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Oil pressure low at idle


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Recently rebuilt engine on 46 Buick Super from head manifold up. After getting it running and going through the post rebuild rides I find that after several miles the oil pressure gage goes way down at idle but seems fine once moving. I know this could be a function of several things but don't have the troubleshooting skills to determine if I'm harming my engine. What could be causing this?

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Which oil gauge, the original 1946? Buy an oil pressure gauge and tell us what pressures you are actually getting. How high does your pressure get, and how low is it at idle? Modern gauges work exactly the same as original, but they may be a lot more accurate. Use the new one to verify your old gauge. What oil viscosity are you using? It's normal for oil to thin when your engine warms up. Give us more info.

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I had this same problem...if you or your assembler installed a gasket between the bottom plate of the oil pump and the body, this will mean too much clearance and wil cause low oil pressure. I took mine out and immediately oil pressure was normal.

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What shape are your bearings in?!?!?!?!?!?!?!?!? You may want to look at them if your oil pressure really is good; this could be a sign that your bearings are about to go; this is EXACTLY what both my cars did before "tick, tick, tick, clackity, clackity, clackity,...Awwww s**t, I know that sound, f**k!!!!!!!!!!!!!!!!!! After that you get to pull the whole motor apart and get all the filings out, before your rebuild, Happy, happy, joy, joy!

Jaybird

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<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Mike.Belforti</div><div class="ubbcode-body">Recently rebuilt engine on 46 Buick Super from head manifold up. </div></div>It sounds like you never took the oil pan off.

Ok, does this mean you did a valve job and rebuilt the carb? Is it a 6? or a V-8? The major players are rings, main bearings, rod bearings, valves, and seals. Then, after careful inspection, a new oil pump (or at least new rotors) might be in order.

How many miles are on this engine? When you pulled the head, was there a pronounced ridge at the top of each cylinder wall? Mike, did you do the work?

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They were an option on the 46 so a lot of times there is no oil filter. If you have one, it sits at the top of the engine right next to the head and in front of the distributor. It looks like a cannister with 2 tubes going to it and a big bolt in the top.

Float on a oil filter, I have never heard of that, there is a screen that you could check.

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The pickup floats in the oil and has a pivot on it...If it were stuck, you'd have zero oil pressure at all times...if pressure rises above idle, it's not stuck. You could easily have a plugged pickup though. You can pull the oil pan in the car...I've done it several times...a real mess, but it can be done. Be prepared for an inch of sludge in the oil pan.

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<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: simplyconnected</div><div class="ubbcode-body">How many miles are on this engine? When you pulled the head, was there a pronounced ridge at the top of each cylinder wall? Mike, did you do the work?</div></div> You never answered any of these questions.

Here's my concern: If you did a valve job, and your rings are 100k-old, the newly restored pressure will dramatically shorten the life of already-worn out rings. In other words, they won't last long. It happened to me. I didn't have enough money to do the whole job, so I just did the heads. Oh, no. It only lasted a couple thousand miles, and started blowing smoke for failed rings. I ended up doing the whole engine but bought gaskets twice.

Do it right, or leave it alone.

I reccommend you pull the engine and do a true rebuild. I assume you already did the head. Now, change the rings, bearings, and seals. PULL THE OIL PUMP APART, AND LOOK AT THE ROTORS. If you see imbedded steel, get new rotors or replace the pump. Measure the cylinder walls for wear. If you have score marks or a pronounced groove at the top, have them bored .010", buy pistons, and rings to match. In the end, your engine will be better than new, with proper oil pressure.

If you are doing the work, you will need a good shop manual with all the specifications. Taking lots of digital pictures along the way will help, too.

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If it's only low at idle, it's probably an indication that you have some wear, bearings, rockers, etc. You don't really need much pressure at idle though. General rule of thumb is 10 psi per 1000 rpm. I had a '65 Impala with a 283 that the oil light would turn on at idle on hot days, it was old and worn. I ran it that way for years, thousands of miles, no problem. If it's not making noise, I wouldn't worry about it until you are ready to do a full rebuild. Again, that's assuming you have good pressure at speed.

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Sometimes you can get away with a head only job...I think the big problem with lower end wear after a head job is that people let the antifreeze from the block drain into the cylinders after pulling the head...you can never drain all the antifeeze out of the pan, and antifreeze is acidic and can eat bearings. It also is not a great main or rod bearing lubricant!

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My feeling is that if the valves are worn to the point that it's smoking or not sealing well, then the rest of the motor isn't far behind and that explains most of it. I've heard the increased compression explanation my whole life, but I'm still skeptical. Actually cylinder pressure varies widely as a function of throttle position and engine rpm. If you are cruising down the road at x mph, that requires x lbs of torque, which equates to x psi at a given rpm. Improving your valve sealing isn't going to change that number. Now, improved valve sealing would affect peak cylinder pressure at WOW, and maybe that has an affect if yo drive it that way. I suspect contamination likely also plays a role in many subsequent ring issues.

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<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: rlbleeker</div><div class="ubbcode-body">I've heard the increased compression explanation my whole life, but I'm still skeptical. Actually cylinder pressure varies widely as a function of throttle position and engine rpm.</div></div>

Reconditioned valves make good use of compression ratio, which increases pressure all by itself from the piston top, through both rod & main bearings. Restored compression ratio (pressure) allows for MUCH more combustion than the engine has seen in a very long time. If the rings are worn, they have more space behind them in the piston groove, causing the ring to slam becase hot gasses are flowing around the ring. It always gets worse because other components are affected. (Hot gasses flowing around the ring increases piston and engine temps.) Upon ring inspection, you can see a wavy wear pattern on the ring tops and bottoms. New piston/ring clearances are tightly sealed, have very little blowby, and can withstand any pressure, seemingly forever. Worn-out engines are fragile and very inefficient. All components need to be restored together or left alone, because it is a matched system.

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Ring groove depth is generally at least .005" greater than the radial thickness of the ring. To wear "behind" the ring would require the ring to leave the cylinder wall by more than that amount. It is the lower ring landing that wears. This allows the ring to move up and down when the piston changes direction which can lead to broken rings and/or lands. On the compression stroke the ring will already be against the lower land. Compression/combustion pushes the rings out against the cylinder walls, this is why proper ring seating on a new engine requires running under varying loads.

I generally agree, leave it alone or do it right. I don't however, hesitate to fix a burnt valve, for example, on an older motor that otherwise appears OK. I've done this many times with good results.

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Ever see an engine billowing smoke like a medieval dragon? As rings wear, two things happen: Since they want to expand against the cylinder wall, the end-gap widens AND they get smaller in minor diameter (which puts more air gap between the piston groove and ring. Even if the end-gap widens to .040-.050" all that intake-stroke oil isn't going through that little gap. A big portion is going around the ring in the groove.

Proper ring seating (mating the ring face to the cylinder wall) has little to do with 'running under varying loads.' Hastings Piston Rings suggest LOADING new rings to break them in:

"Make a test run at 30 miles per hour and accelerate at full throttle to 50 miles per hour. Repeat the acceleration cycle from 30 to 50 miles per hour at least ten times. No further break-in is necessary."

Here's the link:

http://www.hastingsmfg.com/ServiceTips/breakin_procedure.htm

I guess 426 Hemi's better find a long, steep, mountain or pull a heavy trailer. Otherwise, they will never get loaded. You can see why chrome rings take so long to break-in. - Dave Dare

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Yes, that is exactly what I meant by "under varying loads" - meaning not just idle or cruise, but heavy throttle to get cylinder pressures up. I guess I wasn't real clear there.

I get what happens as cylinder walls wear, and if it's puffing a big blue cloud of smoke, I agree, it's time for a rebuild. What I'm skeptical of is that a valve job will turn a non-smoker into a smoker in a short length of time. I've heard this as common wisdom for years, but I've not seen it in my own experiences nor seen an authoritative source for it.

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I was dumbfounded to learn that a smooth cylinder causes rings to hydroplane, resulting in, you guessed it, an oil burner. Like grooves in the highway under slick-highspeed conditions, the oil has somewhere to go under the ring face. I always knew the grooves held oil, but I thought it was for lubrication. Makes sense, too. New cylinders with sharp and deep crosshatch use a small amount of oil, then it gets better for over 100k miles. When the crosshatch is worn to a mirror finish, the engine uses much more oil.

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