I've been racking my brain trying figure this out, and have been through this forum, but still am not clear on my problem.

Ok, here goes.

I have a 2000 Expedition 4.6 with about 140k miles. Has run great for me, but a few months ago, I noticed that the oil pressure guage was intermittently going to "no pressure", but then kicking back to "normal pressure" after a short period. I had assumed it was just a sensor issue, and ignored it, since it always went back to normal after a little while. Of course, after reading this forum, I know that this is only an "all, or nothing" indicator. I had expected to see the guage drop slowly if there was a real issue.(IE: not just on/off)
So, I drove in this fashion for a couple months with no problem. Then one morning a couple of weeks ago, I started up the truck and was greeted with horribly loud knocking sound, which quieted a little after a few seconds. I drove a couple blocks to the gas station, and added a quart of oil, which quited the "knocking" further, but was still noisy.
It had been a while since my last oil change (yeah, yeah, I know :-( ), so I thought that perhaps my filter was clogged, and oil was not getting through the entire engine.
Based on this theory, I changed the oil, and filter (with a Fram, of which I now know is garbage).
Upon draining the old oil, It did seem a quart or two low.

Now, when I started it up, I am still greeted with a very loud knocking for a couple seconds (I assume this is before the oil has a chance to circulate), after which It quits slightly. It is coming from the left side of the engine, but I can't quite be sure if it is coming from under the valve cover or not.
When Reving the engine, the knocking gets very loud. I am afraid to drive it until I know for sure what is going on. I don't know if it is "Piston slap", or failed "lash adjusters", or something else. I have let the engine run for about 15 minutes, but the knocking does not go away..

Hopefully, based on the known "low oil" factor, someone can tell me the most likely culprit.

Thanks Much.
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Welcome to the forum

It's not piston slap. That sounds more like a diesel. The first thing you need to do is hook up a mechanical oil pressure gauge and see what kind of pressure that you actually have. It sounds like it could be a bad lash adjuster, but it's hard to say for sure at this point. Does it run smooth or is it rough?

 

Thanks for the initial reply. It runs smooth, except for the noise.
btw - and it is not an injector. It is much too loud for that, and I did disconnect them to be certain. It seems to be loudest around 2000 RPMs.

 

Sounds to me with the engine being low on oil in the first place and most likely the oil pump is getting weak, that you threw a bearing on the crankshaft or a rod bearing.
Sorry about that, but if it is a loud knocking sound coming from the bottom of the engine, it sounds like bearings to me.

Brian

 

really sounds like you should get ready for the worst. The guage rising and falling was the tell-tale sign of the low oil, as the pump picked up the oil to supply the engine there wasn't enough left in the pan to keep pressure; then as some oil ran back down to the pan the pump would pick back up and show pressure. Sorry to say, you should take these things more seriously.