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  1. #1

    Default engine loses oil pressure when it hits 200 degrees

    I have a new rebuilt motor 69/390 bored 60 over, balanced, ported 58cc heads, comp cams 280h, push rods, lifters and springs with scorpian rollers.
    Everything is new in the motor except for the crank, and the timing chain cover. I replaced the oil pump gears, checked the walls of the pump and they seem to be fine. I am considering replacing the timing chain cover to see if that will make a difference. But if any of you have experienced this problem, please let me know. I would hate to have to tear this engine down.

    Car runs fine down the road but once it gets a little warm the motor loses oil pressure. Also there is no knock or lifter noice when pressure drops off. And the oil pressure is at 0 at an idle in gear, and goes to 20 when you have it in park. I also changed oil pressure gauges.

  2. #2

    Default

    Hi, it sounds like the filter bypass valve is stuck open... When your engine oil warms up to operating temp, the viscosity drops to 10w thus oil pressure loss.

    Since you have a fresh rebuild... did the engine dude use non-detergent 30w for break-in? 1,000 miles of variable cruising speeds... today's hustle and bustle drivers get annoyed easy by it.

    If your still running it passed break in period you can have viscocity breakdown also... since they do not make off the shelve straight 30w without additives anymore.

    I learned to kill the bypass valve along time ago as a race trick. It is realitivly simple and not recomended for those who are not changing their oil filter with every oil change.
    Last edited by 304_dude; 08-03-2005 at 03:07 PM.

  3. #3

    Default

    I will check the oil bypass out this weekend dude thx for the tip. The motor has only been run for like 35 miles or so. I used crappy 30 weight oil when breaking in the cam , which according to comp cams is 2000-2500 rpms for a half an hour. Then I changed the oil and filter. Used straight castrol 30 weight. I will let ya all know what's happening with my progress.

  4. #4
    Join Date
    Aug 2005
    Location
    Lufkin, Texas USA
    Posts
    13

    Default Engine Oil Pressure Loss

    Hi, it sounds like the filter bypass valve is stuck open... When your engine oil warms up to operating temp, the viscosity drops to 10w thus oil pressure loss.
    FYI - The 10W-30 means that at 210 degrees F or at 100 degrees C the viscosity is SAE 30. The "W" is taken at a colder temperature on a scale used depending on which viscosity being tested. It measures how much a Motor Oil thickens when it drops in temperature for cold starting purposes.

    The "W" has nothing to do with anything after a motor has warmed up. It is measured at cold temps, say 20 degrees F for example.

    The SAE 30 and 10W-30 oils are the same viscosity at 210 degrees F. The difference between a straight weight pertroleum based motor oil and a 10W-30 is that if the straight weight were tested at colder temperatures it would be called a 30W-30.

    If you do away with the by-pass valve in your system you may need to read this. Even a stock motor at idle by-passes some oil from time to time to prevent oil pressure loss. If you do have a problem with the by-pass valve it would be better to fix it if at all possible. If you do remove it, better use a high end oil filter that comes with a built in by-pass valve to be sure to get full oil flow. Just a thought.

    A local guy with a potent AMX Drag Car had a oil pressure loss and just changed the oil filter and the pressure came back. It was a new engine which may mean that debris collected in the filter and clogged it. I don't know if you tried that yet.
    http://www.upmpg.com/flash/nascarflag.gif'77 Gremlin
    Currently drive a '77 Gremlin
    I have owned:
    '70 Mark Donohue SST Javelin; '72 Gremlin X; '73 Pierre Cardin AMX; '76 Hornet Sportabout

  5. #5

    Default

    Quote Originally Posted by Gremil
    FYI -
    A local guy with a potent AMX Drag Car had a oil pressure loss and just changed the oil filter and the pressure came back. It was a new engine which may mean that debris collected in the filter and clogged it. I don't know if you tried that yet.

    switching to a new oil filter (i went from a fram wich had 0psi at anywhere to a wix with very good pressure even at idle) worked for me too, 74 jav 360, only mods are removed air pump and a holley 600.

  6. #6

    Default Oil PSI

    Quote Originally Posted by amxer
    I have a new rebuilt motor 69/390 bored 60 over, balanced, ported 58cc heads, comp cams 280h, push rods, lifters and springs with scorpian rollers.
    Everything is new in the motor except for the crank, and the timing chain cover. I replaced the oil pump gears, checked the walls of the pump and they seem to be fine. I am considering replacing the timing chain cover to see if that will make a difference. But if any of you have experienced this problem, please let me know. I would hate to have to tear this engine down.

    Car runs fine down the road but once it gets a little warm the motor loses oil pressure. Also there is no knock or lifter noice when pressure drops off. And the oil pressure is at 0 at an idle in gear, and goes to 20 when you have it in park. I also changed oil pressure gauges.
    You also might want to think about the bore on the engine. I have been told by some AMC builders that you do not want to go over the 40 over mark on the engine bores. Anything much over that will make the engine run too hot and the 60 over on the boring will run the pistons too close to the water jackets. Just a thought

  7. #7
    Join Date
    May 2005
    Location
    Illinois
    Posts
    17

    Default

    I too am having the same problem and I flushed the motor with kreen first. Then I dropped the pan to look at the pick-up to see if it was blocked. It was clogged with a bunch of old gasket material, the jerk who owned the car before me was lazy. Next I rebuilt the oil pump and replaced the oil pressure gauge. I too am using a fram oil filter. My oil pressure seems to drop severly after the car has warmed up after about 15min. Could my problem be the relief valve or the filter? If it is the relief valve where is this located so I can check it out or disable it? If it is the filter what filter do you recommend or where can I get a wix filter. Thanks
    1970 Dodge Charger R/T SE 440/727 (Work in progress)
    1968 Plymouth Sport Satellite 383/727 (Work in progress)
    1970 Plymouth Valiant Duster 318/904
    1972 Buick Skylark 350/350

  8. #8

    Question Something else to check

    Just a thought here, my 360 kind of went thru the same stuff with the oil pressure drop off when the engine got warm (operating tempature). It ended up being the clearances for the bearings and rods were set at GM specifications and this screwed up everything in the lower part of the motor. This was due to the machine shop not knowing that AMC specs are very tight down on the crank and the such. Like a dummy, I did not check to make sure that everything was correct. Different story with the 401 that went back into the car. Not wanting to scare anyone here, I would check your clearances and see if they were set by the machine shop to GM specs. A lot of shops do not have the specs for AMC's anymore and instead of asking the customer if they have them, they will use the GM specs so they won't look stupid or something.

  9. #9

    Default Break In Oil

    Just one thing on your engine, make sure you are breaking the engine in with non-detergent oil, if you don't you will run the risk of the rings not seating and oil consumption problems down the road. I agree with the post on the by-pass valve that is the first thing I would look at.

    Quote Originally Posted by amxer
    I have a new rebuilt motor 69/390 bored 60 over, balanced, ported 58cc heads, comp cams 280h, push rods, lifters and springs with scorpian rollers.
    Everything is new in the motor except for the crank, and the timing chain cover. I replaced the oil pump gears, checked the walls of the pump and they seem to be fine. I am considering replacing the timing chain cover to see if that will make a difference. But if any of you have experienced this problem, please let me know. I would hate to have to tear this engine down.

    Car runs fine down the road but once it gets a little warm the motor loses oil pressure. Also there is no knock or lifter noice when pressure drops off. And the oil pressure is at 0 at an idle in gear, and goes to 20 when you have it in park. I also changed oil pressure gauges.

  10. #10
    Join Date
    Feb 2007
    Location
    Ozark, AL
    Posts
    2

    Default engine loses oil pressure when it hits 200 degrees

    I own a 68 AMX with a mildly modified 390, and noticed the exact same thing about the drop in pressure. When the oil is cool, I get about 20 psi on the gauge. Once the car warms up, the gauge reads about 12 psi. From what I have read, 10 psi is about the norm for gen 2 AMC V-8 engines. Yes, I thoroughly cleaned the screen - no debris, and replaced the oil pump gears. What did you ever figure out. How did you increase the oil pressure? I'm curious.
    Thanks

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