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Ok so I will do my best to keep this short and to the point. I have a 93 chevy truck w/5.7L V8 with about 306k miles. about 6 months ago I replaced the intake gasket because I notice some steam and water bubbling out of a pin hole up by the pass valve cover. All went well except when I pulled the intake off, a large glob of hardened congealed oil fell out from under the driver side head. I cleaned and vacuumed as much as I could. I did a "pretty" good job of maintaining the gasket shavings but in hind-sight I should have placed a towel or something in the valley (I'm not a mechanic), and I found that the port for the EGR valve was completely clogged so I cleaned it out.
I put everything back together and put all fresh oil and filter back in/on. Next day driving to work the SEL comes on. After a few miles I finally noticed that the oil pressure gauge was down around 30 psi and slightly fluctuating. Throw it in neutral and rev it and the psi climbs a bit but less and less the more I drive it. I park it at work, come back out an hour later and crank it and the light is off and the pressure is back up to about 55. Drive it home that afternoon and same thing, SEL back on and PSI starts to drive, same symptoms as an old engine with worn bearings, right? Ok, so here is the monkey wrench with that conclusion, the my motor has ALWAYS had good oil pressure (not great) but typically between 40-50 psi. So what I figure is that some of that junk made its way down and clogged the oil galleries... or something. My thought is that the oil is pumping to the top of the motor but not making it back down.
Ok, so here is what I have done: I pulled the oil pump to clean it. No difference. Replaced the oil pump. No change. (I will say that a motor with that many miles was impeccably clean inside). Drained all oil and ran a flush through... multiple times. and put fresh oil and filter back. I pulled out the fuel pump fuse, put kerosene in the motor and turned the motor over several times for a few minutes each time allowing it to drain back down through the galleries and put fresh oil and filter. No change. Pulled and cleaned the pressure sensor. No change. I tried cranking it and checking the oil level at crank vs the level after the psi starts to drop below 30 psi and the difference is only 2 holes on the dip stick, so now I'm wondering if my original assessment is correct. I put (at a friend's suggestion) Marvel Mystery Oil. I will say that now the PSI nearly peaks 55-60 psi when I first start it but after 10-15 minutes it's back to it's old tricks. Sometimes when I drive it short distances (too afraid to drive long distances) the SEL will come on, and other times it doesn't but I can't see a difference in performance to determine why. I can't run a code reader cause the light is always off by the time I turn the key back on. I ahve called about 100 shops to see if anyone does a "high pressure flush" and no one does it anymore because it voids manufacturer warranties and some says it damages seals. On a positive note, I have NO more oil leaks.
I'm at a loss and people are starting to tell me that the motor is just old and worn out and that I should get it rebuilt or replaced, and if it always had poor oil pressure then I would be on board. I'm not ready to give up on her. I need some ideas. Thanks in advance for any suggestions...
 

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I can't run a code reader cause the light is always off by the time I turn the key back on.
You should be able to run a scanner or do the "paper clip" trick and see what code it's throwing. These OBD1 ECMs will throw a SES light and then it'll go out but, it will store the code for 30-40 key cycles (or you disconnect the battery). I don't think the light is for low oil pressure since they usually don't throw a light until you go below 10PSI IIRC. If I had to guess it's probably your EGR which has nothing to do with your oil pressure issue. Just in case you don't know how...


I think you have a faulty pressure sensor!
Let's hope that's all it is.🤞
 

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It’s all good. I was open to being wrong. My Explorer has a check gauge light that occasionally lights on my way to a gas station.
 
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