Thread: 7.5 professional rebuild 15K miles no oil pressure at hot idle

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Hi guys,
I need your opinions on this one. I had a 95 7.5 out of my F250 totally gone through at a professional shop. No corners cut. After I put it back in I changed oil every 1K to 3K miles and used Shell Rotella oil. Runs great. Around 15K miles I noticed that I had 0 oil PSI at idle with a hot engine. Changed sending unit and oil/filter. Still 0 PSI at hot idle. Checked with a mechanical gage 0 PSI at hot idle. Now the part that drives me nuts. 1K rpm and the same great PSI as when just rebuilt. Bring RPM up to 1K and the gage snaps right up to 60 PSI.
What do you all think?
I should also say that when the thing drops to 0 PSI there is no valve noise no difference in sound what so ever.

Ford Truck forums point to the stock oil pressure guages after '87 being glorified idiot lights with a pressure switch sending unit. Once above a set point minimum the gauge reads mid-range, but has zero analog accuracy. If pressure is below set point the gauge reads zero, and if it's above minimum it reads mid-range, which seems to be 60psig for yours.

Just for diagnostic purpose, pull the sending unit and hook up a mechanical guage. No need for a full installation in dash and everything, just be sure the tubing and gauge are safe from fan belts and exhaust and such, and see what readings you get.

Originally Posted by twolaneblacktop

Changed sending unit and oil/filter. Still 0 PSI at hot idle. Checked with a mechanical gage 0 PSI at hot idle.

Something may be stuck in your oil pump's bypass.

I had a cam wipe out in the Ford replacement engine that was originally in the truck when I bought it. One reason I bought it was that NEW dealer installed engine. swapped out the cam and could not get the engine cleaned up by oil changes or Rislone. Spun a rod one day after. That is why I put this fresh rebuild in. Built by reputable shop in Beavercreek Ohio.

IMHO, I would not run the motor until I found the reason for the zero pressure at idle. The first thing I would do is examine the oil and cut the filter open, looking for particles. If nothing in the oil, it could be a broken or jammed bypass spring in the oil pump or clogged oil pump pickup tube screen.
BTW, I've heard the formula has changed for Rotella oil and may no longer have enough zinc additive to protect a flat tappet cam.

if your getting a 0 reading with the mechanical then i think 36 sedan is right. it is in the oil pump. can it idle for an extended time with no oiling issues ?

I have heard about 460 oil pumps mounting boss cracking. Maybe that is the issue. Engine has a new Melling pump but it doesn't have a beefed up mounting boss like some do. Thanks for the Ford oil gage info. That is why the gage goes from zero to the same level each time. Sorry but the only way I can describe my feelings about this fake gage is Rat Bastards. There is still an issue just a different way to think about it. Looks like a tear down is in order. I was afraid that maybe I wiped out a second cam but there is no valve noise. If the cam was gone there would be valve noise all the time.

God I hope not. If that is the case I may have done in another cam.

Originally Posted by twolaneblacktop

God I hope not. If that is the case I may have done in another cam.

Don't jump to conclusions to fast, check the oil and oil filter for metal particles first. It may just need the pan removed and the pump repaired.

Originally Posted by twolaneblacktop

If the cam was gone there would be valve noise all the time.

Not necessarily so, I wiped a couple of lobes in a 400 sbc and I don't remember valve train noise, just extremely low on power. BTW, I lived in Lebanon until I was 8 years old and my dad, in the early 40's, was the night manager at the Golden Lamb.

Originally Posted by 36 sedan

BTW, I've heard the formula has changed for Rotella oil and may no longer have enough zinc additive to protect a flat tappet cam.

I believe that's true. New regulations on diesel emissions led to a drop in ZDDP in Rotella's oils a year or three back. If I were running a flat tappet cam I'd spring for Brad Penn's oil. Some say that once you've got a good break in that any major oil is OK, but I'd say that's playing Russian Roulette with your engine - at what point is it going to fail?

Ive also heard that newer synthetic oils are not good in older designed motors as it is too slick and thin for their bearing designs and doesn't hold oil pressure well in them.

Small world! Lebanon is a nice place today. Jazz and country music festivals in the streets in the summer. Santa comes to town in a Christmas horse drawn carriage parade in December.
Got to get my truck fixed. I can't get my Camaro to the strip with out it.

no synthetic here. I made that mistake before. Flat tappet engines do not do well with that stuff even after break in. I think that is what happened to the Ford replacement engine that was in the truck when I bought it.