1990lumi Posted May 25, 2014 Report Share Posted May 25, 2014 I have a 1990 Lumina 3.1l MPFI 2 door base and the fuel pump has been staying on even after i turn the car off pulling the relay does nothing, pulling the fuse turns it off checked all wiring no breaks or groundings do i have a bad fuel pump? or maybe ecm? Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted May 25, 2014 Report Share Posted May 25, 2014 could remove all 4 ECM connectors and see if that shuts the pump off, i don't think it will though based on pulling the relay and still running. if pulling the relay doesn't even shut it off, but pulling the fuse does, it sounds like your oil pressure switch/wiring is permanantly conducting. that is the only other source of power for the pump that doesn't rely on the relay. Quote Link to comment Share on other sites More sharing options...
1990lumi Posted May 26, 2014 Author Report Share Posted May 26, 2014 THANK YOU!!!! I unplugged my oil pressure switch and connected my battery just to test to see if my pump would turn on as soon as i connected the battery and it didnt turn on BUT my car turned on when i turned the key and the pump turned off after i turned the motor off...THANKS again for letting me know about the switch saving me money from a new ecm or the time and money for a new fuel pump...now to go spend the 10$ on a new switch Quote Link to comment Share on other sites More sharing options...
pitzel Posted June 13, 2014 Report Share Posted June 13, 2014 1990lumi, I think my oil pressure switch might be sluggish or something (see my other post, it does not shut the car off when I pull the relay! And I have horrific cold start problems). Did you have to drain the engine's oil to replace yours? Or was it literally get underneath and 5 minutes later with a wrench, its replaced? Quote Link to comment Share on other sites More sharing options...
Imp558 Posted June 14, 2014 Report Share Posted June 14, 2014 I can't understand why they did that oil pressure switch to begin with, any GM PCM works fine to enable the FP relay without it. You could just cut the gray wire and INSULATE BOTH SIDES AS THEY BOTH SEE POWER and forget about it. Quote Link to comment Share on other sites More sharing options...
pitzel Posted June 14, 2014 Report Share Posted June 14, 2014 (edited) I can't understand why they did that oil pressure switch to begin with, any GM PCM works fine to enable the FP relay without it. You could just cut the gray wire and INSULATE BOTH SIDES AS THEY BOTH SEE POWER and forget about it. The oil pressure switch business is as protection against an accident where the motor/fuel lines are severed, but the wiring harnesses fail to be destroyed as well. Wouldn't want the tank-mounted fuel pump to be pumping fuel into an engine compartment fire (or even a fire caused by severance of the fuel lines in the middle of the body), now would we? Its likely a certification requirement as well. As I understand, the FP relay's purpose is to merely light the pump up in the absence of oil pressure, ie: at start-up. RobISaar, please correct me if I am wrong though. Edited June 14, 2014 by pitzel Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted June 14, 2014 Report Share Posted June 14, 2014 key-on, the fuel pump will run for 2 seconds and will continue to be commanded on by the PCM as long as crank sensor pulses are detected. if no more pulses are detected(engine off, for whatever reason or a dead crank sensor), then the pump will run for up to 2 more seconds by PCM control. if the oil pressure switch is above its activation threshold, then it will continue to supply power until under the threshold(~4PSI or so). with thick oil at low temps, i've had a pump run for ~5 seconds after key-off, which is weird to hear. with the engine running, and above the oil pressure threshold, both supply current for the pump. i don't know how many, but more than a few applications into the late 90s and early 00s had seperate airbag modules that had no interaction with the PCM at all, so it can't control or detect fuel pump action, nor would the PCM know to stop it. Quote Link to comment Share on other sites More sharing options...
Imp558 Posted June 14, 2014 Report Share Posted June 14, 2014 After the engine stalls The PCM will shut it off anyway Quote Link to comment Share on other sites More sharing options...
Imp558 Posted June 14, 2014 Report Share Posted June 14, 2014 Wow! With a little delayed off timer circuit we could cascade the both of them and the oil pressure switch would kill the engine if it lost pressure despite the PCM. Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted June 14, 2014 Report Share Posted June 14, 2014 Wow! With a little delayed off timer circuit we could cascade the both of them and the oil pressure switch would kill the engine if it lost pressure despite the PCM. it would be easier to run the part of the circuit that feeds the fuel pump relay through the switch first, then to the relay. if either open the circuit, pump goes off. if a couple of second turn on at key-on is necessary for whatever reason, that circuit is pretty simple as well. Quote Link to comment Share on other sites More sharing options...
Imp558 Posted June 14, 2014 Report Share Posted June 14, 2014 Yes but that could make for hard starts. I picture a timer to enable the fp relay and then turn off after a few seconds. Then fall back on the cascaded controls. Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted June 14, 2014 Report Share Posted June 14, 2014 I picture a timer to enable the fp relay and then turn off after a few seconds. Then fall back on the cascaded controls. that's essentially what i was getting at.... even then, if the fuel pump check valve is functional, you retain a lot of rail pressure after the engine is cut. enough to fire the engine in my experience. Quote Link to comment Share on other sites More sharing options...
Imp558 Posted June 14, 2014 Report Share Posted June 14, 2014 that's essentially what i was getting at.... even then, if the fuel pump check valve is functional, you retain a lot of rail pressure after the engine is cut. enough to fire the engine in my experience. That's true, could also solder a diode across from the stater wire to the fp relay wire. and another inline on that wire so the PCM doesn't see the starter power Quote Link to comment Share on other sites More sharing options...
1990lumi Posted June 15, 2014 Author Report Share Posted June 15, 2014 thank you for the replies it was the oil pressure sensor and for some reason gm must have screwed my car up sense it was a early production 1990 3.1l because my dash is a light only dash for oil pressure but the motor in my car is a with sensor motor and it is the original motor but i put in the new sensor and everything works great now (so nice not having to pull the battery every time i turn my car off) Quote Link to comment Share on other sites More sharing options...
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