low oil pressure at idle

Discussion in 'GM Powertrain' started by willardearl76, Sep 14, 2011.

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  1. willardearl76

    willardearl76 New Member

    After my truck warms up the oil pressure drops and the check gauge light illuminates. When the engine is cold at start up the pressure gauge reads 30PSI plus. After it warms up and at a stop in drive the pressure will drop to approx 5 PSI and will even dip to 1 or 2 PSI.

    Oil being used is 10W40.

    Mileage is approx 140 000

    Idle is a little rough but not too bad.

    Engine does not burn oil. A little smoke on start up but not bad.

    I am thinking of replacing the oil pump, but will that be a waste of time/effort.

    Vehicle info:
    1988 C1500
    Automatic Transmission
    5.7 liter
    140 000 approx miles

    Thanks for any advice or assistance.
  2. Mile High Assassin

    Mile High Assassin New Member

    I would get an OBD II reader and confirm that the oil pressure is really this low. Seems these gauges are not that accurate. I have also heard of the oil pressure sending unit going bad/out.
  3. Boonduff

    Boonduff New Member 100 Posts

    A OBDII reader won't work for you being your truck has OBDI, but Mile High is right, verify the oil pressure before you start replacing parts.
  4. willardearl76

    willardearl76 New Member

    I have changed out the sending unit thinking that may be the problem. So to check, do I have to plumb in a mechanical gauge?

    Thanks
  5. gmcman101

    gmcman101 New Member

    I hate threads that end this way.
  6. Caddiac

    Caddiac New Member 1000 Posts 100 Posts

    It does tend to leave one hanging doesn't it.
  7. nadcicle

    nadcicle New Member

    I'd like to know the results myself because I have a slightly almost same problem with my 97 burb.

    When I start it I have 3/4 of the gauge oil pressure. After I drive it for approximately 20 minutes the gauge slowly starts declining over the 20 minutes to the point that when I stop at red lights or stop signs it goes to 0 and says check engine. I also don't notice and valve shudder or engine stumble. I don't get any excess smoke from the tail pipe or anything. I have been running 10w40 as well and am just about to try thicker viscosity to see if it helps for now. I don't want to go throwing money at this thing again and have it out of commission as I have just took the course of 3 months to change the intake manifold gasket. I heard changing the sending unit in my model requires jacking up the engine to drop the pan so I'm kind of worried about doing something like this myself, but if it might be the sensor plug in the back of the engine on the driver side firewall I will replace that since it is easier and cheaper.
  8. marko54

    marko54 New Member 100 Posts

    I honestly think if your sender/guage is O.K. ,at 140,000 miles your assumption that the oil pump needs replacing is probably correct. your main bearings may also be shot.
  9. Ethan

    Ethan New Member 100 Posts

    Question 1, how often do you change your oil?
    Question 2, do you live up north, and why are you using 10W40?

    I use 10W30 in my truck, and don't have a problem at 168000 miles. I do know those trucks have a problem with the oil pressure sending unit being wildly inaccurate, so that just may be the issue. If you changed your oil every 3000 miles religiously, then your oil pump is fine. The smoke at start up is most likely your valve stem seals leaking, since these trucks are known for the umbrella style valve stem seals getting hard a brittle and cracking, not too much of a problem, and a easy fix.

    My oil pressure gauge will show 60 psi at cold start up, then drop to 40 psi when warmed up and driving, and drop to 20 psi or less when sitting idol with the engine warm. But I know the engine is in perfect condition. Manual says the pressure should be 40 psi at idol, but the gauge is inaccurate.

    You can test it by T-tapping a pressure gauge in line with your oil pressure sending unit, and see if you are actually running the correct pressures.
  10. nadcicle

    nadcicle New Member

    Here recently I can honestly say I haven't kept track of miles because I just changed my intake manifold and I didn't spare any expense with the oil changes. I have probably done 3 in about 600 miles as I have been experimenting after the first change. The first change was to get any residual out of the system from doing the intake manifold gasket swap. Immediately I just happened to notice that once I drove an estimated 20 minutes or so the gauge for the oil pressure slowly dropped to the point that at stop lights it was at 0 and the CEL was on.

    At the point of CEL I thought maybe something might have gotten caught up in the screen or oil pump somewhere so I changed the oil again without putting 60 miles on it from the last change. I drove it around for maybe another 200 or so miles and noticed that there was no change in the behavior of the gauge so I thought ok it has a lot of miles (eg 229k) so maybe it just wants a thicker viscosity as others have said that older engines require it because of worn internals. Since changing to the 10-40 the pressure is higher at start, but over driving it for the 20 minutes or so the pressure still goes to 0 at stop lights. I have just sorta tossed my hands up right now and kept driving it to a minimum, but can honestly say it doesn't seem to be pressure as when it drops to 0 there is absolutely no engine noise or excess shudder. Oh, and I live in Tennessee.

    I will say that I am about to do 1 last oil change back to syn from this cheapo castrol I have been using as I was just using it to "flush" everything else out from having done my manifold gasket swap. I just didn't want to have to change it again if it was my sending unit and I had to drop the pan to do so.

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