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Oil pressure gauge in red, then normal

Twice in the last year my 1999 Ranger (@138,000 miles) has started running very loudly: I checked dash and see oil pressure gauge says no oil pressure. First time this happened, I pulled over, had it towed. At garage they start it up: gauge normal, they check oil says its OK. I take it to the dealship, they replace sending unit, but can’t explain why it was running loudly. Today same thing happens: I checked oil pressure gauge, it’s OK. After an hour, starts running loudly, gauge in red. Park car for 15 - 20 minutes, start it up: all is well: gauge OK, running quietly. This is amy ‘spare’ car, so it onoy gets drivven 1 -2 times per month, short distances.

To paraphrase, you have experienced two incidences of low oil pressure and you could actually hear mechanical noise from the engine. The reports are that at the time checked your engine had normal oil pressure but a sending unit was installed because it was thought that the unit could be intermittently failing. No one can explain your reported engine noise. You want to know what to do about this?

If you did not report engine noise I would continue on the path of a false oil pressure indication. You do not really describe what an engine operating with no or low oil pressure sounds like, you just say “loud”. This inaccurate report may be because you don’t know how else to describe what you heard. I say get the oil analyzed to see if any bearing material is present in the oil. I must believe you actualy had 2 events of low oil pressure and your engine ws actually making noise. If the report comes back clean (or even before the report comes back) perhaps a oil pressure check with a mechanical gague, or even have a gague installed. We are on a data gathering mission. At the time the sample of oil for analysis is taken inspect the filter carefully, perhaps even cutting it in half looking for bearing material. Lots of work but we must believe your report.

Which engine is in your Ranger? The 3.0L Vulcan engine has some issues from time to time with the oil pump drive failing, leaving the engine with no oil pressure. Normally they will make noise before failing, though, and not fail intermittently. If you have a 3.0L V6, this is a possibility to look into.

Yes, it has the 3.0L V6 engine.

By running loudly I mean having a pronounced chatter or clatter (what Iimagine is meant by the term ‘piston slap’, but I could be wrong), and it seemed to have a noticable vibration. It was loud enough on both occasions to attract my attention even though the radio was on.