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1994 Buick Roadmaster (LT1 engine) STFT not responding properly
Hello guys, This one is part of a classic cars fleet i usually service. The vehicle´s owner replaced the old carbureted engine with this buick engine/ecu system.
Available tools: Scope, amp clamp, gas analyser, OBD1 scanner
Driver is complaining he smells gasoline at middle/hard acceleration (I even smell it at idle). I hooked up scanner to notice that when engine was warmed up, one of the upstream O2 sensors (or sometimes both) stayed above 850mV and STFT was stuck at 128 (never lower/never commanding lean mixture). If fan cooler turned on, O2 voltage bounced between 200mV and 600mV and STFT increased to about 132, then went back again to 128 and O2 to 850mV, once fan cooler turned off.
I took brake booster hose out (so i made a vacuum leak) to notice O2 voltage falling to 150mV and STFT increased in few seconds to 150. Same thing happened if i unppluged one of the injectors.
I checked O2 voltage directly with my scope (heaters, signals and grounds), so scanner is not lying to me. I also checked powers and grounds at PCM (less than 80mV difference at both power and ground).
I noticed this engine doesn´t have de right distributor (No high res. signal/ only Low res. signal) and EGR system is canceled. My understanding tells me both are not related (PCM is taking rev. reference from low res. signal and EGR system is programmed to not operate at idle for driveability reasons).
I´d apreciate any idea/input from your side. Thanks in advance.
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