Much better oil pressure after oil change.

[OneofFew]

Background: 245k miles, bought neglected, sat a lot and oil was well overdue for changing based on time.
Not long after the 1st oil change I had to replace the oil filter again due to oil pressure drop offs. After changing the filter, all was well. I have been using Mobil1 10w30 synthetic and the A/c delco oil filter. PF47 I think, switched to the larger capacity PF61 after the initial oil pressure drop off issue. The second oil change occurred within 3k miles as it seemed to be accumulating debris.
now that I have put about 15k miles on it, it is becoming clear that my oil pressure is much better right after an oil change and it starts to drop off pretty quickly after. I have been changing the oil at 5k intervals since.
Initial oil pressure is around 40psi hot idle and 55psi at 2k rpm. It does not take long for the oil pressure readings to drop to 25/40 respectively with some fluttering of the needle.

The only thing I can think of is that the oil is getting diluted, but I don't see any evidence of a fuel injector leak. The oil seems fine when I change it. I may have to pull the spark plugs to check for indications of an injector leak. No Mil lit. Fuel mileage is always around 15mpg with a 5.3 4x4 with 50% highway, 50% windy mountain roads and dirt roads, some requiring 4x4.

 

[mattbta]

Any leaks? I think my rear main is leaking as I have noticeable oil under the truck and everything but that has been replaced. I also noticed oil pressure drops significantly after 400-500 miles after an oil change. I'm thinking it is due to the leak. I need the check my level and see if it is getting low and top off to see if pressure rises.

 

[Rocket Man]

I would send a sample to Blackstone Labs for analysis. They may be able to shed some light on it.

 

[Larryjb]

Pop off your valve covers and look for evidence for cracked heads. These symptoms are very similar to mine. Now I have rebuilt heads and oil pressure stay up for the duration of a normal oil change interval.

I didn't realize this at first, but a small coolant leak involving Dexcool causes oil sludging, not milkshake oil.

 

[OneofFew]

Pop off your valve covers and look for evidence for cracked heads. These symptoms are very similar to mine. Now I have rebuilt heads and oil pressure stay up for the duration of a normal oil change interval.

I didn't realize this at first, but a small coolant leak involving Dexcool causes oil sludging, not milkshake oil.

I do need to replace the valve cover gaskets anyway, so I may do that soon. It has not been using coolant at all, so I doubt any coolant is getting into the oil. After 5k miles, the oil seems to be in good condition, no sludge, just a bit dark- which I attribute to washing out of old debris from years of neglect.
But as I said, I will pop the valve covers and check for sludge. Thanks for the suggestion.

 

[OneofFew]

I would send a sample to Blackstone Labs for analysis. They may be able to shed some light on it.

That would probably be a good idea. If there is fuel/oil dilution beyond the typical rates, it would definitely show that.

 

[OneofFew]

Any leaks? I think my rear main is leaking as I have noticeable oil under the truck and everything but that has been replaced. I also noticed oil pressure drops significantly after 400-500 miles after an oil change. I'm thinking it is due to the leak. I need the check my level and see if it is getting low and top off to see if pressure rises.

I add maybe a quart between oil changes, which I think is good. It doesn't leak more than a few drops, but I have never heard of an oil leak reducing oil pressure unless the level drops way low to where the pickup is drawing air.

 

[OneofFew]

If I was adding more oil between oil changes and the pressure drop was more linear over time, I would suspect engine blow by diluting the oil with fuel.

 

[Rocket Man]

That would probably be a good idea. If there is fuel/oil dilution beyond the typical rates, it would definitely show that.

Also will tell you of any unusual engine wear ie bearing material, coolant, silicones, etc. They're very thorough.

 

[Larryjb]

Take a look at post #31. This what you could be looking for. It's not a fast leak, so you might not notice any coolant loss before it sludges in the sump. The filter would prevent sludge from getting back to the heads.