I have 97 F150 with a 4.2 V 6 with 75000 miles. The oil pressure was fine at start up but when warmed up the pressure gauge would drop showing no pressure at all while stoped at a light. I changed the oil sending unit,oil pump and now have installed a mech.gauge. With the new gauge in it shows 15 to 20 lbs pressure while driving but when stoped in gear at a stop light it reads 4 to 6 lbs of pressure.If i put it in park it will go up to 10 lbs.
What to do now???Not looking to rebuild or anything costly.
Thanks ,Mike

 

Sounds like either your oil pump pick-up is clogged or you have excessive bearing clearances.
You can try running a heavier oil to see if the pressure increases. Maybe 10w40 or 20w50.

 

Will try heaver oil and post reply.thanks

 

I changed the oil to 20/50 and the pressure is still low.Howwould the excessive bearing clearances be repaired and what would be an average cost?

 

I had a 1989 F150 300six And the oil presser was always reading low. The mech told me to bang on the dash and it should bring back the right PSI. Well it did and I always came back so try that before you spend any money on this project. take it from me it cost me $50.00 to find that one out that made me mad.

 

He installed a mechaical gauge. This rules out a gauge problem.

 

is your oil level maintaining full capacity if not you could have a leak in a seal or the pan.

 

Mike, your situation indicates a clogged or dirty intake at the oil pump. Going to the heavier oil would make the situation worse. As the oil heats up it gets thicker and is more resistant to flow. Viscosity is exactly that, resistance to flow. I would suggest that you change your oil again using 5w-20 which is approved by Ford for use in your truck. I would also suggest a cleaning agent like Auto-Rx or Seafoam. Auto-Rx would be my favorite but avoid the quicky flushes as they do nothing more than reduce the viscosity and are basically solvent based. Auto-Rx is ester based similiar to the esters used in synthetic motor oils and won't strip the engine. Make sure to follow the instructions with the cleaner that you use.

 

I had the exact same problem. Mechanic thought it was a clogged screen. He thought it was the oil pump. It wasn't any of those inexpensive things . It was the throw out bearing and I had to have the engine replaced. The 4.2 is notorius for being poorly designed from the start and Ford won't own up to it. Sorry this happened to you and MAYBE you may find a cheaper way out , but it sounds like you have the same problem I did. Try putting it in park and reving it up to high RPM's. If you hear a knock you"ll know its the bearings.
Cigar

 

Mike 420 I know this is an old post but, I was wonding what was your fix for this problem? I have the same problem.

 

Mike,

Sorry to hear about your problem. Now the bad news. In my case I had to replace the engine , at around 128,000 miles. Cost a little over 4 Grand, from Jasper. The good news is I have almost 100,000 miles on the new engine and 225,000 on the truck itself. In my case, as you may have heard, Ford put some faulty gaskets in the early 4.2 engines. The mechanic said it was not my fault, as I changed the oil every 3,000 miles.
I hope this was of some help. By the way I did this about 4 years ago, I intend to keep the truck at least two more years, so for me it was a worthwhile investment.