Oil pressure and knock
Thread Starter
Advanced
Joined: Aug 2015
Posts: 55
Likes: 1
From: Hanover PA
Oil pressure and knock
Today I took my '81 to have the alignment done. First time it has been on an extended run since I got it running. As I was coming home, I noticed my oil pressure was dropping down to about 20 and I could hear what sounded like a knock. When the engine is cold, it sits around 50 with no knock. I have a SB 350 and am running syn 20w/50 Castrol GTX oil. Could the oil be the problem here? I took it out again this afternoon to see if it did the same thing, and yep it does. No noise nice pressure when cold, low pressure and knock when hot.
TIA
TIA
Pro
Joined: Mar 2015
Posts: 681
Likes: 31
From: Nebraska
There are a few things that can cause a knock, but I would be wondering why there is 20w50 in the crankcase. Did you just buy it and the PO had 20w50 in it?
It is conceivable that the 20w50 caused wear that could result in a knock. When the engine is cold, that oil will not flow into place as needed like the 10w30 does.
I would change the oil and then begin to evaluate the knock.
It is conceivable that the 20w50 caused wear that could result in a knock. When the engine is cold, that oil will not flow into place as needed like the 10w30 does.
I would change the oil and then begin to evaluate the knock.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
As your '81 has an electric oil pressure gauge that uses a sender its best to check the pressure with a good mechanical gauge. Chances are your sender is going bad and causing your gauge to read low. And the knock could be your imagination because of the low pressure reading. If a mechanical gauge also reads exceptionally low you can start worrying.
Last edited by toobroketoretire; Sep 3, 2015 at 06:49 PM.
Race Director
Joined: Jul 2015
Posts: 13,082
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From: SW Florida.
just cuz engine was rebuilt 2k ago doesn't mean it was done right. did he give you paperwork from the rebuilder? if not, it was probably bubba's auto repair that did the "rebuild" and i would start worrying now...
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Le Mans Master
Joined: Oct 2007
Posts: 7,451
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From: RI, Now Franklin/Nashville TN
I have about 1,500 miles on my rebuilt L-82 355 with a roller cam and use Mobil 1 0W-40 European Formula (1,000 PPM ZDDP), the only grade of Mobil 1 that is a true PAO group IV synthetic, and I can assure you that Buba did NOT do the rebuild...all quiet at any temperature.
Le Mans Master
Joined: Apr 2011
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From: Hermosa
I have about 1,500 miles on my rebuilt L-82 355 with a roller cam and use Mobil 1 0W-40 European Formula (1,000 PPM ZDDP), the only grade of Mobil 1 that is a true PAO group IV synthetic, and I can assure you that Buba did NOT do the rebuild...all quiet at any temperature.
20W50 should be much too thick for a fresh rebuild. Not only that, but dino oil has a bad habit of shearing out of it's viscosity rating.
Polymers are added to 20W to get the 50W rating when hot. Those polymers get sheared or shredded after a fashion. Then all you have is cold 20W oil and hot 20W oil. When hot you're gonna see the oil pressure drop due to this shearing of the polymers.
Drifting
Joined: Jun 2010
Posts: 1,781
Likes: 15
From: Oxford Ohio
If it's a rod, locate the cylinder in question. Hook up a timing light to each spark plug wire one at a time. Continue until the flash matches the knock. Drop the pan and check that rod bearing. While there mic the crank or plastigage that rod to see what the clearance is. You can guess for days and days. Your time will be better spent just digging in and locate or eliminate the issue. May just have been rebuilt, but all rebuilders aren't equal.
Race Director
Joined: Jul 2015
Posts: 13,082
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From: SW Florida.
At keast with the engine set back stead of over suspension crossmember, pan is pretty easy. If one is knocking, check and plastiguage all rods. Read part number, date, off the bearings. Aftermarket will say sumpn like cb663p .010 or std or .040 if it was a high volume engine mill like jasper. They bore and cut everythi g to the max so they don' hafta change on tge machines. If orig bearings sumpn like 6 digit number and 11-81 or 73 or whatever year engine was assembled.
Le Mans Master
Joined: Oct 2007
Posts: 7,451
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From: RI, Now Franklin/Nashville TN
If you have a flat tappet cam, Mobil 1 15W-50 (ZDDP-1,200 PPM) is made specifically for these type of motors The only grade of Mobil 1 synthetic recommended for Flat tappet cams BTW)...I used it for years in the OEM L-82 and the cam lopes and lifters looked brand new when I tore down the engine.....FWIW.
Last edited by jb78L-82; Sep 4, 2015 at 09:02 PM.
Burning Brakes
Joined: Feb 2014
Posts: 1,045
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From: Cape Neddick Maine
2020 C3 of the Year Finalist - Modified
knocking with warm or hot oil and low pressure is not good.
Did you ever pull off the oil filter and check the relief port in the filer adapter? I usually plug these to filter my oil 100% of the time
The way they are set up is a fail safe that if you have high pressure it wil BYPASS the filter and send dirty oil to your engine instead of no oil.
By plugging it, if the filter gets dirty you may get pressure drop or loss of oil going to the bearings.
Any one that does this really needs to be careful and aware of their oil pressure and oil changes.
I run MOBIL 1 10w30 also with a roller cam
I modify all my oil adapters and never had any problems If I am not mistaken the GM relief only filters the oil when the pressure is below a certain psi and only filters a certain percent.
Did you ever pull off the oil filter and check the relief port in the filer adapter? I usually plug these to filter my oil 100% of the time
The way they are set up is a fail safe that if you have high pressure it wil BYPASS the filter and send dirty oil to your engine instead of no oil.
By plugging it, if the filter gets dirty you may get pressure drop or loss of oil going to the bearings.
Any one that does this really needs to be careful and aware of their oil pressure and oil changes.
I run MOBIL 1 10w30 also with a roller cam
I modify all my oil adapters and never had any problems If I am not mistaken the GM relief only filters the oil when the pressure is below a certain psi and only filters a certain percent.
Thread Starter
Advanced
Joined: Aug 2015
Posts: 55
Likes: 1
From: Hanover PA
Changed the oil to Mobile 1 10w30. I also put on a new K&N oil filter. noise is still there. Pressure dropped a bit, but not as bad as yesterday. From listening to the noise, it sounds like a rocker arm or two are loose. I'll probably take valve covers off tomorrow and check all the rockers. Does anyone know the torque spec for the rockers?
Le Mans Master
Joined: Dec 2006
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Changed the oil to Mobile 1 10w30. I also put on a new K&N oil filter. noise is still there. Pressure dropped a bit, but not as bad as yesterday. From listening to the noise, it sounds like a rocker arm or two are loose. I'll probably take valve covers off tomorrow and check all the rockers. Does anyone know the torque spec for the rockers?
Le Mans Master
Joined: Oct 2006
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Back the rocker off until it really taps loud, then to 0 lash plus 1/2. Can't miss. Messy, but can't miss.
Never hurts to do a good visual inspection while the covers are off. I've seen problems with rocker arms, pushrods, valve springs and retainers.
Instructor
Joined: Jul 2007
Posts: 245
Likes: 2
From: Hope Indiana
Changed the oil to Mobile 1 10w30. I also put on a new K&N oil filter. noise is still there. Pressure dropped a bit, but not as bad as yesterday. From listening to the noise, it sounds like a rocker arm or two are loose. I'll probably take valve covers off tomorrow and check all the rockers. Does anyone know the torque spec for the rockers?
Valve noise sounds more like a tick (sharper sound). Rod knock usually sounds more like koncking on a door (more dull, not a "tick).
This link describes engine noise pretty well.
http://www.onallcylinders.com/2014/0...engine-noises/
