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Date: 1 Jul 2003 08:53:44 -0700
From: alancemornopsamo.com (Lance Morgan)
Subject: 2500 - 3000 RPM Bucking (long)


2500 - 3000 RPM bucking problem, at steady state throttle, on level ground or even more when going down gentle grade. Always reproducible when engine is hot (milder and not as reproducible when engine cold). No noticeable flat spot when accelerating, and no bucking when gas held steady from 3000 RPM to redline Basic tune items good/checked Key wiring ran, and grounds/earths clean and secure All sensors within specs No Check Engine Light codes Fuel pressure steady & O2 sensor healthy/fast V fluctuation (w/DMM) during bucking Injectors working with CEL test, but can't be sure what they're doing at problem RPM -- 2 possible faults found: - no ground at AMM #4 (should be one, per Bentley, and indicates LH ECU is not internally grounding AMM #4: both LH ECU grounds are good, as is continuity between AMM and LH ECU) - ignition timing good at idle, however, when rpm held steady ~2700 RPM, timing _retards_ (< 10 BTDC) instead of advancing, suggesting problem in EZK control module (and/or may be faulty signal from ECU to EZK). Timing advances to + 20 BTDC with faster throttle blips thru problem rpm. KS is putting out voltage, and no apparent problems above 3K RPM Any thoughts/suggestions? Seems like ECU and EZK failures are pretty rare, and usually don't manifest themselves this way... 91 900S 5spd LH 2.4.2 Thanks, Lance p.s. there appear to be 4 wires that connect 6-pin LH 2.2 AMMs to ECU, and 3 wires for 5-pin LH 2.4+ (w/the 4th wire being a ground, and 5th being batt V/power) One of the 3 or 4 wires (pin #3 on my 91 n/a [Bosch 019]), AFAIR, sends a variable non-linear voltage to the ECU - something in the 2.2V - 2.9V range - depending on engine RPM Does anyone know what the other 2 wires do?

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