Losing Oil Pressure
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Losing Oil Pressure
I have a '79 short bed 4x4 with a 351M. It has about 30,000-40,000 on a rebuild. Earlier I went out to my in-laws and got a full load of dirt. The bed was down just above the tires. We were out in a field to get the dirt and I had a pretty good climb to get out of the bottom area I was in. The truck climbed it no problem and drove all the way home without a hitch. The only problem was the oil pressure dropped down just a shade below the normal range. It's usually at the upper end when cold, then warms to between the low normal area and half. I shoveled all of the dirt out by hand and went to move the truck and the pressure was back in the normal range which tells me that as it cools, it will be back up there. My question is, is this normal? Should I put a heavier weight oil in? Thanks in advance for any advice.
Brian
Brian
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Joined: Nov 2004
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From: Ashland, KY
I had the same problem. Mine is a 302 in a 79. I planned on doing a freshing up on the motor and I have it tore down know and everything looks good. Sorry I can be of anymore help so I'm going to wait on some advice also.
Senior User
Joined: Nov 2006
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From: Oklahoma City
My oil pressure would fluctuate considerably on my 78 351M with 82,000 original miles until I put a diesel grade motor oil in. Now the random tappet sound is gone and pressure has stayed consistent.
Post Fiend
Joined: Jul 2004
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From: Northern California
These oil pressure sending units can fail or read low when they get hot. After they cool, they read normal again.
Most likely replacing the sending unit will fix it.
Also do what "79F2506.6L" suggests and test it with a mechanical gauge at the very least.
Most likely replacing the sending unit will fix it.
Also do what "79F2506.6L" suggests and test it with a mechanical gauge at the very least.
Cargo Master
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From: hinckley ohio
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Thanks for all the advice. I'll try your suggestions. I just fired it up and have normal oil pressure.
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From: Ashland, KY
Pulled the main bearings today and found the problem. I was going to put new in anyway but I should have better oil pressure. I thought about going to a thicker oil also. My motor only had 85,000 miles on it, but the way the bearings looked it should have had 285,000 miles on it.
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From: Duncan B.C.
A word of caution about changing to heavier oil to correct low oil pressure. When I did this on a 78 351m ( went to a 20/50 weight ),the oil pressure did increase, but shortly afterwards, it started throwing distributor gear roll pins. I think it was due to the increased drag on the oil pump. And when I tried going back to a lighter oil, it had developed a knock. Engine is now un-runable. I have heard of this happening to others, too.
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