Oil Pressure

Discussion in 'Fox 5.0 Mustang Tech' started by SoFlStang, Jun 19, 2013.


  1. SoFlStang

    SoFlStang Member

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    Hope I am not killing you guys with all the questions, and I appreciate all the help you have given me with getting my fox up to par. So, now I have a new issue....

    As some are aware, my temp and fuel gauge do not work. My battery gauge and oil pressure gauge have appeared to work. Today while driving I noticed my oil pressure gauge was about 1/4 way up, then when I stopped at a light, it went to 0. When I accelerated from the light, it went back to 1/4 to 1/2 way where it normally sits. Did it again at the next light. Other then oil level, is there something to check or could my gauge just be messing up? I thought if the oil pressure went to 0 the car would stop running. Didn't notice a change in RPM or engine noise. As always, your help is greatly appreciated.
  2. MFE92

    MFE92 Active Member

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    Get a real oil pressure gauge, even a diagnostic one you temporarily hook up to the sender port, and see what you find.


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  3. 89stang1

    89stang1 Stroked and Juiced

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    if you didnt hear any engine noise i wouldnt worry but i agree with MFE92 get some real gauges especially if you plan on modding! stock gauges are not accurate even when they do work try Autometer gauges its what im using so far no complaints
  4. MFE92

    MFE92 Active Member

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    Just because it made no noise doesn't mean "no worries". The stock gauge drops to 0 when it senses less than about 10 psi. So either the sender is bad, or he's really making less than 10 psi at idle. A diagnostic gauge will tell the tale. A healthy 5.0 should be making no less than 30-40 lbs at idle.
  5. 89stang1

    89stang1 Stroked and Juiced

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    lol really my point is if the gauge is reading o and its not running any different with 0 oil pressure then obviously the gauge is sheot!
  6. MFE92

    MFE92 Active Member

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    No, it's not obvious at all, because the real point is, just because it's not making bad noises doesn't mean everything's OK and it's just a gauge issue. Like I said, the stock gauge, properly functioning, will drop to 0 if the engine is producing anything less than about 10 psi. Say it's really making 5 psi, that's not low enough to cause it to make noise but it's definitely a sign of a real problem.
  7. 89stang1

    89stang1 Stroked and Juiced

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    your not getting my point all im trying to get thru is if the car is running normal and the gauge is reading 0 i wouldnt worry its not 0 im not saying there is no problem there like i stated earlier i would get good aftermarket gauges and not rely on stock ones!
  8. SoFlStang

    SoFlStang Member

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    I get what your both saying, makes sense. I went an picked up an engine oil pressure tester today, and will order a quality gauge in the next day or two, as no one local has one, just the sunpro gauges at autozone. Im hoping its just the sending unit or gauge, and hoping the no weird noises or change in engine running means that. Ill be testing it tomorrow with the tester. I do have a question about using the tester. Since I noticed the drop to 0 occurs after the engine has been running a bit and getting heated up, can I hook up the tester and then zip tie it in place, drive a bit and then check it? Of course thats after verifying the pressure at startup idle. Seems it only happened after driving 35 miles or so.

    So I should show 30-40psi at startup idle and again when I idle after driving for a bit, correct?
  9. 89stang1

    89stang1 Stroked and Juiced

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    usually when you first start car idle oil psi will be a little higher than after car is up to temp idle will be lower, for instance on my autometer gauge startup idle pressure is 45 psi then up to temp 39 psi On and average 30 to 40 is normal
  10. SoFlStang

    SoFlStang Member

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    So I went out to do the testing and when I really looked at my oil pressure sending switch, I noticed its not like the one the part store shows for my car or others have shown. It looks like this:

    [​IMG]

    Can I put the correct one in from the parts store, which has the bell look, according to Summit they are both 1/4 - 18npt ? Or is this the correct one? If so, does anyone know what model it comes from so I can buy a replacement? Now I am going to try and get it off the car.

    Edit: upon some searching the part store site, it appears the part I have is from the 90-93. I know I have the pre 90 gauges. Is there a way to determine if my 88gt has an 88 engine or a 90-93 engine or if a swap to something else was done?

    Edit 2: The parts are not interchangeable. I must not have an 88 engine in mine, since the part for the 88 (the bell looking one) will not fit. Thread size is the same, but the clearance is not. Only the long one will fit. Back to the parts store.
  11. SoFlStang

    SoFlStang Member

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    Took the one above off and hooked up the tester gauge, showed 40psi, so I am guessing that the oil pump and engine are ok. Going to try to install the new sending unit now, and see how it goes. Any suggestions on the aftermarket gauges? I keep reading Autometer are good gauges, should I go mechanical or electrical?

    Edit: See above, new unit was the wrong one :(
  12. 89stang1

    89stang1 Stroked and Juiced

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    Im using autometer sport comp gauges in a 3 gauge dash pod a monster tach with oil pressure and coolant temp gauges very pleased with them. Im happy to hear oil pressure is ok!! buying those gauges is money well spent good luck with car!
  13. SoFlStang

    SoFlStang Member

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    Well I think I have a problem. I don't think I left the test gauge on long enough. None of the local parts stores had the replacement switch, so I decided to grab a set of the sunpro gauges after reading some reviews that they were ok and alot better then the stock gauge. Since I have a plan for how I want the final car to look, and the gauges for that are pricey, I thought why not, get the gauges and save for the exact ones you want. So I got the oil pressure gauge hooked up, and these were my readings: 40psi at startup, I ran the engine around 2000rpm for a bit and then let it idle, as the car warmed up, the pressure appears to drop to 32psi. After running it some more and then idling, it seems to be at around 12-15psi, about half way between the 0 and 25 marks on the gauge. Rev the engine and the psi goes up. Now I noticed the line to gauge is semi-clear, and I can see the oil fill the line. However, the oil does not fill the entire line, only about 3/4 the way or so. Should the oil fill the entire line? I know the line the gauges come with is not ideal, I plan to upgrade the line, but want to see where I am at with this engine first. Also, the previous owner said this engine was rebuilt less then 1000 miles ago and stroked to a 331, not sure if that matters in what is happening. I did not hear any abnormal noises from the engine while doing the above.
  14. 7991LXnSHO

    7991LXnSHO Active Member

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    The pressure is less than ideal, but is hardly fatal for a motor with plenty of miles. But if it was redone 1000 miles ago, I would be suspicious of build quality. 15 is a lot better than 0 psi.
  15. Grabbin' Asphalt

    Grabbin' Asphalt Advanced Member

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    I put a heavier oil in mine, I went to 20/50 full synthetic and it raised my psi considerably. I'm in GA though and it's stupid hot here. Put in a high volume oil pump & hardend oil pump shaft later on to help throw it around. But I never had any problems when I did either one and it was a LONG while before I updated the pump/shaft. Some cat may have put 5/30 in your motor and its breaking down fast, .......just a thought.
  16. Grabbin' Asphalt

    Grabbin' Asphalt Advanced Member

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    Absolutely, ......and yes hearing something on TOP of that would be even worse, cause we already know stock gauges are known for being, ....well ......stock.....lol
  17. SoFlStang

    SoFlStang Member

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    I am in south florida, so I know what you mean about the heat. It is very possible they used 5/30 or whatever when they did the last oil change. I will do the 20/50 synthetic this weekend and hopefully that helps too. As for build quality, I am becoming skeptical about what was actually done to the engine. I know it has BBK headers and an Edelbrock manifold, but I don't think there is a way I can confirm the rebuild was done or it was actually stroked. Wish I would have known then what I know now. The next Fox I get, I will be alot more knowledgeable about.
  18. Bullitt95

    Bullitt95 Active Member

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    If the engine's healthy the oil pressure (cold) should be at least 40psi at idle and 50psi at 2500+rpm. As the engine warms up the oil thins and the oil pressure drops, but it should still be at least 20psi at idle and 40psi at 2500+rpm. A hot oil pressure of 10-20psi at idle is lowish but still viable, but less than 10psi indicates excessive bearing clearances.
    An oil pressure that doesn't rise to at least 40psi at 2500+rpm indicates either a worn oil pump or a weak oil pump pressure relief spring.
    Installing a good quality, accurate oil pressure gauge will help you diagnose your problem and find the correct fix.
  19. SoFlStang

    SoFlStang Member

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    I went ahead and changed the nylon line to copper today and got the gauge mounted inside. On cold start, the oil psi is 40psi and raises if I press the gas. I drove it around a bit and got the engine warmed up, which I can tell now with my new coolant temp gauge. Driving around town, with an rpm of 2000-2500 I am seeing just under 40psi. When I stopped and idle, the pressure drops to around 25psi and as it warmed up more, the idle psi was about 15ish. Im gonna drive it work tomorrow and see what the readings are and then tomorrow night or sunday I'll do an oil change to 20/50 synthetic.
  20. Bullitt95

    Bullitt95 Active Member

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    Those oil pressures are consistent with a high mileage engine that has some degree of wear but not enough to warrant a rebuild. A thicker grade of oil could raise the oil pressure by ~5psi and buy the engine some more time.
    If you decide later on to add performance heads/cam/intake, it would be adviseable to refresh the bottom end anyway while the engine's apart.

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