New M20 Won't Rev Past 1100 RPM

[s_hemphill]

Hello,

My rebuilt M20 won't rev past 1100 RPM. Seems to idle fine, but at ~1100 RPM the engine will start backfiring through the exhaust and act like it has a massive vacuum leak (it doesn't). It also threw an engine temperature sensor code, which cleared. I cleaned the C191 connector pins thinking a bad connection to the injectors and temp sensor, but that didn't help. The Crank Position Sensor is new with the rebuild. Have any of you experienced something like this with a bad CPS? Any ideas?

Sounds remarkably similar to this:

https://www.r3vlimited.com/board/forum/e30-technical-forums/engine-drivetrain/m20/381905-m20b25-backfiring-and-won-t-rev

Thanks

[Tinkerer007]

Does it run better if you unplug the MAF?

I recently replaced my M20B27 with a 2.9 stroker. It ran crappy till I changed to a M20B25 MAF.
(It ran better with the MAF unplugged.)

[s_hemphill]

No change unplugging the MAF. Checked all sensors at the ecu plug and found the CPS was open (infinite resistance). I’m going to assume the new CPS I bought is bad, though I have no idea how the engine started with a bad sensor.

[s_hemphill]

Well, I replaced the CPS and cut out and spliced the C191 connector, which was rotted out pretty bad. Fired the engine up and no change, except that it will run quite a bit better with the AFM unplugged (no backfiring, will rev as much as it wants, etc.) The AFM has 5 volts going to it, and resistance changes when i move the flap. Any ideas?