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GlennLever's avatar
GlennLever
Explorer
Feb 27, 2014

Low oil pressure / inconsitant oil pressure

I'm a little concerned.

This is a 6CTA, Ref # 53-34874-004 engine # 45358278, model C8.3-325, read that as 325 HP.

When first cranked (weather cold -12, or hot 110) the oil pressure gauge on the dash pegs (all the way to the right, in the white above the green, not pressure indicated on the dash gauge, just red, green, white)

When fully warmed up and running at 1500 RPM about 55 MPH, it will sink to within 1/8 inch of the line between the red and green, never goes into the red (going down the road).

When idling it does fall into the red, but as soon as you accelerate away from a stop light it immediately climbs into the green.

If you bring the RPMs up to 1500 when sitting with no load and fully warm it pegs the top of the green.

When going down hill and at 1500 rpm it pegs the needle to the top of the white.

Low oil pressure warning only comes on before cranking and never while running.

I have it at a Cummins authorized shop right now. They have have explained there are two oil pressure sensors, one for the computer, and one for the dash gauge. They will be putting it on their computer to see what that says.

There are several entries on this when you do a google, but no resolutions offered, just a post describing sort of the same thing.

I have driven this from AZ to NY, Ny to NH and back, NY to Ky and back, and it has acted like this consistently. Chassis voltage is at 14 volts all the time when running.

Thoughts?

I have heard back from the shop, they could not put a computer on the engine because of the age, but did put a mechanical gauge on the engine and say it is "operating normally" what ever that is.

28 Replies

  • I liked the "For Amusement Purposes Only"

    That's a sarcastic quip only a pro would make IMO. Methinks you are in good hands.
  • GlennLever wrote:
    Mandalay Parr wrote:
    Maybe you can install a regular gauge.


    If it ever warms up here,-5 tonight and that is not wind chill, I will run a separate ground to the gauge, check for corrosion on the terminal of the sender, and then if all else fails replace the sender, and then the gauge to see if I can get it to work.


    No, Mandalay was talking about installing a MECHANICAL GAUGE.

    This is done by adding a "TEE" which will allow you put original sender back and have a mechanical gauge connected at the same time..

    I have personally had multiple oil pressure senders fail over the years. Each time the sender would give erratic pressure reading from cold to hot..

    A mechanical gauge doesn't rely on electric senders, instead it has tubing from the engine directly connected to the gauge and is a direct read of the pressure.

    This will eliminate the sender, wiring or even gauge faults.

    If pressure stays steady on the mechanical gauge then your engine does not have a fault. The fault would lie in the sender, wire or dash gauge.

    If the mechanical gauge does the same as the dash gauge then you do have a fault with the engine and further investigation may be warranted.
  • A mechanical gauge will be the only way to know... but you are still betting on the accuracy of the gauge. Generally they are pretty good.

    On my '93 GMC, the oil pressure gauge would fall to about 5 lbs at warm/hot idle, and would go to mid-gauge (40-45) at cruising speed. I always felt the gauge or sending unit was at fault, so I eventually replaced the sending unit with a dealer part. It read 25 lbs at cruising speed and just over 0 at warm idle. Disgusted with that, I put the original back in, then a couple years later I bought a Napa sender. The gauge reads 55-60 at cruise, and 15 at idle.

    The point is that 3 different senders gave me 3 different values.

    I trust the GM gauges to give me a general guideline, but I've both heard and now experienced the excuse for gauges Ford provides. On our MH, the temp gauge will come up to about 1/3 of the range as it warms up. When encountering hills, the gauge never moves but I know the water temp is gaining when the clutch fan kicks in. I've installed a ScanGaugeII, so now I can watch the actual temp as reported by the computer.

    Ford's gauges are not even gauges, they are idiot needles in lieu of idiot lights. Once the needle moves into the upper end of the range, the damage is already done. The only useful Ford gauges are fuel & speed.

    Does your coach have an J1939 and J1708 protocols (the diesel equivalent of ODBII) or is it too old for that? ODBII started in about 1996.
    Looking at the ScanGauge website, the ScanGaugeD monitors oil pressure.
  • Mandalay Parr wrote:
    Maybe you can install a regular gauge.


    If it ever warms up here,-5 tonight and that is not wind chill, I will run a separate ground to the gauge, check for corrosion on the terminal of the sender, and then if all else fails replace the sender, and then the gauge to see if I can get it to work.
  • dons2346 wrote:
    The pressure sensor that feeds the computer is the important one however either sensor not reading pressure will shut the motor down. With tolerance of 10% for the sensor and 10% for the gauge, it is possible to have an error of 20% on the low side. Cummins told me that 10lb of oil pressure at idle is plenty.Some one else in the business says that the dash gauges are for amusement purposes only.


    It drives me craze when I look down and see it though.
  • The pressure sensor that feeds the computer is the important one however either sensor not reading pressure will shut the motor down. With tolerance of 10% for the sensor and 10% for the gauge, it is possible to have an error of 20% on the low side. Cummins told me that 10lb of oil pressure at idle is plenty.Some one else in the business says that the dash gauges are for amusement purposes only.
  • The only part that might be a worry is the needle going into red at idle but an engine does not need much oil pressure at idle. If the oil light was coming on it would be of greater concern as that usually is set to come on at a lower level than is indicated by red on the gauge. If you really want some piece of mind consider adding a mechanical gauge. Since the shop said it was “operating normally” I would think they measured it with some sort of gauge and did not see a problem. You might ask them how they made that determination. If you were seeing low pressure at higher RPM’s it would be more of a concern.

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