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Old 09-23-2002, 08:36 PM
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Post WEIRD oil pressure

I just drove my car to work for the first time after a putting in new rings and bearings in the motor. I always had used the stock electric oil pressure guage and it seemed to always work well. A friend convinced me it would be wise to purchase an aftermarket mechanical guage, so i did , it was a inexpensive one that uses a plastic line. My oil pressure at idle is about 60psi and when it warms up it is about 55psi. When i was driving it I mashed the throttle down in second gear about half way and the oil pressure dropped to about 30psi. When i gradually increase the rpm the pressure increases but it the needle on the guage stutters and move back and forth a little. The plastic line doesn't seemed to have much oil in it and looks to have air trapped in it. The line is sold in a role so it is curled up. The motor is a fresh motor that was built buy a local proffesional engine tuner that builds dirt track motors. Is the guage just a piece of junk or is the problem in the line. I guess I'll put the electrical one on and see if it does the same thing. Suggestions a plenty please!

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Old 09-23-2002, 09:38 PM
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What kind of motor is it?? Your pump might be running the sump dry, I know this is a promblem with some engines, especially if you have a ton of oil pressure (i.e. 65 psi @ Idle!!!)
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Old 09-24-2002, 05:04 AM
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A high volume oil pump will such the pan dry. All the oil is up in the heads and the top end. You may meed more oil capacity to solve rectify this. I would try a better guage first. I would suggest Autometer or VDO.

Steve
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Old 09-24-2002, 09:15 AM
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That is just the nature if a mechanical guage.
It sounds identical to the way my motor was when I put it together(SBC) The plastic live works from pressure in the line not the amount of oil in it.
You are fine. The only way that you are going to get a filled gauge line is to disconnect the end at the gauge and start the motor until you purge all of the air out...
Later,
WEIMER
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Old 09-24-2002, 09:26 AM
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I would not take this as lightly as Weimer is, this has some serious potential to ruin your motor. He may be right, but you need to find out for sure. If you only have a standard 5 quart pan, chances are you are running it dry, and needless to say, that is not good. As for "the nature of a mechanical guage" mine never does that, a mechanical oil pressure guage is by far the most reliable.
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Old 09-24-2002, 10:08 AM
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What I actually should have said...that is the nature of the plastic line on mechanical gauge.
I used the plastic line several times...but I actually prefer the soft copper.
Your gauge line doesn't measure the oil pressure from the pan, measures it from the oil galley near the distributor. 90% of oil in a running motor is in the topend and heads. If the pan was near full while running THEN I would be worried.
Ever notice how close the pickup screen on the pump was to the pan?
I could be way off on this one, I am just talking from my own experiences.
Later,
WEIMER

[ September 24, 2002: Message edited by: Weimer ]</p>
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Old 09-24-2002, 11:02 AM
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Now I see what you were saying, I don't like the plastic line either, the metal is more durable. My concern is with as much oil pressure as he said he has, it could easily fill the valve covers and valley with 5 quarts. Then, as soon as the pickup on the pump is not submerged, the pressure will begin to fluxuate.
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Old 09-24-2002, 12:11 PM
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The only thing that comes to mind is loose mains, maybe not loose enough to hear, but loose enough to cause this pressure problem. You said that the pressure dropped when in second gear and mashed about half way, and then came back with an increase in RPM. If this is the case then it cant be oil trapped in the top end (oil pressure would never come back at any engine speed higher than when the pressure first dropped). Loose mains however will allow pressure to rise again once the load is off the engine (manifold vacuum normal range) At normal vacuum (idle/cruising)the crank is being held up in it's bore by vacuum acting on the pistons. When it is held up it is hugging against the oil feed holes that feed the mains. But when the engine is under load (low manifold vacuum) the crank falls in it's bores away from the oil feed holes allowing more oil to escape through the main bores. Causing the oil pressure to drop when the pedal is mashed but come back again.
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Old 10-11-2002, 08:43 AM
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Chris:
I'm having the EXACT SAME PROBLEM with a 350 that my son and I built for his 67 Nova. I posted a question entitled "Small Block Chevy Oil Pressure Problem" on July 24th. To this day, I have not gotten a really good explanation for the cause of the problem, although the post by "bstmech" absolutely makes the most sense. I have tried hi and standard volume pumps, thicker oil, windage tray, and nothing helps. We haven't torn down the engine yet, but I believe that I'm gonna find excessive bearing clearance to be the problem. (Even though I plastigaged every bearing during assy, and they were all within tolerance) I think we both will find that we'll have to turn the crank and get a matching set of bearings to fix the problem. If you already have the problem solved, I'd be interested to know what you did. I see your post is about a month old. Have you fixed the problem?
Regards
Tom
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Old 10-11-2002, 09:07 AM
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Purge the air out of the line like Weimer said it should be fine.
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Old 10-11-2002, 09:14 AM
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Quepas00:
That was one of the first things we tried. In fact, we bought a new Autometer gage, purged the line and fired up the engine, only to have the same results. I wish I could find someone who has experienced this problem and found the solution. I get a lot of guesses from well-meaning people who are trying to help, but NO ONE has told me that they've experienced AND FIXED the problem. I appreciate all the suggestions, but again, I'd like to find someone who has actually gone through this before. Thanks!
Regards
Tom
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Old 10-11-2002, 11:38 AM
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Sorry Knizet I did'nt see your post. I was talking to Chris. It did'nt sound like he purged his line. As far as what bstMech was talking about does'nt make sense. If that were true even on a properly clearanced engine the oil pressure would fluctuate or go to a higher pressure when you got on and off the gas. The pressure relief valve should hold the pressure steady unless your way down on pressure. But I've been wrong before. You may want to ask this question on Chevytalk.com
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Old 10-11-2002, 12:00 PM
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Quepasoo: you just answered your own question on why it doesn't make sense about what I posted. Think about it, the difference between PRESSURE and VOLUME. If you have loose tolerances in your motor the relief valve may never be off it's seat. However, if you have tight tolerances, the relief valve will actually function.
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Old 10-11-2002, 12:03 PM
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steve here i had that same problem with my 440 your guage if fine some motors need a windege tray in the oil pan i have one still lost oil pressure what i found out was when i built the motor the pick up screen was to close to the bottom so when it needed oil at high rpms it would cavatate make my guage go crazy all i did to fix it was pull the drain plug and put a small but long screwdriver in the hole and pushed the screen up have not had a problem sence hope this helps steve gonzales67@msn.com
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Old 11-18-2002, 02:13 PM
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I'm having the same type of oil pressure problem you all are describing in my 67 firebird 400 (the pressure drops 10 psi or so when a certain RPM is reached). I tried bleeding the oil line connecting the gage and although there was a bit of air released, this had no effect on the gage reading. Before bleeding the air line I saw the air bubble actually back up toward the engine when the magic RPM was exceeded. I also had a thought that the high volume pump installed in my last rebuild was sucking the pan dry faster than it could return. Does anyone think that a larger capacity oil filter might help? Maybe the filter is the choke point in the flow of oil??? I just read the post suggesting pushing the screen up through the drain hole with a screwdriver. That seems worth a try.
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