Hi my 1988 XLT 4x4 2.9L truck can't make up it's mind. Oil light comes on when ever it wan'ts. I replaced it with a new one to make sure it wasn't bad. My lifters start clanking then they'll stop making noise when ever . Oil light goes out.Runs great. I let it Idle and once warm oil light pops on again. Rev it up light goes out then comes back on at higher rpm. Let Idle lifters clank again...and so on. seems erratic. Weak oil pump???? slop plugging pick up screen??? I'm running 20w50 oil is has 188,000. It ran today for 1/2 hour no oil light no noise then Here we go again..clanking valves ,oil light on...

 

Welcome to FTE.
Did the oil pressure warning light & noise begin After you began using the Much heavier than specified crank case viscosity lube?
This time of year, where you live it would likely appreciate a 5W-30, or a 5W-20 that Ford has back specified in 02.
Crankcase lube with that high a viscosity won't drain back from the top end to the oil pan sump quickly enough to keep the sump level properly topped up as designed. So the sump level can drop, the pump suck air under high rpm demand when you rev the engine, because too much remains in the top end, because it's too vicious to quickly drain back, especially when cold. If the oil sump level gets low & the oil pump sucks air, it'll aerate the oil & the engine won't like that.
Aeration can happen on the top end too, as the valve train can whip the thick oil into a froth. Lots of reasons Not to use that viscosity in that engine.
Any sludge deposits under the valve covers?
When you last changed oil, were you able to rake any sludge out the oil pan drain hole, using a ? mark formed piece of wire?

 

There was a time when the oil pan gaskets installed disintegrated and left little 'pellets' in the bottom of the oil pan. Said pellets would gang up and plug the oil pump pickup screen. To my knowledge this was more prevalent in the 2.3 than the 2.9.
The 2.9 had cam bearing problems, I think caused by the oil flow required by hydraulic lifters taking some of that which had previously been directed at the cam bearings.
Given the erratic behavior, I would also suspect a possible sticking pressure relief valve built into the oil pump.
Were it mine... I would temporarily attach a mechanical oil pressure gauge to get readings cold, warm, hot, at idle and at speed(1.5-2K rpms). You would also be able to check how quickly oil pressure dropped when the engine is shut off. Slower pressure drop tends to indicate closer or tighter bearing clearances.
So, if it drops like a rock, has low (l.t. 10psi per 1k rpms) oil pressure hot, I'd point to the cam bearings, only from what I have seen reported. Dropping the pan and putting in rod & main bearings might bring the cam pressure up to acceptable readings. It might not.
If you have looked, the lifters installed are 'gem sized' as that engine was designed for mechanical valve train back when, so they had limited room for hydraulics. I *think* the lifters may become leaky or sticky with age. If they leak too much oil, they'll clatter.
tom

 

Worn engines tend to have problems maintaining enough oil pressure at idle when the engine is hot. Generally going to a thicker oil like 20W50 is the right thing to do because the thicker oil does not leak from the worn bearings as fast, allowing some pressure at hot idle. Your suggestion that the oil pressure problems occur when the engine is hot, sounds like a worn bearing problem.

I agree that a mechanical gauge would allow you to see what is going on. Ford started using an idiot light sender in trucks in the '87 time frame, even in trucks with oil gauges. Those gauges will read either mid range or zero. I believe the switch over point is about 5 psi. You would like to have 7 or 8 psi at hot idle, or better.

If the mechanical gauge shows a problem only at hot idle, you might be able to add an oil thickener like Lucas oil stabilizer to keep enough idle oil pressure. If so, I would caution you to go easy on the revs while the engine is cold, and use a good oil filter that will pass a lot of oil, like Motorcraft, Wix, or NAPA Gold. If you are in an area that gets very cold in winter, this very thick oil might make starting a challenge on cold mornings.

 

Since it is not a constant problem, that seems to indicate a sticking or malfunctioning oil pressure relief in the oil pump or possibly blocked screen on oil pump pickup. A worn oil pump should not cause the higher rpm loss of pressure, but could contribute to idle pressure issues.

If you always get the light flickering at hot idle, that indicates excessive bearing clearance as suggested. Thicker oil is just a crutch, it does nothing to address the root cause problem of excess clearance as the thicker oil just causes enough flow resistance.

I agree to add a mechanical gage to really get a better idea of what conditions and exact pressures when the oil pressure problems occur.