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1978 Spider
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Discussion Starter #1 (Edited)
finally got the car running after completely re-doing the entire fuel supply system. I have no recent history on this car since it was bought as a project from another guy and many things like vacuum leaks, bad grounds, gauges were not working properly..

Anyway. Oil light goes out immediately. but the gauge appeared to be dead. double checked the wire and snugged up the female spade connector. Attached are 3 pics, idle oil press, 3000rpm oil press and 4000 oil press. well the gauge does seem to work...or is it starting to fail or the sender starting to go bad...or have the crank plugs come loose?

known car history as follows:
  • Feb 2003: PO emails Wes Ingram discussing oil pressure staying under 30psi and oil light at idle (900rpm), Wes suggests crank plugs.
  • Apr 2003, 87,200 miles: PO mentions low oil press but no number recorded. He suspected lost crank plugs.
  • July 2003, 92,500 miles: PO notes installing new light and gauge sending units. pressure ranges from 45-65psi.
  • May 2004, 106,800miles: PO removed cracked oil pan for repair. No mention of crank plugs.
  • 2008 Alfa BB thread: crankshaft plugs posting #7 PO mentions finding 2 crank plugs in oil pan during repair in May 2004 event. Crank plugs re-installed (implying not new plugs) and peened over. No mention of Loctite. States "oil pressure back up (no PSI recorded) and no light at idle"
No more PO notes past Jan 2007/134,054 miles. Last PO receipt 2014. His journal always stated 20W-50 oil.

I bought the car from an individual whom I believe acquired the car from PO ("Bill" on the AlfaBB here) in Sept 2019? The title is dated Sept 2019 but it's possible he held the title open for much longer before doing the paperwork. But he did state he bought it from a "teacher" last year. "Bill" is a professor. I do not believe this new owner drove the car much, if at all, considering the poor to non-existent running condition. I've only driven it once...straight home about 3 miles. It has not been driven since.

Before I drop the oil pan and search for missing plugs, what should I investigate first? Sending unit? Gauge? How does one test either? Obviously both are working to some extent. But how to tell if they are fully functional or are starting to fail/go bad and not giving accurate readings. Not dead but not good. somewhere in between? Or do these just either work or they do not?
 

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But Mad North-Northwest
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The gauge sender is notoriously finicky. Best to test with a mechanical gauge, then you'll know where you're really at. Most auto parts stores will loan you one for free.

When hot you should have min 7psi at hot idle and approximately 10 PSI per 1000 RPM when revving.
 

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ground the sender wire to make sure the gauge goes to max...that shows the gauge is working ok
the senders are a weak point, they fail.
The oil light sender is a different type and much more reliable.

I'd just go for a new sender before dropping the pan and worrying about crank plugs!
 

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I agree that the senders are the weak link. Even with a new properly functioning sender the gauge needle will be near the '0' at idle with warm oil. But if checked with a mechanical gauge it is probably more like 20 psi. When you switch off the needle drops way down to its rest - that is more like where 0 should be. (I've thought of taking off the glass and adding a '2' in front the zero just for peace of mind...)
 

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My notes indicate the following sender resistance range
0 psi - 340 ohms
57 psi - 127 ohms
114 psi - 8 ohms.

So if you connect a 100 ohm resistor in place of the sender then the gauge should read a little over 50%.
 

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1978 Spider
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Discussion Starter #7
My notes indicate the following sender resistance range
0 psi - 340 ohms
57 psi - 127 ohms
114 psi - 8 ohms.

So if you connect a 100 ohm resistor in place of the sender then the gauge should read a little over 50%.
so if I ohm check the sender it should be around 340? anything much higher and it would be sending an artificially low signal PSI to the gauge? Anything much lower would be artificially reading high on the gauge?
 

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But Mad North-Northwest
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You can check resistance, but it likely won't tell you much since you can't tell if the pressure is actually low or if the sender is bad. The senders have a short life and conk out all the time: I would just suggest replacing it regardless.

Here's what I'd do given the car's history of oil pressure weirdness:

  • Buy a new sender
  • Rent an oil pressure gauge
  • Remove old sender (you need a thin wrench, there are threads on how to do this) and throw in trash
  • Hook up analog gauge, warm up the engine, see what the oil pressure actually looks like (the dash gauge, even when working properly, is not particularly accurate)
  • Install new sender
This way you'll only need to fark around with the sender once, you'll have an accurate measure of your actual oil pressure, and you'll have a new good sender in there
 

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Hi there, Dom mentioned this above, ground the sender wire to check for full scale. Very recommended to do this, super quick and easy. Grounded should be full scale one way, open full scale the other way.

Cheers,
 

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Push hard and live
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I’ve got somewat low pressure in both the 115 and 116 I’m refurbing right now. Already have a new sender to diagnose.

Most of us grew up driving cars with only an idiot light. I’m reminded of a “Car Talk” episode where a call-in lady was complaining about a red warning light on her dash. I forget whether it was Tom or a Ray, but their advice began “got any black tape....?”
 

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1978 Spider
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Discussion Starter #11
well the state governor is not helping....

I just called Harbor Freight to see if they had the oil pressure test kit in stock....yes they have a few...but they are about to get shutdown (like any minute now). Texas apparently is ordering ALL non-essential businesses to close???? ok, pretty much most businesses are essential to daily life in some manner. They provide goods and services that keep other thing operating...
 

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My notes indicate the following sender resistance range
0 psi - 340 ohms
57 psi - 127 ohms
114 psi - 8 ohms.

So if you connect a 100 ohm resistor in place of the sender then the gauge should read a little over 50%.

I have one of those gauge testers that you can dial the resistance into.
Time to experiment!!!
 

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1978 Spider
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Discussion Starter #13
scored an oil pressure test kit at HF....and invented several new bad words while trying to get a wrench on that **** sensor unit....why oh why did they stick it behind the starter???

Anyway...grounded the wire and got full pressure on the gauge so that seems ok.

Tried to get a continuity reading on the sensor but couldn't get any no matter how much I scratched around on the body for a ground. I "think" I got 180-ish trying to check it still installed. But after removing it I could not even get a flicker on the ohmmeter.

Plugged in the tester and getting about 10psi at idle. Max I could achieve was about 50psi at 5500rpm. I didn't take the engine any higher. But that is notably lower than the PO's recorded notes of 45-65psi BEFORE replacing 2 crank pins a few months later.

What is the normal psi? Or probably more relevant is what PSI does the relief valve operate at?
 

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In addition to replacing the oil pressure sending unit; I like the idea of dropping the oil pan on a recently purchased used car. Not only will you know if any crank plugs have been pushed out of the crank but you may find an assortment of nuts, washers or master link pieces from the timing chain that previous mechanics have dropped. Also you'll be able to clean up a lot of dirt and gunk that has settled to the bottom of the oil pan. Your oil will look cleaner longer and you can seal the sump with Ultra Grey (and/or a new gasket).

Mark
 

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1978 Spider
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Discussion Starter #16
yeah..saw where Gubi posted that...but at some point there will/should be no more PSI increase as the PRV comes into play...what is that value? At the moment I'm not comfortable pushing the engine to redline just to see if I can hit 65-70 psi.
 

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1978 Spider
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Discussion Starter #17
So why is that **** sensor in such a hard spot to access? Is there any particular reason you couldn't say "swap" the sensors? put the "reliable" oil light sending unit on the right side of the block behind the starter? you are not likely to need to replace that one too often. And move the gauge sending unit over to the left side where it's open and easy access? swapping the wires is a breeze.

Does that oil light one need to be closer to the pump? Is the gauge at the end of all the oil passages & galleries? should that even matter? I guess pressure drops the further you are away from the pump. But then shouldn't the warning light be at the point of lowest pressure in the system?

If I can't swap the sending units...maybe I'll just install put a high pressure hose and 3-way adapter and relocate the gauge sending unit along with a remote mechanical PSI gauge somewhere on the firewall/fender?
 

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I ground down and shortened a wrench. Easy after that.

Location of the senders isn't a factor. Hell, you don't even know if the Hobo Freight gauge is even accurate.
Or your dash gauge for that matter. You only know it pegs when grounded. How accurate is it between no ground and full ground?
 

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1978 Spider
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Discussion Starter #19
I'll move the tester over to the other side and see what readings I get there...if same at both locations then I will know it does not matter...so I can just swap the senders for future ease of access...I bet I can even use the same wires and just swap the wires at the gauge & light inside. easier than running extension wires back and forth across the engine bay.

Then there is this thread Testing old 60507261 oil pressure senders, etc... about using a Bosch sender as a replacement for the troublesome 605.07261 unit. It's a cheaper unit too...even after getting the 1/8-M10 adapter. But I see a clearance issue given the stock location on the Spiders.
 
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