No oil pressure after rebuild

[rcv]

I know this topic has been discussed quite a few times (and usually starts with this same sentence).

I just finished rebuilding my L24, and after getting it in the car and filling it up with oil I decided to give it a test crank to make sure I could build oil pressure. Nada, zip, nothing, 0 PSI.  Here's what I've tried so far:

• Filled the oil filter to the brim
• Pulled the spark plugs
• Put ~10PSI of compressed air down the dipstick tube
• Replaced the sender wire connector with a brand new one
• Tried with both the old sender and a brand new one (https://zcardepot.com/products/oil-pressure-gauge-sender-240z-260z-280z-70-77?_pos=2&_sid=c56c8c526&_ss=r)
• Cranked in 15 second increments ~10  times

The good news is that when I pull the valve cover off there's oil in the head, and the assembly lube I drenched the cam in seems to be replaced with oil.  Also, when I remove the oil pressure sender fresh oil quickly oozes out of the hole.  This gives me hope that it's just a sensing problem and not actually a pressure problem?.  

To test the gauge I grounded the connecting wire, and the needle shot up to full 140PSI, which indicates that the gauge itself is working properly.  Then, just on a whim I tested the voltage on the unplugged connector.  What's weird is that I don't get a solid voltage between the unplugged connector and ground. It seems to be oscillating somewhere between 0-12 volts. I don't have an oscilloscope at home and my crappy $20 multimeter can't give me any more details but it's definitely not a solid signal.  Is this expected? I assumed the gauge was basically working as a DC voltage divider which would mean that signal should be a solid voltage. 

 

Also, how worried should I be about cranking the engine like this? I used a ton of assembly lube when I put everything together, but it's been almost 6 months since I put the crank in and buttoned up the oil pan.  I've probably accumulated a little over 2 minutes of cranking time in 15 second intervals so far.

Edited September 6, 2020 by rcv

[Zed Head]

I don't know much about how, exactly, the gauge and sender work but I do know that at cranking RPM pressure will be very low (it will be low at idle speed also when you do get it running), and the gauge does not react quickly.  So you have a slow moving needle, which is barely going to make it to 10 psi, maybe, once it starts moving.

The best way to feel comfortable is to get a mechanical gauge and either install it temporarily or "T "it on to the port with the sender.  But, if you decide to do that you'll need to either get a BSPT to NPT adapter or booger up the threads on the gauge to force it in to the hole.

You're right, it's a common worry.

[240dkw]

I think the gauge has a bimetallic strip in the circuit that heats up with current flow and opens than closes as it cools, if you put it straight to ground with no sensor it with heat up quickly. Take Zed Head’s advice and check it with a mechanical gauge.

[rcv]

Good idea guys, thanks - I just ordered a mechanical gauge and some adapters.  Is 10PSI good enough at cranking RPM to consider the engine good to start?  In case you can't tell, I'm super nervous about starting it for the first time so I want to do as much groundwork as possible.

[Mark Maras]

  The pressure reading from the auxiliary gauge is the best test but I'd have started it up by now. Oil flow up to the head indicates the oil pump is pumping. On initial start-ups I normally leave the valve cover off  to check the oil flow to each cam lobe and watch the pressure gauge.

[rcv]

@Mark Maras Doesn’t oil go flying everywhere? I’ve thought about building an acrylic valve cover like that Restoration For Beginners guy: 

 

[Diseazd]

Did you fill the oil pump with oil before installing?......very important.  Just cranking over doesn’t produce much oil pressure. If you watch the gauge carefully when cranking over with plugs out, any movement, however minuscule of the needle,  means you’ve got oil to the head. That’s assuming that you didn’t forget to install the oil pickup LOL. 

Edited September 6, 2020 by Diseazd

[rcv]

Yup, I filled the pump with oil and turned it a few times before installing it. I took pictures of the pickup installed too, so I’m pretty sure it’s there unless a gremlin stole it. 

[Mark Maras]

3 hours ago, rcv said:

@Mark Maras Doesn’t oil go flying everywhere? I’ve thought about building an acrylic valve cover like that Restoration For Beginners guy: 

 

 A small amount escapes at idle. A towel along the edge is enough to catch it. Don't rev it with the cover off though (VOE), you'll spend the rest of the afternoon cleaning up the mess on the fender well.

[rcv]

I pulled the valve cover off and cranked it for a few seconds, and it looks like I'm actually getting some oil pressure.  What do you all think, should I just stop worrying and start this thing?

 

 

[Patcon]

I'm with Mark, I would have already started it.

I have always been under the impression that you should start it and hold it at fast idle for a period of time to break the cam in????

[Mark Maras]

The oiling looks OK to me. Did you clean the oiling tube? The holes can become restricted with sludge. A torch tip file works wonders for opening them up.