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Thread: Low Oil Pressure Indicator Light After Oil Change

  1. #1
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    Default Low Oil Pressure Indicator Light After Oil Change

    Happy New Year-

    Changed the oil today in my 1990 Century w/the Iron Duke 2.5. Car has about 78K miles and I drained out 5W-20 and put back 2 quarts of 5W-30 and the rest (appx. 1.75 qts) of 5W-20, long story why. Replaced filter too with some Chinese POS I got from Rock Auto.

    Drove vehicle on the highway for about 5 miles and got the oil light flickering at low RPM's when returning to town, revving vehicle probably above about 1000 RPM's makes it go out. WTH?

    My Hayne's manual makes no mention of the oil pressure sender other than in the wiring diagram. I have not gotten back underneath to see where it is located. Is this just a strange coincidence of the oil change and now this? Open for suggestions, to begin with where is the sending unit located on this engine?



  2. #2
    Senior Member 85_Ciera_Rebuild's Avatar
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    Later day GM's oil pressure sensors were a three wire setup for engines with electric fuel pumps, where one wire went to dash unit, and the other two were for the fuel pump; if not enough oil pressure existed, the fuel pump would quit. See this forum for pics.

    Oil Pressure Senders were made for those with a Gauge, and those without a Gauge; I don't know which you have, but both work the same...not enough oil pressure, and fuel pump quits...unless someone "hot" wired fuel pump.

    I always replaced these sensors when I acquired an used vehicle, and replaced ECM temp sensor and radiator's thermo sensor (if radiator had one).

    I can't say what's happening in your situation, but myself, I would replace oil pressure sensor unit.

  3. #3
    Senior Member Zaloryan's Avatar
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    Oil filters typically have a bypass valve which, when activated, causes oil to skip the oil filter. In theory, when the oil filter media has become clogged with deposits, the bypass valve in the oil filter senses this restriction and engages, causing oil to no longer pass through the filter. This is to ideally ensure that the engine still receives oil at all times, although it is unfiltered when the oil filter bypass valve has engaged.

    If your symptom was not present prior to your oil change, I would hypothesize that your problem is likely a combination of incorrect oil viscosity and a potentially defective oil filter. I would recommend another oil change with the OEM recommended oil viscosity (probably 5W30 or 10W30 for North Carolina) and installing an AcDelco or Purolator oil filter.
    What is this & what does pulling it out do?

  4. #4
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    The oil filters for the 2.5l are cartridges, no bypass valve. Short of the filter media turning to stone would this still cause the issue? Also oil viscosity on the cap is 5W-30 and technically my brew in the crankcase is lighter, again in theory should not cause this issue. Appreciate all the input. I have not driven it since as this vehicle lives at the beach. May be a flaky sensor.

    https://www.ebay.com/itm/For-1988-19...-/293373384300

  5. #5
    Senior Member Zaloryan's Avatar
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    Quote Originally Posted by Bdevil View Post
    The oil filters for the 2.5l are cartridges, no bypass valve.
    I did not realize that and now stand corrected. Thanks for teaching me something new about the Iron Duke.
    What is this & what does pulling it out do?

  6. #6
    Senior Member CorvairGeek's Avatar
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    Fyi, the FWD 2.5 switched to a cartridge filter in '88, but it looks like RWD version could be had with a spin on through '93.
    Jerry

  7. #7
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    Update (of sorts): Drove car yesterday. In my community to get any stores, etc. you need to drive on the highway 3-5 miles. Its about a 5-7 minute drive within the neighborhoods (25-45 MPH) to get out on the highway. While driving out to the highway no light flickering at low RPM's or idle. Make my highway run at 70 MPH and light flickers at low RPM's/idle once in town and again on the way back home. It would appear to be easier to change the oil sensor than the oil filter. On the picture of the Fiero the motor orientation is different than mine. Sensor is not on the front of the block. I guess it's on the back and I'll need to remove air cleaner to get to it? Thoughts?

  8. #8
    Senior Member CorvairGeek's Avatar
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    I just changed the oil pressure switch on my '92 3300. Getting mine wet started making the light come on at idle immediately after washing the engine to change the valve cover.Even though it was obvious it had oil pressure, from the sound, it still startled me. Just as a quick double check, I unplugged the fuel pump relay and drove the car to see if it would stay running with the light on (it uses a 4 pin switch, one pair for the light and one pair as the back up in the event of fuel pump relay failure, just a longer cranking time). Sure enough, the light was on but the car would stay running. I later unplugged the switch and cleaned / dried it with some WD40 and it went back to normal, but I replaced it anyway. The Weather Pack connector seal was distorted. I would assume it was getting close to failing and leaking oil.
    Jerry

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