oil pressure question

bagdranger · Dec 9, 2009

Alight fellas, i got a set of sunpro gauges to watch my vitals and i recently put a new crank in my 85, and if I run 10-40 oil the oil pressure wants to run bout 20 psi while cruising. Before the crank install I was running 20-50 royal purple it would stay around 40 on the highway at about 1600 rpms. Why does this motor want to run that kinda pressure at crusing? When i run the 10-40 weather its synthetic or not it wants to run low psi but with the 20-50 its higher and stays there. Oh right now its got 20-50 oil from autozone and even still it wants to stay about 25 psi while crusing. Just wanna make sure this is normal or if somethin could be wrong.

Eric Kropp · Dec 13, 2009

I am not so sure that is normal. Is the gauge you are using mechanical? I was never sure about my oil pressure with the stock electric gauge so I put a mechanical one on and mine idles at 40 psig and runs at 60 psig, which is what the manual says it should be. My engine has over 320,000 miles on it.

Eric <><

bagdranger · Dec 13, 2009

Well i just changed the oil today to synthetic and it still does the same thing but the oil pressure reads about 10 psi more, and yes its a mechanical gauge. The other thing i didnt mention in the origional post was if I'm cruising and release the gas and let it coast it builds back up to round 40 psi. Ill see what she does tomorrow on my way to work because its a 45 minute drive so itll have time to warm up all the way.

Bob Ayers · Dec 13, 2009

Too much clearance on your main and/or rod bearings will cause low oil pressure.

snoopdoggie · Dec 13, 2009

Give some more detail about what was done when the crank was changed.

bagdranger · Dec 13, 2009

well seeing that when the motor was rebuilt at the begining of the year my machinist matched the crank rods and bearings after shaving the crank. And i guess putting a crank kit in this time it has a hair more clearance with the rods being machined from before. If thats the case what can i do to fix this.

Eric Kropp · Dec 13, 2009

I am no machinist, and I may be some what of a hair ball mechanic....but I would think that if the crank/bearings were fitted in such a way that you loose half of your oil pressure, I would think you would be hearing some noise from that area. I tend to wonder if you have a bum pump. I replace the pump on mine when I had the engine out just because it seemed like a logical thing to do, but even my old one ran at 40psig idling and 60 cruising. The bummer is, I really don't think it is possible to change the oil pump w/o removing the engine...

Eric<><

bagdranger · Dec 13, 2009

When i built the motor at the begining of the year it was replaced. I didnt replace it this time because nothing was wrong with it. The night before the motor came out it was reading 40 psi at idle but number 6 rod bearing had spun. But theres no noise coming from the lower side of the motor.

brokedown · Mar 10, 2010

Either a pump or bearings and my guess would be bearings. You could add some Lucas Oil Treatment...I did so in my '84 only because I do not want to pull the engine and rebuild or replace it yet. My pressure is now running about 30 psi low and 45-50 with a load. Compression is good and the engine runs well though I do not drive it much as I am purchased it last summer and am going through it in my spare time when I have spare $$$. (Which isn't often these days).

bagdranger · Mar 10, 2010

Wow this is an old post and I forgot this was here. Well bad new on the low oil pressure. It was excessive clearence on the rods and sad to say but either #5 or 6 went through the side of the block. So V8 swap here i come.

kimcrwbr1 · Mar 11, 2010

I would talk to the machineist who matched the bearings just to see what he says. I allways plastiguage rod and main bearings prior to putting it back together to be sure it is within tolerance. And while I have it apart there is no reason not to put a new high volume pump in.

brokedown · Mar 11, 2010

Amen! I have rebuilt two VW 1.8 motors and always played close attention to all the bearings as well as a new oil pump regardless. For the minimal cost it is like insurance. Both ran like a champ, had great compression, and the oil pressure was well within specifications. Sorry to hear about the motor but am geeked about the V8. Good luck.

bagdranger · Mar 12, 2010

I am too. Starting to find parts now. Hopefully within the year itll be ready to be on the road again.

jester77 · Mar 13, 2010

wow- I have the same problem- well not quite- My engine has 360,000 miles on it- the pressure gauge was reading low- so I unhooked the sending unit. Now I don't worry what the gauge says_ i just drive, when the thing goes T-U- I'll think about the 302 or 351 swao

oil pressure question

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