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Old 11/30/04, 7:10 AM   #1
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Whenever my RPMs drop below 1000 on my 91 LX I get a sharp drop off in oil pressure. Basically the gauge drops right down to "L." I notice it when I am stopped or driving very slow with it in either drive or reverse. If I am stopped and put it in neuteral it brings the RPMs back up above 1000 and the oil pressure shoots back up to normal. I am not 100% sure that the gauge is accurate b/c I only bought the car a few months ago. It has been doing this for a while now and at first I thought it was a faulty gauge b/c it seemed to be moving around so much but when I took a closer look I could see the pattern I just described.
Is it definatly my oil pump or could it be something a little easier to fix? Please let me know what you guys think it might be.
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Old 11/30/04, 2:33 PM   #2
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install a Motorcraft filter and check from there
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Old 11/30/04, 2:59 PM   #3
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My first Capri had a 1993 Cobra long block with under drive pullies, but still Speed Density, and I had an Autometer oil pressure gauge and a red warning light attached to the gauge to alert me of low oil pressure. If my RPM’s dipped below 1000 the oil pressure would fall and from time to time the warning light would come on. (Much like your scenario.) I would just tip the throttle to bring the RPM’s up and my pressure would go back up.

I never thought twice about it and the car ran like a scalded cat. My other two Capri’s had stock engines without “real” oil pressure gauges and they never dipped to the “L” position. So maybe my first Capri had something wrong, but it ran like it didn’t, and I never really thought twice about it.
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Old 11/30/04, 3:34 PM   #4
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I know the stock gauge can be very inaccurate and it is odd how sharply the pressure dips off at the 1000 RPM mark. I think I am going to replace the oil pressure sender unit b/c I heard they are notorious for going bad.
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Old 11/30/04, 8:32 PM   #5
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My bet is on the sending unit being bad.
I too have had that experiance you talked of.

You will need a special socket made just for removing such units.
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Old 12/1/04, 8:04 AM   #6
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I know the sending unit is located where I circled in this picture but I have never changed it...any advice on how to do it? Also, I am thinking about adding an autometer oil gauge for a better reading. Anyone ever do this? How hard is it?
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Old 12/1/04, 9:58 PM   #7
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Thats different from the ones I am used to seeing.
But, I would spray some PB or WD 40
Maybe even the day before but atleast 20 minutes before tackling it.

You may want to talk to Auto Zone about the possibility of special tool to do the job. But from the picture it appears to me he will tell you vise grips.

Good luck

After thought, you might want to change Oil filter at the same time as it may make it easier to access from the bottom.
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Old 12/2/04, 3:54 PM   #8
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What I am going to do is take the unit off completely and hook up a mechanical oil guage to get an accurate reading. I have talked with a mechanic I know and he is thinking it may even be a drop in voltage at the low RPMs that is causing the stock gauge to drop.
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Old 12/2/04, 10:05 PM   #9
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Quote:
Originally posted by I8URVTEC@December 2, 2004, 3:57 PM
What I am going to do is take the unit off completely and hook up a mechanical oil guage to get an accurate reading. I have talked with a mechanic I know and he is thinking it may even be a drop in voltage at the low RPMs that is causing the stock gauge to drop.
Ok, the mechanical guage is the best thing you can do. The factory sending unit is just a sensor that completes a ground. The more oil pressure the more the sensor is grounded. Not sure how to test the factory sending unit, but say like the water temp unit can be taken out with the sending wire still attached and another seperate wired touching one end of the sensor and the other end touching say the intake. With the key in the on position (don't try something like this with the oil pressure sending unit) when the sensor is grounded it will make the temp guage go all the way to the right reading a way high temp. Replacing the sending unit is the first step to try. Either way once you have the oil pressure sending unit out you can them hook up a test mechanical gauge and see what it reads. Then you can make a better determination as to what the problem is. The oil pressure sending unit can be taken out with a (cannont remember what size 21-26 or so) mm socket. Just make sure the screw off the little white cap off the sending unit first. However, on to the bad news. I have been working on these cars for years. When the factory oil pressure guage starts acting like a mechanical guage there has been (in my experience, almost everytime) a more serious problem than the sending unit. In some cases it could be the oil pump. However, more than likely it could be the rod and main bearings. Once these bearings are left without a sufficient amount of oil for a minimum amount of time damage is done. So then the bearings start to grind down (turning into metal shavings). Where do you think the metal shavings go? That's right, they drain into the oil pan. Then are stinked into the oil pump pick-up screen and tube. The shavings that get through just do more damage to the motor, the ones the don't clog up the pick-up screen (which starves the pump and the motor of oil). This leads to oil pump damage, and motor damage. All of which is a sign of low oil pressure and the inaccurate readings by the factory guage. I really hope for you that it is just the sending unit or a old altenator and not the pump or bearings. If there is anything I can do to help just let me know. Good Luck.
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Old 12/3/04, 7:41 AM   #10
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I will let you know what happens when I hook up the mechanical gauge to it but doesn't it seem a little wierd that:
a.) it only happens when the car is warmed up
b.) It only happens when in Drive or Reverse (when put in park or neutral the gauge shoots back up to normal)
c.) The change is so instantanious. When you hit the lowest running RPM in Drive or Reverse the needle falls quickly below the "L" and when you throw it in neutral or tap the gas the needled quickly shoots back up to "Normal."

It just seems so irratic to me.
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Old 12/3/04, 1:42 PM   #11
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Quote:
Originally posted by I8URVTEC@December 3, 2004, 7:44 AM
I will let you know what happens when I hook up the mechanical gauge to it but doesn't it seem a little wierd that:
a.) it only happens when the car is warmed up
b.) It only happens when in Drive or Reverse (when put in park or neutral the gauge shoots back up to normal)
c.) The change is so instantanious. When you hit the lowest running RPM in Drive or Reverse the needle falls quickly below the "L" and when you throw it in neutral or tap the gas the needled quickly shoots back up to "Normal."

It just seems so irratic to me.
Yeah, the best thing you can do is hook up that guage and get a true reading.

a) the oil pressure will read high before the motor warms up. The oil is cold as this point and is still thick. The warmer the motor gets the warmer the oil gets and less thin.

c) I knoe exactly what you mean. I had a 94 Gt do the same thing. This was several years back so at the time I just changed the sending unit and thought it was fixed. But nope, the irractic readings came back shortly. Then I checked the oil when I was changing it and the filter. The oil looked like I was panning for gold. Bearing material all in it (by the way check the oil too, look for anything that looks like metal). I tore down the motor to find some of the most worn bearings I have ever seen. Good think it was a Ford, cause before I rebuilt it and tore it down I drived the crap out of it waiting for it to go. Never did. But it was only a matter of time. Good luck to you. Again, keep me posted and let me know if you need anything.
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Old 12/3/04, 10:04 PM   #12
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I was browsing ebay tonight and saw something that maybe of interest.
I mean if your thinking expanding with gauges
This might be a real cool addition Pony if you don't already have it? B)

http://cgi.ebay.com/ebaymotors/ws/eBayISAP...&category=46100
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Old 12/4/04, 8:29 AM   #13
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Quote:
Originally posted by I8URVTEC@December 3, 2004, 7:44 AM
I will let you know what happens when I hook up the mechanical gauge to it but doesn't it seem a little wierd that:
a.) it only happens when the car is warmed up
b.) It only happens when in Drive or Reverse (when put in park or neutral the gauge shoots back up to normal)
c.) The change is so instantanious. When you hit the lowest running RPM in Drive or Reverse the needle falls quickly below the "L" and when you throw it in neutral or tap the gas the needled quickly shoots back up to "Normal."

It just seems so irratic to me.
a) typically oil pressure will drop anyway when warmed up as the oil is thinner and moves more freely.

B) When in drive or reverse there is a load on the engine....when in park or nuetral the RPM's rise and therefore the oil pressure will rise...that is normal as well.

c) I would have to see it to see how "instantaneous" it is doing this....

The FIRST thing I would do is replace the factory sending unit...any change gained by doing that will tell you if it is faulty or not. I have had many of those fail with age and do similar things as you are describing as well as failing altogether.

Do you hear any valvetrain ticking? low oil pressure will show itself there often....The new sending unit is probably around $10 and a cheap fix....easily removed and replaced with either a 1/2 or 9/16 wrench right under the "bell top".


Your descriptions do not indicate to me that there may be an electrical issue....
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Old 12/6/04, 7:29 AM   #14
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I am going to either replace the sending unit or put a mechanical gauge on it some time this week to see how the oil pressure is really doing.
To futher elaborate on "instantaneous" I would say it takes less than a second for the gauge to drop from normal down to hanging the pin below "L" when I am stopped and in drive after the car is warmed up. Then when I put it in neutral or park it takes about a half a second to shoot back up to "normal" pressure.
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Old 12/6/04, 7:34 AM   #15
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Are you running underdrive pullies? Just curious . . (especially on your alternator?)

The reason for asking about the pullies/alternator is that if you are running pullies your alternator would be running slower than normal and might cause the volts to dip a little on the sending unit. Which could be a reason it doesn't take very long for the pressure to decrease and increase with a little tap of the throttle. That cause your alternator to spin faster sending more "juice" to the unit.

Just a thought . . .
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Old 12/8/04, 2:29 PM   #16
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This might sound wierd but I don't "think so." I bought the car a few months ago from a second owner and he said that everything in the engine compartment was stock but then again he didn't seem to know much. I have never done underdrive pullies and this is my first 5.0 so I wouldn't know the difference.
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Old 3/27/05, 2:06 PM   #17
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Ok so what about at ildle she runs at normal PSI but when Accelerating, pressure drops a considerable amount and will stay there even at a constant speed. At Ilde pressure is normal giving it gas drops the pressure or bettter said the gauge drops when fuels is applied. A note I have replaced the Sending unit already

Don
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