I posted this same problem on another website (with good results) and looking here also for additional comments/suggestions.

I have just recently replaced the head gaskets in a 193k mile 4.2 auto F150 4X4. The engine has been running great but now after driving a distance getting the engine warmed up. The oil pressure will fluctuate when the truck is stopped and in gear (low RPM). The idle RPM dont seem lower than normal and the O.P. fluctuated aprox mid range. If you put it in park or neutral with the engine idling the oil pressure is fine, or when the oil is cool its fine. It has never done this before as its always had steady, non fluctuating O.P. It has aprox 1000 miles since the repair.
The engine has fresh oil installed, new oil filter and is not low of oil. Should I change the oil pump or is there something else that could be causing this that should be verified first?

I finally got some oil pressure numbers with the engine warm and hot. This was with a calibrated gage routed into the cab. I also lowered the gage to aprox the same heigth as the oil press port on the engine to see if it effected the pressure indication, but it was about the same readiings.

ENGINE WARM Idle @ 800 RPM - 18 PSI, Idle in-gear @ 600 RPM - 13 PSI, 1500 RPM - 36 PSI, 2000 RPM - 47 PSI, 2500 RPM - 48 PSI.
ENGINE HOT Idle @ 800 RPM - 12 PSI, Idle in-gear @ 600 RPM - 8 PSI, 1500 RPM - 27 PSI, 2000 RPM - 40 PSI, 2500 RPM - 44 PSI.

With the oil pressure (indicator) flickering* around 8 PSI I'm assume that is where the O.P. switch is preset to trigger at. I raised the Idle to 700 RPM (in gear) which put the oil press to 10 PSI hot until I look further for my actual problem with the engine.The oil seems some what dirtier than it should be. I guessing possibly due to the oil is probably flushing the old oil/collant that maybe didnt get fully drained? Could this be the reason for the lower oil press?

Any thoughts for the next thing to do with this engine/truck?

Thanks

 

motor is just old and a little tired. 8 psi is not too bad at idle when hot. new filter and oil might give you an extra 1-2 psi and keep the light off. If you using 5 w 20, you could change to 10 w 30 and maybe get a couple psi. 10w30 is not too thick.. Also a different pressure switch might be set more like 7 psi instead of 8 ?? just a couple ideas.