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Oil Pressure

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CamJam View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote CamJam Quote  Post ReplyReply Direct Link To This Post Topic: Oil Pressure
    Posted: Feb/08/2016 at 7:31pm
I don't have a lot of miles on my 390 since the rebuild, but have taken a couple of 20 and 30 mile runs previously. Today I took a short drive, at moderate speeds and ambient temperature of about 60 degrees F, and for the first time I had the oil pressure light come on whenever the engine revs dropped below about 1,200 rpm, i.e. at idle. Whenever I had to stop for a light I made sure to keep the engine revved up a little to keep the light off. Obviously I'll need to check the sending unit, but when these things are operating normally at what psi does the light come on?

Water temp was below half and I'm running Valvoline VR 30W racing oil.

EDIT:  Correction... oil viscosity was actually 20-50W


Edited by CamJam - Feb/09/2016 at 11:03pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rebel Machine Quote  Post ReplyReply Direct Link To This Post Posted: Feb/08/2016 at 8:39pm
Oh boy.... I've seen this movie.

A few years ago I had my 401 rebuilt and experienced the same problem you're having. Instead of the low pressure light it's got a gauge. I think the light comes on around 7 lbs.

When the engine was rebuilt the machine shop set the main bearing clearances between 0.035" and 0.040", which is twice what it should be. The book they got the information from was wrong.

The symptom was dropping oil pressure after the engine had been at operating temperature for more than 20 minutes.

If you end up taking your engine apart check the cam bearings. If the babbit has been stripping off check the main and rod bearing clearances.

-Steve-
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Post Options Post Options   Thanks (0) Thanks(0)   Quote CamJam Quote  Post ReplyReply Direct Link To This Post Posted: Feb/08/2016 at 9:00pm
Thanks for the response, Steve. Since my post I've since read a few older posts with similar issues. Sounds like there are any number of issues that could cause this.

Of course, things can happen with any engine, but mine was built by a very knowledgeable AMC guy who is active on this forum, so at least I'm confident that was built by someone who knows what he's doing when it comes to AMCs. The engine has maybe 250-300 miles on it now and the problem just started, so if the oil pressure is indeed low I'll check the obvious things like oil filter first.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ADAM12MATADOR Quote  Post ReplyReply Direct Link To This Post Posted: Feb/08/2016 at 9:15pm
First thing you should do is install a mechanical oil pressure gauge. Just temporary so you can actually see what the pressure is. What brand, weight of oil are you using? What oil filter? Rule of thumb is 10 lbs for every 1000 rpm. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sweatlock Quote  Post ReplyReply Direct Link To This Post Posted: Feb/08/2016 at 9:18pm
I think a decimal place was left off there for the main bearing clearances listed above.
Anyway, if you're lucky and the engine was built right, it could just be the sending unit - they can and do go bad. Pull it out and thread in a known good pressure gauge. That will determine if it's truly a low oil pressure issue - it's just a matter of what's causing it at that point. Also check for really obvious & silly stuff, like a low oil level due to leaks or oil usage. Occam's Razor

Best of luck
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amxdreamer Quote  Post ReplyReply Direct Link To This Post Posted: Feb/09/2016 at 12:19am
My fingers are crossed for you Cam, hoping it's nothing serious. Have you got a gauge you can install to confirm what it's actually making for pressure? It would be a shame to have to pull the engine now.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AMXFSTBK390 Quote  Post ReplyReply Direct Link To This Post Posted: Feb/09/2016 at 1:21am
The engine may have insufficient oil viscosity. If so, and the engine let's say has 10w30, replace it with 10w40.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 401MATCOUPE Quote  Post ReplyReply Direct Link To This Post Posted: Feb/09/2016 at 5:02am
Just my opinion Cameron, at 60 deg OAT, 10w30 should be more then adequate for a fresh 390......10w40 would just be a band aid, especially when your summer temo gets well over 100 deg OAT.  1. Check that oil level, fresh engines CAN consume some oil....and that would be normal, especially if you had moly rings but in, my 401 in Jeep took 3 oil changes to calm down the oil consumption..that is why I no longer use Moly Rings, most of these engine will never see lots of miles.     2. Check the oil pressure (like others are saying) with mechanical gauge.  3. If good just change the sender; if bad go back to the engine builder...I know who did it and he will make it right.  Did he give you a spec sheet on the bearing clearances when it was assembled??  If he did check it, you spent more then enough on it, it should be right.  Be methodical.....step by step.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Boris Badanov Quote  Post ReplyReply Direct Link To This Post Posted: Feb/09/2016 at 5:42am
Originally posted by Rebel Machine Rebel Machine wrote:


Oh boy.... I've seen this movie.

A few years ago I had my 401 rebuilt and experienced the same problem you're having. Instead of the low pressure light it's got a gauge. I think the light comes on around 7 lbs.

When the engine was rebuilt the machine shop set the main bearing clearances between 0.035" and 0.040", which is twice what it should be. The book they got the information from was wrong.

The symptom was dropping oil pressure after the engine had been at operating temperature for more than 20 minutes.

If you end up taking your engine apart check the cam bearings. If the babbit has been stripping off check the main and rod bearing clearances.

-Steve-





X2, you got a minor internal leak, like excessive bearing clearance.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Feb/09/2016 at 6:17am
Was a valley oil line installed? A leak in that will drop pressure. As others have been saying -- a mechanical oil gauge needs to be installed, even if just temporarily. If you don't want to do that right now swap out the oil sending unit with a new one. Good chance that's all it is, but could be something internal. A plugged oil filter can drop pressure, but the bypass should function in that case. Won't hurt to change the oil. See if the shop that built it can analyze the oil or know a local place that can -- that can tell you a lot!
Frank Swygert
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