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Discussion Starter #1 (Edited)
Driving last night I noticed that my oil pressure was abnormally low. Below 0 at idle and around 25 PSI when the engine is rev'ed. I am running 20/50 oil and the levels are fine. I am not burning oil either.

The prssure is normally near the middle of the guage when the engine is warm.

This AM when the engine is cold it is only reading around 25 PSI.

I am leaning towards the pressure sensor going out.


Question...
1) Where is the sensor located?
2) Are there other things that I should be looking at?

The car runs very strong

The car is my '90 Graduate

Jim
 

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The sensor for the gauge is on the right side at the back of the block. You may need a short thin wrench to unscrew it. I don't recall if it is 14mm or 17mm though...



The sender for the idiot light is on the left side - low on the middle of the block.
 

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1966-2013
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Not to be the fly in the ointment, but it could be a crank plug decided to pop out on you.

Key on then ground the sensor wire to see if the gauge pegs. If so, at least that part of the circut is OK.

Beyond that you could only test with a different sensor, or a mechanical gauge kit temporarily installed.
(though there 'might' be some specs around here somewhere for testing the old sensor itself witha multimeter based on electrical impendace)
 

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I hope so too.
Still its plausible that the dummy light wouldn't come on if the pressure were at some minimal amount above 0.
(any idea about where it actually does activate btw?)

Actually, being monopod, it could be that a solder point or trace in the PCB is failing.
In any event, I'd not be running the engine a whole lot while trying to figure it out.
 

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Discussion Starter #6
The guage works fine ... ground tested it.

Just started the engine ... approx. 25 PSI with the engine cold at idle

Engine cold at 3000 RPM maybe 28 PSI.

Still leaning towrds the sensor.

However, with all ALFA mechanicals you must have infant hands to access the sender. Any suggestions. Is it more accessible form the top or bottom
 

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If the gauge tests fine but the needle's going below zero at idle, yeah, that most likely points to the sender. Only way to tell for sure is to either replace the sender or test with a mechanical gauge.

Easiest to get to it from the top. I bought a 14mm cone wrench (basically a flat stamped wrench) from a bike store and cut it short enough to fit it in place on the nut. Then I rested a ratchet extension on the end of the wrench and tapped it with a hammer from above to turn the sender.
 

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my money would be on the sender; however, it could be a crank plug as well. the sensors are fairly poor in my opinion. I replaced mine about a year and 1/2 ago due to low pressure reading..btw, it is 14mm...get to it from the top but not easy..i have several 14mm open ended wrenches and eventually using two differing lengths was able to chage it out. it worked great for about a year but pressure is now dropping lower and lower. somewhere i read that the diaphragm inside of them leaks a bit of oil and then the liquid on the wrong side of it doesnt allow the diaphragm to move correctly but have no idea if this is correct. IAP has them for about $45--a bit pricey for a sender but i would change it out before i drove much.
 

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Borrow a master mechanical pressure gauge and screw it into the warning light sender location at the front LHS of the engine ( I think it is there on your engine, it is on later blocks). Then you can compare your electrical gauge readings against a calibrated one. My money is on the sender too!
 

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Just as a reference, comparing a mechanical oil pressure gauge readings to the stock oil pressure gauge readings on a S-4 Spider. It was in the low 80's at the time.

 

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71 Berlina 74 GTV 17 Giulia Q4
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If it's time for an oil change chances are if a plug did come out of the crank it will come out the drain hole when you change the oil. They are aluminum so you have to keep and eye out for it instead of using a magnet. If it happened all of the sudden I'd lean towards a crank plug, if it happened over a period of time I'd lean towards the sender.
 

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If it's time for an oil change chances are if a plug did come out of the crank it will come out the drain hole when you change the oil. .
Chances of it coming out the drain hole are slim. They get caught up in the baffles in the pan. The best way is to remove lower pan and see if one is there! But I'm with others leaning' to the sensor
 

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The senders are notoriously unreliable and to have a sudden reduction in reading is "normal" in my experience.....which is why I bought the calibrated master gauge!!!!
 

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Discussion Starter #14
Finally replaced the sender and the pressure is back to normal.

Thanks for the advice
 

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71 Berlina 74 GTV 17 Giulia Q4
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Someone must be living right:) Glad it was an easy fix and it's spring time driving weather!
 

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good news....but out of interest use the mechanical gauge to compare the electrically sent pressure. You will be amazed at the difference.
 

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Discussion Starter #17
Stuart ...

Could you (or anyone else) please send a picture of exactly where you connected the mechanical unit?
 

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My first oil pressure sender failed only 2 months after buying my Duetto NEW in 1967! I had just left LA on a summer cross-coountry trip to Philly, and had just passed Palm Springs when the oil pressure started drifting lower and lower. In near panic, I limped into the Alfa dealer in Mesa AZ. They replaced the sender and changed the oil in 30 minutes.

In the 34 years since, I've had dozens of these crummy senders - they get replaced almost as often as the oil filter (well, maybe 3-to-one...).

In many years of driving and racing Alfa's, I've heard about the crank plug issue, but never had one come out, nor knew anyone that did either. But everyone has had senders fail.

Ind I do not understand why Alfa nor VDO ever improved this device. Same one in some Ferraris, and they have the same problem.

Robert
 
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