I’ve put together a step-by-step guide to help solve low idle situations with the LS swap using the ACE kit. It is usually fairly simple to isolate the problem. Happy to answer any questions about it and will add to this if I find out more from others. It is important to note that the low idle issue can cause a limp home (M&S lights) issue. In most cases, if the idle is going below 600 at any time, the idle is causing the M&S lights -- and its not the other way around. With the standard ACE kit, the idle RPM should be set around 750.

Sometimes the VSS is the cause of the issue, sometimes it is other things. Generally, it is important that your LS ECM has a VSS signal to get out of the idle tables and get the best fuel mileage, but that can be tuned out as well. If you ever get any codes from the GM side, check into those before doing this step-by-step.

Steps:
1. Plug in an OBD2 monitor and drive between 0-60 mph. Monitor your MPH signal from the OBD2 monitor from the GM port. Does you speed show any non-zero number (does not have to be correct)? It may be important that your OBD2 monitor can show partial MPH. Ultragauge does this.
A. Yes. But its incorrect - your ECU tune is not correct, but this is not the cause of your low idle issue - move to 4.
B. Yes. Speed is approximately correct - move to 4.
C. No. Speed always shows 0. Your VSS wiring is a problem, move to 2.

2. Are your VSS wires properly soldered? Is the polarity correct?
A. Yes. Are you sure? Move to 3.
B. No. Properly solder and retest at 1.

3. Redo the wires to go directly from GM ECU to ACE box (not through the harness, which increases resistance). This should shorten the length significantly. Try to keep the VSS + and VSS - wires twisted. Retest at 1 - does MPH shows up on OBD2 monitor?
A. Yes. My idle issues are solved.
B. Yes, I have MPH, but I still have idle issues. Go to 4.
C. No. Contact ACE for a new ECB box.

4. Do you have the EVAP purge valve/line hooked up to the intake?
A. Yes. Go to 5.
B. No. Go to 7.

5. Do you have the EVAP purge line hooked up to the LS? Temporarily unhook the EVAP purge valve line from the intake and plug the hole in the intake. Unplug the purge valve. Drive around normally. Is the problem gone?
A. Yes. Your problem is in the EVAP system. Go to 6.
B. No. Go to 7.

6. Did you follow the ACE emissions hookup guide?
A. Yes. Are you sure? You probably have a leak in the EVAP system. It is causing a vacuum leak when the EVAP system is active. It could be the purge valve or any of the related lines.
B. No. Verify emissions hookup according to the guide. If the Rover EVAP system does a test while the LS is running it will cause a major vacuum leak that can cause the engine to stall.

7. You probably have a vacuum leak after the throttle body. Check all orifices on intake.

The problems could also be tune related, but that is fairly unlikely if the tune is relatively stock.