Oil pump low oil pressure and has a loud knock in the lifters

Lifter noise is an indication of low oil pressure, that can be from low oil level and other things like a defective oil pump or plugged oil passages or excessive engine main and rod bearing clearances. Please have the oil pressure checked with a special tool, DO NOT DRIVE THIS WITH LOW OIL PRESSURE.

Check this information about "engine noses"...

ENGINE CLICKING NOISES
A clicking or tapping noise that gets louder when you rev the engine is probably "tappet" or upper valvetrain noise caused by one of several things: low oil pressure, excessive valve lash, or worn or damaged parts.

First, check the engine dipstick to see if the oil level is low. If low, add oil to bring it back up to the full mark. Is the engine still noisy? Check your oil pressure. A low gauge reading (or oil warning light) would indicate a serious internal engine problem that is preventing normal oil pressure from reaching the upper valvetrain components. The cause might be a worn or damaged oil pump, a clogged oil pump pickup screen or a plugged up oil filter. Using too thick a viscosity of motor oil during cold weather can also slow down the flow of oil to the upper valvetrain, causing noise and wear.

COLLAPSED LIFTER NOISE
Worn, leaky or dirty lifters can also cause valvetrain noise. If oil delivery is restricted to the lifters (plugged oil galley or low oil pressure), the lifters won't "pump up" to take up the normal slack in the valvetrain. A "collapsed" lifter will then allow excessive valve lash and noise.

VALVE LASH NOISE
If you can rule out lubrication-related problems as a cause, the next step would be to remove the valve cover(s) and check valve lash. On older import engines, mechanical lifters require periodic valve lash adjustments (typically every 30,000 miles). Too much space between the tips of the rocker arms and valve stems can make the valvetrain noisy -- and possibly cause accelerated wear of both parts.

To measure (and adjust) valve lash, you need a feeler gauge. The gauge is slid between the tip of the valve stem and rocker arm (or the cam follower or the cam itself on overhead cam engines) when the piston is at top dead center (valve fully closed). Refer to a manual for the specified lash and adjustment procedure. Also, note whether the lash spec is for a hot or cold engine (this makes a big difference!).

On engines with hydraulic lifters, oil pressure pumps up the lifters when the engine is running to maintain zero lash in the valvetrain. This results in quiet operation. So if the rocker arms are clattering, it tells you something is amiss (bad lifter or worn or damaged parts) or the rocker arms need adjusting.

DAMAGED ENGINE PARTS NOISE
Inspect the valvetrain components. Excessive wear on the ends of the rocker arms, cam followers (overhead cam engines) and/or valve stems can open up the valve lash and cause noise. So too can a bent pushrod or a broken valve spring.

RAPPING OR DEEP KNOCKING ENGINE SOUND
Usually bad news. A deep rapping noise from the engine is usually "rod knock," a condition brought on by extreme bearing wear or damage. If the rod bearings are worn or loose enough to make a dull, hammering noise, you're driving on borrowed time. Sooner or later one of the bearings will fail, and when it does one of two things will happen: the bearing will seize and lock up the engine, or it will attempt to seize and break a rod. Either way your engine will suffer major damage and have to be rebuilt or replaced.

Bearing noise is not unusual in high mileage engines as well as those that have been neglected and have not had the oil and filter changed regularly. It can also be caused by low oil pressure, using too light a viscosity oil, oil breakdown, dirty oil or dirt in the crankcase, excessive blowby from worn rings and/or cylinders (gasoline dilutes and thins the oil), incorrect engine assembly (bearings too loose), loose or broken connecting rod bolts, or abusive driving.

Bearing wear can be checked by dropping the oil pan and inspecting the rod and main bearings. If the bearings are badly worn, damaged or loose, replacing the bearings may buy you some time. But if the bearings are badly worn or damaged, the crankshaft will probably have to be resurfaced - which means a complete engine overhaul or replacing the engine is the vehicle is worth the expense.

ENGINE PINGS OR KNOCKS WHEN ACCELERATING
The cause here may be Spark Knock (Detonation) caused by an inoperative EGR valve, overadvanced ignition timing, engine overheating, carbon buildup in the combustion chambers, or low octane fuel.

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Before driving it anymore I would have someone test the actual oil pressure using a mechanical test gauge to see how much , or little, oil pressure you actually have. The knocking noise could be a hydraulic lifter that is partially collapsed due to low oil pressure or a rod bearing that has been damaged from low oil pressure. If it is just a lifter that is making noise then there's a pretty good chance that the lifter will quiet back down once the oil pressure is normal again. If it turns out that your oil pressure is normal both at an idle and at highway speeds then the noise may be unrelated. You can cause some serious damage by continuing to operate any engine that is low on oil pressure even if the dipstick shows it to be full of oil. Hope this helps and good luck!

If you need an oil pump you will be experiencing problems that are associated with low oil pressure. The most obvious sign would be an oil pressure warning light coming on. Other symptoms would be an oil pressure gauge that is reading very low (if your vehicle is equipped qith a gauge). Valves and valve lifters continuously ticking or loud knocking noises in the engine are some of the more severe symptoms.

if this engine has hydraulic valve lifters, then it could be a weak hydraulic lifter that pumps up once the oil warms up. Hopefully you dont have low oil pressure. Does the oil light stay on for a while after you start the engine. If it does that could cause your engine to knock

I doubt it....It could be a totally worn out engine.

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what I would try first is replace the oil pump. you say you're not getting oil to your lifters. no oil pressure is a sign of bad oil pump. I had a ford fairmont do the same thing, and once I replaced the oil pump; put a WIX oil filter on. and used a blended(syn and reg) oil, it fixed it. don't start anymore until you've done this.

Are you sure it's the lifters? I had a problem with an 86 Subaru, part of a solution at one time was to run 90w gear oil in the engine in an attempt to get back missing oil pressure which was the root cause of my problem.

And then the problem developed a bit more such that I had air in my oil and that caused the lifters to always click and clatter no matter how clean or thick the oil was. And then I discovered the front seal on my oil pump was dangling on the input shaft not sealing a darn thing.

After the new seal, I got rid of my air and the lifter clatter and now I'm also back on Rotella 10-40 oil. Darn old Subarus always run low oil pressure so no real relief there.

What's your oil pressure? Where is the oil pump on that thing? Any chance the pickup tube is sucking air? It's amazing how just a little air can make the lifters just go ape.

Does the oil light come on or the knock occur during the engine warm up after a cold start?
There could be a couple of problems causing the issue. You could have an oil pressure pump or pump pressure regulator sticking. I would have a mechanical oil pressure gauge installed and verify oil pressure at idle and 2500 rpms to make sure oil pressure is in normal ranges.
You could also have excessive piston rod bearing, crankshaft bearing or cam shaft bearing wear that will cause oil pressure to drop too low to prevent the rod knock your hearing at 2500 rpm.
You said a knock and not a ticking noise,so I don't believe a lifter is the issue at 2500 rpm.
Start with the oil pressure check and go from there.
Let me know.
Regards,

It all depends how loud the knock is. A tapping valve can be caused by low oil level, or dirty oil. Topping up or changing oil will solve the problem. A really loud valve tapping is abnormal. Cars sometimes make what you call a spark knock when accelerating hard. Extremely loud knock coming from deep within the engine is a bad sign, which means the engine has failed. Further examination would reveal metallic residue on the dipstick.