fresh Cadillac 500 oil pressure drop during initial startup - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-10-2004, 08:34 PM
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Arrow fresh Cadillac 500 oil pressure drop during initial startup

Everything goin fine, adjusting carb and then about 15 min. into the initial break-in, oil pressure begins to drop slowly, took about 30 sec and then I shut the thing off when it got around 10 pounds of pressure.

this is a freshly rebuilt motor, I have cam crank and all kinds over other goodies in the engine,

What could cause this?? I was told it could be a clogged oil filter??

When I first started it up, it had around 60 lbs.

thx for the input!

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Old 01-10-2004, 08:53 PM
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Did you replace the oil pump with new or rebuild the old one, if you rebuilt, did you replace the plate gasket with a new one?

What all did you do with this engine when rebuilt, as far as plastigage, and checking bearing clearances?

Were any oil galley plugs removed.

Did you use assembly lube for break-in, and do you think the oil filter could be plugged with the lube, causing the filter bypass to open?
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Old 01-10-2004, 09:49 PM
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I had the thing built for me, tey were supposed to do everything, I assume rebuilt oil pump, could a clogged filter cause this?
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Old 01-10-2004, 10:05 PM
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If your filter is clogged your bypass will open and send unfiltered oil throughout the rest of the engine where it shouldn't be.

The bypass shouldn't affect oil pressure though.

Was there any sign of a leak anywhere?
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Old 01-10-2004, 10:23 PM
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I guess the first question would be: Are you using a good mechanical oil pressure gauge to get these readings? Many times, a factory electric gauge is inaccurate, and a fresh motor with higher oil pressure will cause a stock used sender to go bad.

If the gauge is good, a problem exists.

An oil filter can cause oil pressure to drop.......Some filters don't have a bypass......Cheap and easy to check by installing a new filter.

Before you fire the motor again, I would re-prime the system with a drill motor. You will be able to see, without damage to the crank and bearings, if your pressure comes back to the 60psi. Dan

PS, Was this motor put together by one of the Cadillac vendors, or a local shop?
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Old 01-11-2004, 11:42 AM
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MTS parts and locally built
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Old 01-11-2004, 05:52 PM
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I'm using an autometer mechanical gauge

I just changed oil and filter today and spun the oil pump around with my drill and got nothing, not even a little resistence, so I think I found the problem, busted oil pump,

Are these fairly easy to rebuild??

Is this a common thing to happen to oil pumps? (spinning with no resistence and no press.??)

thx
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Old 01-11-2004, 05:57 PM
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I'm not familar with the Caddy engine, but the Chevys HAVE to have a dist housing installed in the block in conjunction with the primer shaft as the oil comes in from behind the dist and goes around the housing in a groove and then on to the rest of the engine.

Take a look at your dist to see if it has a similar setup. Or just look down the dist hole while you're priming it and see if you see oil pumping out and running back down into the block.

Mark
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Old 01-11-2004, 06:28 PM
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The Cad oil pump is external. Easy to get to. The filter spins on it.

First thing to check on the pump is the pressure relief valve. If that valve's stuck open....You aint gettin no pressure. A single grain of sand is all it takes to stick one......You can pull the pump to make it easier to check pressure relief spring and valve, as well as the gears for abnormal wear. Make sure you use a new oil pump gasket when it goes back together to maintain proper end play.

I doubt you will find anything wrong with the gears themselves. It is uncommon for any oil pump to go bad. I have seen broken springs and stuck valves before.


You can also run your drill without the filter to check that there is oil flowing threw the pump. A catch pan under the pump minimizes the mess. If it puts oil out, you know for sure the pump has a prime....You problem must be after the pump......If you don't get anything out of the pump.....Your suction line would be th likely suspect. There is an O-ring where the pickup tube bolts to the block. Hopefully you will find a little trash in the relief valve and your problems will be solved.......Dan
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Old 01-11-2004, 06:43 PM
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thankyou so much, this all helps a lot

I will get into that thing tomorrow,

so I can pull the pump without dropping the pan??

that would be very nice
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Old 01-11-2004, 06:56 PM
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Yep, pump bolts to the outside of the block. If you can take the oil filter off, You can get to the pump to take it off as well.....Dan
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Old 01-11-2004, 08:42 PM
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I'll toss this out for the heck of it. Years ago I rebuilt an old Ford inline 6. It came in complaining of low oil pressure and eventually smoked a couple of bearings. Come to find out that the pick up tube had a hairline crack in it and as it warmed up, the crack opened up a tad and the pump began sucking air and provided a nice foamy mess to all the bearings. Worth a look I guess.

Mark
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