new to the site and need info on oil pressure its a 1990 xj with 169xxx 4.0 20w50 fram filter replaced pump verified gauge is right more the less stock when it is cold 75 psi at 700 at temp 40 at 700 rpm my problem comes in on long trips after awhile the pressure drops to 20 at 2000 rpm the valves rattle when put under load at temp they dont when it is cold and body got any ideals thanks
What brand oil are you using. What is the API spec on the oil you are using?
Most of the 20W50 oils drop in pressure due to sheer loss of viscosity. They use polymer additives to get the 50 weight viscosity when hot spec, but under extended drives the polymers sheer and the oil pressure drops. It is normal, it is the oil doing it, not the engine. Some brands, and the older API oil specs (like SL, instead of the newer SM), hold up better. I have found the old API-SL spec Havoline gives me better hot oil pressure on long drives, but I found an old inventory 2 years ago, and stocked up. Autozone has a 20W50 that is still SL spec, that I use on my 89, but it has less miles.
Jeep minimum oil pressure spec is 13 psi (with an accurate gauge!!!!), so if you have a real 20 PSI hot after long drives you are OK.
The noise is another issue entirely.
You might want to read up on ZDDP (zinc....) additive that has been removed from the new oils, that we need in our older engines. There are several long threads, debate about it and flat tappet problems when it is not in oil we use. STP is one one additive that has it that is easy to find that might help.
I used Lucas on mine for 2-3 years ago with MMO together, as I cleaned up my every old (280,000 mile 87, 4.0) and worked the kinks in the valve train (loud ticks and knocks at cold start up), but know I run straight 20W50 Havoline (old SL spec, old inventory), as the noisy valve train is now quite. Mine runs about 58 psi cold and 21 psi in the summer after a 1 hr drive, at hot idle in 100 F weather.
I also never use any oil filter other than K & N now.
