Dropping oil pressure

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Echomirage

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I just went over the entire thread, and could not see how many miles are on this engine. That would be useful information when diagnosing mechanical condition. You were going to pull the pan anyway, and I think this is something you should do.

Lubrication of bearings is all about oil flow, not pressure. Low pressure just indicates either that the pump is not flowing enough oil, or that the bearing clearances are too big, leading to a lack of resistance to flow.

A clean sump and pickup, clean oil filter, enough oil, and a good gauge. That should be the starting point to determine if there's a problem here. If you get 25 psi at normal rpm hot, I think your engine is ok.


150K, im the 3rd owner. the 2nd owner did very little in terms of maintenance, unless it was broken. he owned it 10 years and only put 20k on it in the 10yrs. im not sure he changed the oil much at all.....the filter was rusted. of course i immediately did the oil and tune up. it was seriously neglected during his ownership. i dont think it was ever washed or vacuumed, let alone had preventive maintenance done. it was running on 7 cylinders, with #8 plug caked up solid.

i dont hear any new or bad noises when running. no ticking, slapping, grinding, etc. i went for a longer drive yesterday, and saw the dash gauge fluctuate again. it did it a little erratically; it wasnt a steady drop. it wavered and shook, so it could be a bad sending unit.

since the oil sat for so long with no use, im thinking sludge in the pan. i also looked up what it takes to pull the pan, and i doubt id be able to do it, especially alone. i dont have a soul to help me here. i may just buy a high pressure pump from rockauto, and bring it to a place and let them mess with it. i might shop around for a better quality gauge also, but even a cheap unit couldnt be that far off of actual pressure.
 

grampadirt

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Just snapped a pic of my gauges.This is at operating temp and maybe 25 miles driving.I'm not sure if it's any help though.The gauge on the far left is the trans temp.

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Echomirage

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so youre just below 20lbs also......is that at idle or cruising? what worries me about mine is the very low pressure at idle. im almost scared to drive it now, for fear of doing damage.

i only paid about 2500 for it, so getting a pump done will hopefully not cost too much.
 

grampadirt

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so youre just below 20lbs also......is that at idle or cruising? what worries me about mine is the very low pressure at idle. im almost scared to drive it now, for fear of doing damage.

i only paid about 2500 for it, so getting a pump done will hopefully not cost too much.
Sorry,I should have said....idling in gear.
 

Mrich0908

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Time for a hone and re-ring , freshen up. If your oil pressure goes to 0 at a red light its going to start knocking soon . After Verify you gauge is right . Most likely is. Plan to Re fresh your motor now before you need a complete rebuild.
Your motor bearings are most likely beyond proper tolerance. Rock auto sells kits for decent money. Consider your motor was running on 7 cylinders your oil was probally gas washed pretty bad. When intakes goes bad this happens also .
Goodluck.
 

Ironhead

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More basics: Has a compression test been done? Before I tore this engine down, I'd want to have more knowledge. Another question: what oil is in the engine now? Mostly thinking of viscosity here. And finally, it seems from reading this thread that there is some sort of inconsistency to the oil pressure readings--they sometimes drop to 0, then go back up. Why? I would definitely get any sludge out of the pan/pickup, and make sure the engine had a clean oil filter.

If I read this thread correctly, the engine does not knock. That usually means that the bearings are OK, or else that the compression is so low that there is not enough pressure in the cylinders to cause the knocking.

Think about it: a compression test is simple and even if you have to pay to get it done, cheap. An oil and filter change is also cheap. We know the maintenance was neglected, so sludge could very well be the issue.

Tearing an engine down to do bearings and rings is NOT cheap.
 

Echomirage

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it of course has fresh oil and filter. maybe 1500 miles on 5w30 and a wix, with a can of seafoam added. i was planning on changing it soon, 1K to 1500 miles, because of the neglect.

im hoping the gauge sender is bad, since the needle will fluctuate. its not always a steady drop and hold, itll jump up and down on its way down. im going to order a high volume pump regardless, and have them take a look at whatever they can while the pan is down. and probably look into a compression test, one way or another.

i figured i would have problems with this. id rather it have 200k steady miles, than 150k neglected miles. worst case ill have to get a new motor, maybe shop around for something with regular use and maintenance.
 

Echomirage

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on amazon now, ordering a melling M77HV high volume, intermediate shaft and pickup tube, along with an innova 6 piece compression kit.

its going to hopefully, eventually, be used for towing, so a high volume pump, regardless of original pump condition, certainly wont hurt it.
 

Mrich0908

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The engine was gas washed . This happens from having a dead cylinders. Or the common bad intake gasket. If you oil drops to zero at idle it means your main bearings are toast .
Your motor WILL knock soon and WILL need more than a refresh .
Oil pump wont help if your bearings are bad. They lost there tolerances.
Refresh kits start at 275 at rock auto. Thats pretty cheap . Kit even comes with pistons resell them to recoupe a few bucks.
Bearings could be checked as simple as pulling an oil pan off. They slip right out. I redid a set of main bearings only on an old truck for the same reason in a few hours after work one day.
Lasted a year until I decided to do a full rebuild. I went though exactly what you are. What your going through isnt uncommon it happens all he time. Just google low oil pressure at idle. You will read what I wrote.
Start your motor let it get good and hot . 200 degrees. Pull a valve cover and see if your top end is getting oil. Itll tell you if all your bearings have too much clearance or not. I wouldnt be surprised if you barley have oil going to the top end.
http://www.rockauto.com/en/moreinfo.php?pk=5966613&cc=1166453&jsn=453&jsn=453

http://www.rockauto.com/en/catalog/gmc,1997,yukon,5.7l+v8,1166453,engine,engine+rebuild+kit,5316
 
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Echomirage

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got the oil report back.....

There are some issues here. Potassium and sodium show coolant. There was also enough water present that the oil boiled during the flashpoint test, preventing us from checking for
fuel. Water measured at 2.0% and insolubles are quite high as a result. There is a lot of metal too. Iron is from steel parts. Lead and copper generally show bearing wear, but copper can be from any brass/bronze. The coolant did dilute the additives, so actual wear might be higher than this. Change this oil and make repairs.
 
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