Rebuild or no

[seawalkersee]

So, after the engine warms up I lose oil pressure. Yadda Yadda Yadda, long and short is that my wife has a buddy whose hubby is the diesel instructor for a localish votec. I am still awaiting a price for all new bearings. However, he hit me back in an e-mail and told me he had a set of .03 pistons he would give me.

My question is; Since I have only owned the engine for maybe 2k and the milage was unknown, should I go with a rebuild? I have NO clue what the previous history was. Its a 6.9, but on the cold engine, it went from 250 (I think) to 400 in the compression test. I am just worried that after I get it all together, I will drive it for a few thousand mi. and then the block worm will come visit.

SWS

 

[Fordsandguns]

I think that the 6.9's aren't as prone to cavitation as the 7.3 engines due to having thicker cylinder walls. Even boring it for the new pistons it should be fine.

 

[typ4]

6.9's dont cavitate.

 

[87crewdually]

6.9's dont cavitate.

I wouldn't say don't, just less likely to find one, but has happened. I would do a more accurate documented compression check to see how far the pressures are apart. If the pressures are within 10% of each other and over 300, i wouldn't do a rebuild.
For your oil pressure, hook in a mechanical gauge and monitor it. Your more than likely to have a faulty gauge or sender.

 

[RLDSL]

If you haven't replaced the sender yet, do that first. The little spring gets worn and soft with age and does like you describe

 

[kas83]

If your compression varies that much, I'd spring for the rebuild.

 

[RLDSL]

Try the sending unit first and if that stabilizes the oil pressure, 400 is good, there's a very good chance the low holes are due to sticky rings from soot buildup. Try running a couple of treatments of Auto Rx through the thing before tearing into it. Ive seen that stuff bring holes that were way lower than that to up over 400 after a couple of treatments from rings that were packed up. At the very least, if you do wind up having to go in, you'll have a spotless engine to deal with

 

[Diesel JD]

I'm pretty sure the oil pressure drop is either the sender or something else not related to bad bearings. There is a relief spring in the header of the oil cooler where the filter screws on. If this spring is worn you'll get low oil pressure at idle. Honestly if you're looking at the factory gauge all you know is that the oil pressure is at least 7 psi if it's in that range somewhere. I was told one time that you could get some sort of sender to make that factory gauge actually read somewhat more accurately. If it's in the range and the check engine light isn't on, it's probably pretty normal. With a mechanical gauge you should see a bare minimum of around 7 psi at hot idle, preferrably closer to 15 or 20. The more important reading is going down the road at 2000 rpm and up it needs to be 30 or better. Now if you have holes as low as 250 and as high as 400 it isn't ever going to run right. Would have a rough idle and be tough to start in cold weather would be my guess. So if this guy will do a build for you on the cheap and you trust him and can be without your truck it might work out well for you. RL could be right about just some sticky rings causing the problem. Usually you'll either see a smooth worn out engine from bad maintenance or just lived out it's lifespan or one with all good #s and then one or two that are zero or very low cause something went very wrong! Bottom end problems are rare but not unheard of. Todd C managed to break a crank, first one of those I've seen in about 7 years surfing IDI forums.

 

[franklin2]

General rule is 10psi per 1000 rpm as a minimum.

 

[fx4wannabe]

Warm oil will have less pressure than cold oil also.

 

[icanfixall]

What are you using to determine this low oil pressure... The factory dash gauge just isn't accurate at al and causes lots of issues just like what you may have... An after market gauge is the only real answer and buy one with a 270 degree sweep.

 

[seawalkersee]

Okay, forgot to add that to this thread, the guage is from a mechanical autometer guage. Temp warms up, oil pressure drops to "0". I will go back to my other thread and find what the compression is.

SWS

 

[seawalkersee]

Went back and looked at it, 340-440 was the difference. 340 was indeed the lowest and 440...welll, you get the idea. None the less, the oil pressure drops to zip after it warms so it is going to have to come apart.

SWS

 

[sassyrel]

NOT NECESSARILY--the oil pressure spring, on the backside of the oil filter base--can either get weak as stated--orrrrrrrr, like it did on my brothers truck--it got stuck--OPEN--youd have pressure cold--but as soon as warmth came --ZEROOOOOOOOOO. the oil pump puts out LOTS of volume--but not wild pressure-----take the oil cooler of,may be about due for o rings---drain all coolant, and expect a mess----youll see the presure reg washer and spring on the back of the cooler. we did rods,mains,oil pump,only to find that washer stuck canted--i took it apart---polished it with scotchbrite,fine grit-----NOTHING ELSE--,and put it back together,and staked the backup washer--still going fine when he sold the truck this last summer--200,000 later!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[icanfixall]

Its just as posted above... That spring wears thin because of the drag it sees in the regulater. There isn't any replacements either... Ford sells the rear header for around $300.00... When I rebuilt my motor I shimmed the spring 70 thousands. I idle warm at 25 to 30 and see 75 to 80 on a cold motor... Then I see around 55 on a warm motor at 2000 rpm....