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Thread: 1990 4G64 oil pressure problems

  1. #1

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    1990 4G64 oil pressure problems

    First, I wasn't sure if this was the proper forum or perhaps maybe the Powertrain folder, so apologies on that first if I picked the wrong one.

    After having read a few other archived posts with similar issues, there were a lot of theories but no sound solutions posted, so here goes.

    I've got this 1990 Max with the 2.4L - all stock. I've rebuilt the upper end with a new head, exhaust manifold, rebuild throttle body and rebuilt ECU 9capcitors leaked). I got it running pretty good and then ran into major oil seal leaks on the front housing, so I pulled it and replaced all the seals including new bearings for the balance shafts. Maybe I should have eliminated them - but they're still there (for now) - and with new bearings. All new sensors including oil pressure switch (for idiot light), and I just replaced all the instrument cluster bulbs since they were so old and dim. now the oil pressure light is flickering on now and then and in some cases on solid.

    No ticking, oil level is fine and opening the fill cap I confirm I have oil delivery to the top end.

    I hooked up a gauge to the sending unit port and on cold startup I got 82psi. A bit high I thought. as the engine began to warm for the next few minutes, it dropped to maybe 78psi. FSM (factory service manual) states at idle and at operating temperature, it should be "11psi - or higher" a max level is not specified.

    So I put the sending unit back on and same issue right away, flickering oil pressure light. One post had a theory that the high pressure blew the new sending unit. Another held a theory the oil pressure valve spring was worn.

    If I blew the sending unit with the high pressure, I need to solve the cause of the high pressure before replacing again. Standard doesn't specify the range except to say the switch is activated at a low point of 7psi

    How do you tell if the spring is bad? Are there other things to check/replace for a high pressure condition? I always thought 20-35psi was an ideal range, but I'm thinking 80 even at startup is much too high.

    Thanks for the help!

  2. #2


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    The high pressure is not that big of an issue. Some Mitsu engines will push up to 100 on really cold idle and the oil switches will hold just fine. It is possible to blow the switch if it is a cheaper switch or a faulty one.

    The other thing is if you cold started @ 82PSI, waited a few minutes and read 78 PSI, you were not at operating temp. Go drive the thing around for a while and check your operating pressure again. It will take a very very long time to get a correct operating temp pressure reading if it just sits and idles.

    I would suggest you either have a faulty switch, or the wire has an issue if you have verified the actual operating temp pressure reading.

  3. #3



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    Put on a real gage. mechanical or electric. But get a gage that reads in PSI and see what it's doing.
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  4. #4

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    Quote Originally Posted by camoit View Post
    Put on a real gage. mechanical or electric. But get a gage that reads in PSI and see what it's doing.
    Trust me it's planned. But until then, I'd like to know why something as simple as an idiot light is functioning properly when clearly there's enough pressure. It was a new Standard brand sending unit and cannot fathom why if the unit supposedly only activates at 7psi per specs found online, why at 82psi (per my testing gauge) it's flickering.

    Basically I have two problems... 1st, why the damn light is flickering when there's plenty of pressure. 2nd, what is causing the really high oil pressure. Merrill and a few other posters have said this isnt unusual, and the FSM doesnt specify a max range. But I suspect this to be too high and am wondering if something else is wrong besides the faulty flickering light.

  5. #5

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    not being able to edit one's posts is a real pain - and baffling as it's pretty common on just about every other forum out there.

    I meant to say "I'd like to know why something as simple as an idiot light ISNT functioning properly"

  6. #6

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    Hello, MI-Mitzu. We live in a frustrating winter.

    You'll be able to edit posts once you get a few more posts under your belt.

    The best I can offer is to check the wire that goes to the sending unit, and make sure it's not grounding out by way of grease or oil, or shorting out somehow. I was stumped by the pressure relief valve, too, and the book says nothing about it.

    Essentially I'm just offering a sympathetic ear, and a shade-tree mechanic's view.
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  7. #7

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    Quote Originally Posted by royster View Post
    Hello, MI-Mitzu. We live in a frustrating winter.

    You'll be able to edit posts once you get a few more posts under your belt.

    The best I can offer is to check the wire that goes to the sending unit, and make sure it's not grounding out by way of grease or oil, or shorting out somehow. I was stumped by the pressure relief valve, too, and the book says nothing about it.

    Essentially I'm just offering a sympathetic ear, and a shade-tree mechanic's view.
    Thanks for that. The pressure relief valve is a bit of a mystery as it is the only suspect item in the FSM for high oil pressure... funny thing is the FSM doesnt really specifiy what high oil pressure is. I'm considering jsut spending the few bukcs ordering a new spring, piston, and washer, but as with any other OEM Mitsu part ordered online it'll take a week and a half to get these and it seems to be a pain to change unless you undo all the belts and remove the harmonic balancer. Sockets are too big as are open and closed end box wrenches.

    The assembly diagram for the pressure relief valve is really hard to see, but I assume the piston is going back in correctly.

    The truck does have 270k on the odometer and at this age I guess it's not inconceivable the spring could be original and is likely worn.

    In the meantime, I'll recehcek the wire for a 3rd time, maybe even cutoff the OE plastic clip and crimp on a new known-to-be-good spade connector and give that a shot as well.

    Any other inputs are appreciated.

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