vacuum increases as expected on both the boost gauge (to about 30in-hg) and on the Tuner Studio MAP gauge (to about 12kPa).
After about a second, the MAP in Tuner Studio increases to about 100kPa while the vacuum on the boost gauge remains constant around 30in-hg with no input from me.

The car is a '97. The engine is a '99 with all the '97 sensors and wiring. The MegaSquirt is a 9697 MSPNP that I have been using without problems for the past 3 or 4 years.

Any suggestions on what might be causing Tuner Studio to see the MAP increase when it is not actually happening?

 

In troubleshooting, I connected a second boost gauge where the vac line connects to the MS. This gauge reads exactly the same as the other boost gauge and does not show the spike that Tuner Studio does.

I have attached a screen shot of a datalog that shows the problem. The TP goes to zero when I lift off the throttle and the RPM starts to decrease. The MAP drops to 12kPa briefly and then shoots up to 100kPa where it stays until I touch the throttle.

 

I really don't know what to tell you besides that something is wrong with your MAP sensor. 100 kpa on decel shouldn't ever happen.
I would imagine the MS is dumping fuel, what does your o2 read when this happens*
(assuming you have a wideband of some sort)

 

It does not seem to be going rich, but I think that the ecu goes into overrun at the same time and so cuts fuel off anyway.

It is screwing up the auto tune though, causing the map to be retuned because MEga Tune thinks that it is operating in a part of the map that it is not.

The problem does not seem to happen when I use megatune.