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Thread: Yet Another Oil Pressure Post

  1. #1
    Registered User Oiluj's Avatar
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    Default Yet Another Oil Pressure Post

    Last few days I've been driving my 72' 240, and noticed a couple strange events.

    Observation #1)
    Normal oil pressure at speed, (2/3 to 3/4 on gage), but while sitting at a stop light, gage registers "zero" oil pressure. I blip the throttle up to maybe 15 RPM, and pressure bounces up to 1/3 or higher and stays there... No additional noise from valve train or anything else to indicate a loss or reduced lubrication. Oil level is fine and oil is clean.
    I searched the threads, and yes, I have a gold color sender. Thing is, it's never done this before...

    Observation #2)
    Everything's good, then temperature bumps-up from center of the gage to almost 3/4! This is in-town, stop-n-go traffic. She doesn't act or sound different, (like she's overheating), but again, it's a new behavior. BTW, this does not happen in conjunction with low pressure reading, they seem to be unrelated events.
    I checked water level and while a tiny bit low, is fine. Have not checked operation of the thermostat or radiator cap, though I don't se why they would change...

    Was running with the ignition retarded about 7-10 degrees to see if minor pinging at WOT could be eliminated. No difference, but retarded ignition resulted in intermittent dieseling when shutting engine down. So this weekend, I advanced timing 7 degrees which seems to run better and no dieseling. Sadly, I'm stuck using Kalifornia 91 octane, (if you are lucky), + alcohol fuel, so I guess I'll just not push her when going up steep hills...
    A friend with an older Porsche has a similar issue with low octane fuel.

    Any thoughts?
    Last edited by Oiluj; 09-01-2014 at 12:54 PM.
    Julio
    1972 240Z (in-progress, 95% complete)
    CZC# 15388

  2. #2
    Registered User Oiluj's Avatar
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    Default

    The excessive oil pressure thread got me thinking. I'm running a Fram oil filter. Was thinking that perhaps the bypass valve is open and I'm losing pressure there, enough to lower pressure at idle below the oil pressure sensor threshold...
    Julio
    1972 240Z (in-progress, 95% complete)
    CZC# 15388

  3. #3
    Registered User madkaw's Avatar
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    Default

    #1 - install 20$ gauge from autozone and observe oil pressure OTHER than an electric signal.
    #2 - the gauge is there to warn you of possible troubles. Things do change in an instant with 40 year old cars. Check complete coolant system
    Steve
    71 240z,bw-5sp 2.4-40 over,balanced,e-88,big valves,ported&polished, stage2,header, triple Mikuni's 40's
    3.90 Subaru STI LSD

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    HLS30A 17574 djwarner's Avatar
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    Default

    The oil pressure sender is constructed so that until you reach 6-8 psi the terminal is open. Once pressure reaches the magic number, resistance drops to 60 ohms. As pressure increases, resistance drops down to about 15-20 ohms at 90 psi.

    Remember the oil pump is a constant displacement pump and only generates whatever pressure necessary to push out it design volume each revolution. The bearing in our engine relies on a flow of oil to prevent metal to metal contact - flow, not pressure. Coming to an idle stop after a high temperature run has oil very thin and runny. It takes very little pressure to push the oil out. As the pressure drops below the 6-8 psi range, the sender opens and the gauge goes below zero to the peg.

    This phenomena has existed in our cars since they were new. After 44 years, I guess this is properly classified as normal operation.

    As for your temperature spike, you don't say whether you are running AC which would aggravate the situation. Another cause may be your thermostatic fan clutch allowing the fan to freewheel. Consult the FSM to understand the theory of operation.

    Hint for boosting octane to control pinging. Try adding about 5 ounces of acetone to a full tank. This will boost your octane rating 3-4 points. If it solves the problem, you know where to look. Though run-on is normally caused by carbon accumulation which can be caused by an overly retarded spark.
    Last edited by djwarner; 09-01-2014 at 01:44 PM.
    1971 240Z HLS30A 17574 L24-021025

  5. #5
    Registered User Oiluj's Avatar
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    Default

    I plan to hook-up a mechanical gage to the port this week, though I suspect, as posted above, everything's all functioning normally.

    Am already adding acetone, (mixed with a small amount of Marvel's Mystery Oil), and it does seem to help reduce pinging at WOT or under load going up a steep hill, but not it's gone completely. I have no audible pinging at all in normal driving. I've tried added a several gallons of 110 race fuel to a tank, and no pinging under any conditions, but it's a pain to get and expensive...

    Regarding cooling, I wasn't running the A/C. The fan clutch shows no evidence of leaks, but bears some investigation. I'll check it out.
    I checked the timing today and it was a 3 degrees too far advanced, so I set it to factory spec. This doesn't seem far enough out to cause the engine to run hotter.
    Think I'm going to add a coolant recovery bottle as well. I have a heavy-walled, 1 quart round poly bottle, (Nalgene), I've been planning to make into a coolant reservoir.

    Thanks for the input! Guess I've got some more detective work to do...
    Julio
    1972 240Z (in-progress, 95% complete)
    CZC# 15388

  6. #6
    HLS30A 17574 djwarner's Avatar
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    Just how are you measuring your timing? Static timing or with a timing light?
    And do you still have a stock distributor/ignition?
    And what rpm?
    Is the vacuum advance disconnected?
    If you have a vacuum gage, you might try adjusting timing for maximum idle vacuum.
    1971 240Z HLS30A 17574 L24-021025

  7. #7
    Registered User Oiluj's Avatar
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    I have a Pertronix ignition. Timing checked with a timing light, vacuum disconnected at 750 RPM.
    (My vacuum advance is not connected at this time).

    I do have a vacuum gage and will try adjusting for max vac at idle.
    Julio
    1972 240Z (in-progress, 95% complete)
    CZC# 15388

  8. #8
    Crumudgeon
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    Do not add marvel mystery oil, or any oil to your fuel. You're actually reducing the octane of the fuel and adding combustion by products that increase carbon build up.

  9. #9
    Registered User Oiluj's Avatar
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    Thanks! No more MM oil...
    Julio
    1972 240Z (in-progress, 95% complete)
    CZC# 15388

  10. #10
    Registered User Oiluj's Avatar
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    OK, I verified that I do have oil over 12 psi oil pressure at idle when hot, even when the dash gage indicates zero, so all is good.

    However....this weekend, when travelling down hill on a cool, foggy morning, for an extended time, (10 minutes), my temp gage dropped to about 120 degrees.
    I've previously verified dash gage readings vs. a digital thermometer.
    Temp came right back up as soon as the engine was loaded. Guess I now need to check operation of my thermostat...
    Julio
    1972 240Z (in-progress, 95% complete)
    CZC# 15388

  11. #11
    Registered User conedodger's Avatar
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    Julio! Quit looking at the gauges and drive! :-)
    Rob
    2000 BMW R1100 RT-SE (for sale)
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    1973 Datsun 240Z Restoration project. New paint in original white. E31 head on 2.4 block. Nissan Motorsports header. R200 with Nissan motorsports LSD.

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    Julio - what radiator are you running? Has it been pulled, flow tested or re-cored lately?
    By all means, do a pressure test on the radiator and cap first.

    Carl B.

  13. #13
    Registered User Oiluj's Avatar
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    Will do. At least it's not over-heating.
    Julio
    1972 240Z (in-progress, 95% complete)
    CZC# 15388

  14. #14
    Registered User EuroDat's Avatar
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    Quote Originally Posted by Oiluj View Post
    However....this weekend, when travelling down hill on a cool, foggy morning, for an extended time, (10 minutes), my temp gage dropped to about 120 degrees.
    I've previously verified dash gage readings vs. a digital thermometer.
    Temp came right back up as soon as the engine was loaded. Guess I now need to check operation of my thermostat...
    Sounds like your thermostat could be sticking. If it sticks in a fixed position, say half open, it can cause overcooling (downhill no throttle) and overheating under heavy loads. Its easy to check without dismantling to much and they are not expensive.
    Chas
    Chas
    5/77 280Z HLS30 403100 with some modifications
    Original colour: 305 Light Blue. The PO changed it to Red

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