Oil pressure fluctuation

Cobra_fd · December 1, 2015

I've got a 2005 Z71 4X4 ext.cab 5.3 Sierra, and I have been having an issue with oil pressure readings on the gauge. When I first noticed it, the gauge was at 0 kpa, so I pulled over and checked the level, which was at the top on the stick. Got back in and started it up , and the gauge registered 250 kpa, so I hoped slamming the hood closed fixed the problem. Nope. Went about 500 yards and the gauge dropped back to 0. Back under the hood and did the roadside fiddle-with-wires fix, and sure enough, back to 250 kpa. The trip back home was probably 10 miles, and in that time the pressure dropped to 100, then 175, then 60, then 0.

This seems to be the pattern for the most part, depending on the interval from start to stop to start again. Anything over about 20 to 30 minutes,and I don't get the start up 250 kpa.....unless it is a real cold start. The wire wiggle test and connections all seem to be good and clean. And I have to assume the gauge in the instrument cluster is good seeing as it does register pressure at times. There has never been any top end noise associated, so if it is pump related, there must still be oil movement. The engine temp holds steady at running temp as well.

My question would be...which direction should I be going?

Oil pump?

Sender?

Gauge?

Any help would be appreciated.

Thanks, Steve

Doug_Scott · December 1, 2015

put a tee connection on the engine, and install the stock sending unit on the one port, and grab a cheap mechanical oil pressure gauge from your local auto parts store to install on the other port. Run the line into the cab, and connect to gauge. No need to mount gauge, just drop it in the console cup holder. When you see stock gauge drop to zero, take a look at the mechanical gauge. If it is reading properly, you have just verified the expensive part is good. Now you need to start tracking voltage from the sending unit back to dash. Sending unit may be defective, and gauge in dash could also be defective. If voltage at sending unit is not changing when gauge is reading zero, then it will be a connection issue or the gauge in the dash.

If it were me, I go the mechanical gauge route first. But, then again, I happen to have one just for doing this when I was still on the bench. I bought one decades ago for $7 from a clearout bin at Canadian Tire.

Cobra_fd · December 1, 2015

Thanks Doug, ,was thinking about trying to match the thread to try a mechanical gauge, but never thought about tee-ing it. Now my next question is, why not just eliminate the dash gauge, providing there is no issue with oil pressure just the gauge/sending unit?

This may be a dumb idea but is there going to be an ECM issue if it does not have a good oil pressure signal?

After posting, I searched the forums and found many references to oil fluctuation issues, and I am hoping that I only have a sending issue and not the oft reported O ring on the pick up tube, cause I just changed the oil and filter before this showed up. As long as I have good working pressure, I will just drive it till next change is due I guess. If the pressure is not good, then it won't matter what state the oil is in, it will still run down my sleeve the same......just sooner.

Doug_Scott · December 1, 2015

Don't know about there being an ECM issue, but, since you have already had the truck running with the gauge showing 0 psi and the ECM did not do anything to warn/save you I would think it does not watch the pressure gauge in the dash. But, in the olden days when electric fuel pumps first showed up, they would depend on the oil pressure switch get power once engine was running (they used a "timer" to run fuel pump to start engine". If the sender said oil pressure was too low or zero, fuel pump would shut off. This was not to protect engine, it was to reduce chance of fire in accident. On your truck once fuel pump is shut off, engine will quit just about instantly. On a carb equipped engine, it would stay running until fuel bowl emptied, plenty of time to kill the engine from no oil pressure.

My 2010 did not even come with an oil pressure guage, you had to buy a SLE or SLT to get one in the Sierra line up. Same for Voltmeter.

Have never looked, but, they may sell a "A" pillar pod mount that will alllow you to install a couple of accessory gauges. You may be limited to electrical gauges though. If you are planning in installing a mechanical oil pressure guage inside the cab, make sure the tube does not chafe where it comes through firewall. Oil coming out of a pin hole under 60 psi will most likely cause a fire quickly if it happens to be aimed at the exhaust manifold. The finer the mist, the quicker the fire. Run the tube through a small hole in a large grommet to be safe.

newdude · December 2, 2015

Either the gauge in the cluster is acting up, or the pressure sensor is on the fritz. Run a mechanical pressure test first (with a gauge) to make sure the engine is good though.

Cobra_fd · December 3, 2015

Thanks to all for the heads up. Have determined as you thought Doug, there is no ecm issue, the fuel or anything else does not depend on oil sensor. Except engine,hahaha. We have checked and oil pressure is present, and the gauge does work when we supplied power, so I am changing out the sensor. Without taking the intake manifold off, can not verify whether there is a screen under the sensor or not. Mechanic buddy (40 yrs. pulling wrenches) says he has never heard of one being present. What is the consensus on it. Still have a little skin left on my hand, but trying to get a screen out will take care of the rest I would imagine.

Doug_Scott · December 4, 2015

Thanks to all for the heads up. Have determined as you thought Doug, there is no ecm issue, the fuel or anything else does not depend on oil sensor. Except engine,hahaha. We have checked and oil pressure is present, and the gauge does work when we supplied power, so I am changing out the sensor. Without taking the intake manifold off, can not verify whether there is a screen under the sensor or not. Mechanic buddy (40 yrs. pulling wrenches) says he has never heard of one being present. What is the consensus on it. Still have a little skin left on my hand, but trying to get a screen out will take care of the rest I would imagine.

As odd as it sounds, mechanics with decades of experience do not have anymore experience on the current level of technology than someone with 5 years experience. Don't get me wrong, he has shitloads of mechanical logic experience, but, he sounds like me, some of this new crap they are doing would never even dawn on us to look for.

I think the intakes are dry on these engines, so, should not be difficult to remove. Sometimes people confuse time consuming with difficult. And sometimes saving an hour can cost you many hours later on.

Cobra_fd · December 6, 2015

Now don't get me wrong, and mech buddy fully agrees, years of wrenching means squat if you don't strive to keep up with technology. He just hadn't heard of a screen being present. Could it be a USA only thing? Anyway, because we didn't pull the intake, we just mechanically check pressure, and it was good. So replaced the sensor and had the same results....pressure all over the gauge or not at all. I am now waiting for the delivery of a new step motor and will be new from start to finish except for wiring.

Got the heat turned on in the shop, and cold ones in the fridge....I got time and somebody to drink and tell lies with...life is good. .... ... ...

Doug_Scott · December 7, 2015

Now don't get me wrong, and mech buddy fully agrees, years of wrenching means squat if you don't strive to keep up with technology. He just hadn't heard of a screen being present. Could it be a USA only thing? Anyway, because we didn't pull the intake, we just mechanically check pressure, and it was good. So replaced the sensor and had the same results....pressure all over the gauge or not at all. I am now waiting for the delivery of a new step motor and will be new from start to finish except for wiring.

Got the heat turned on in the shop, and cold ones in the fridge....I got time and somebody to drink and tell lies with...life is good. .... ... ...

I may not have been clear in my post, what I was saying is that two techs in the same shop, one with 30 years experience, the other with 5 years experience, have the same amount of experience on 4 year old technology, provided they both work on that technology daily. It is just one of those things that stand out to me. Don't know why it does, but, then again, I have a weird way of seeing things.

Cobra_fd · January 17, 2016

Just wanted to put this post to bed with a final note. Thanks to all above for their experience and info and ideas. After trouble shooting my way from the sensor back to the gauge cluster, ended up it was the step motor. So after changing the stepper, and re-installing gauges, voila`, I have oil pressure.....but alas, I seem to have no voltage gauge. Turns out these little guys are tender and don't like much man handling. Good thing they came in a set of six. So after pulling everything back out and changing the volt stepper, we are good to go. The only thing I wonder about now is that I got the gauge needles back on in the correct position. Might be going faster or slower than what they are showing, or run out of gas before it says empty . Should be close enough !