After watching a situation develop since I installed ITB EFI and with only 4500 miles on the system, I concluded that the TPS (throttle position sensor) had gone bad and needed to be replaced. By gone bad, I mean the TPS had become increasingly noisy especially at the low % TPS end of the wiper.
With TunerStudio connected to the MegaSquirt2 ECU and the engine running, warmed up (no WUE - warm up enrichment), and at idle (950 rpm), the % TPS would swing from -0.5% to +1.7% and when I SLOWLY opened the throttle just off idle the TPS Acceleration Enrichment would start firing and the idle speed would become erratic and the injector pulse width would start jumping. When I let the throttle return to the idle setting, the system settled back down.
On the freeway, this behavior translated to bogging / hesitation whenever I was at very light throttle cruising along on level road (for example: at 65 - 70 mph (approx 3000 rpm)). Again, with TunerStudio connected, running and a datalog recording, I could see the TPS Accel Enrichment was triggering and the AFR was very rich at 11:1 with a target of 14.0:1.
The Green Spikes in the bottom graph are from the TPS Accel Enrichment firing:
Since the CIS to ITB EFI conversion kit I installed has ITB's from Race Hardware Developments, I went to the RHD website in search of information regarding the TPS used with my ITB's. I found the TPS part number plus a good discussion on why RHD recommends using Alpha-N (TPS) versus Speed Density (MAP sensor) load sensing with ITB's.
The RHD FAQ page also states that the MAP signal can only be used in conjunction with the Alpha-N. In TunerStudio, that option is on the General Settings page and is called "Multiply MAP (Caution!)". I tried running my engine with this setting set to "multipy" but my vacuum signal is weak and erratic due to the large degree of intake and exhaust valve overlap with the camshaft I am running, and I could not control the engine speed below about 3000 rpm so I run my tune with this setting set to "don't multiply".
So, I replaced the TPS with the BMW unit (13-63-1-721-456) listed on RHD’s website.
When I removed and checked the old TPS with a DMM it looked okay at DC, but I knew it was noisy and triggering from the Accel Enrichment spikes on the datalog I recorded.
Anyway, I ordered a new BMW TPS from Pelican which I received today. After checking all my ground and power wire terminations and wire connectors as okay, I installed the new TPS, performed the TunerStudio TPS calibration, checked for fuel pressure/leaks and started the engine.
TPS Calibration dialog box in TunerStudio:
Now % TPS at idle varies much less - +0.5% to +0.7%, I was able to reduce the TPS dot threshold from 25%/s back to 15%/s, and after running auto tune the car cruises down the freeway with the AFR right around 14.2:1.
I also found that with the new TPS and a stable % TPS signal that I had to add 0.4 ms to the TPSdot Accel Enrichment pulse width adder in order to achieve smooth tip-in response without bogging or intake back firing.
Old TPS (bottom, part number and other markings obscured) versus new TPS (top with BMW logo and part number):
