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No oil pressure after rebuild

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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:

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 by rcv

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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.

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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.

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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.

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  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.

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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 by Diseazd

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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. 

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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.

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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?

 

 

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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????

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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.

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I blew it out with compressed air right before I put it on, but I just ordered a cheap torch tip file that I’ll use before I button up the valve cover. 

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8 hours ago, rcv said:

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?

Yes, it is to be expected. That's normal for the signal going out to the sender units (Oil, fuel, and temp).  They are pulse width modulated.

I don't think it directly pertains to your issue (which sounds as if it may be a non-issue after all), but if you or someone else wants a description on how the gauges work, let me know and I'll put something together.

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I’m with you Charles and Mark......fire that thing up. You’ve got oil to the valve train!

Edited by Diseazd

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I had this issue on my rebuild, although i confirmed oil was getting it the head, and the oil pump was primed. After that I just ran it, and the oil gauge showed pressure. I’m not sure is there is an official way to do it.  ?

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Well, I took yall's advice and started it up this afternoon.  Oil pressure jumped to the middle of the gauge (70PSI?) almost immediately.  I started another thread here if anyone's interested in following along with the fun.

FYI before I started the engine I also tried connecting a mechanical pressure gauge in place of the sender.  After cranking for a few seconds, I didn't even get a blip on the gauge.  I then just pulled out the gauge and put my thumb over the hole and saw a little squirt out where I didn't press hard enough.  That was good enough for me, and starting it up was definitely the right decision.

 

@Captain Obvious I'd still love a rundown of how the gauges work if you're up for it.

IMG_2393.jpg

Edited by rcv
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Oops! Sorry!  Memory like a goldfish. Well that, and I kinda wander in and out of on-line consciousness.

I'll put something together. I just took some pics.

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OK, I promised to describe how the gauges work... Not sure it should be here in this thread, or if I should start a new thread for this. I'll put this here for now, but if people think it should be separate let me know and I'll start a new specific thread about gauge theory.

So in order to understand how the gauge system works, I think it would be a little easier to first understand how they DON'T work. I think everyone already has a good handle on this simple system, but it's an important place to start.

Here's a pic of a simple (uncompensated) gauge design. Wrap a heating coil around a bimetallic strip and mechanically attach that strip to the linkage of a gauge needle. Flow current through that heating coil and to a sender unit who's resistance reflects the level of whatever you're trying to measure (temp, oil, fuel, whatever).

As the resistance of the sender unit changes, the current through the heating coil changes and that changes how hot the bi-metallic strip gets.

The lower the sender resistance, the higher the current.
The higher the current, the hotter the bimetallic strip gets
The hotter the strip, the more the needle moves.

Here's a pic:
gaugetheory1.jpg

This simple uncompensated gauge system does "work", but it is subject to a few real-world outside influences that can affect the accuracy:

First, since the whole thing works on the temperature of the bimetallic strip, the gauge will read differently on a hot day than on a cold day.

And second, since the gauges are powered by the battery system of the car, changes in that system voltage will affect the gauge readings. The gauges would read differently sitting with the engine off than they would with the engine spinning at 3000 RPM when the alternator has kicked up the voltage a bunch. The system voltage can vary from about 12V to over 14V and the gauge readings would change as the voltage varied.

So the basic gauge system above sorta works, but these two real-wold effects are undesirable. If the above basic gauge system makes sense, I'll get into how they compensate for those two real-world effects.

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This is great so far @Captain Obvious. I’m out of TV shows to watch, so I’ll be anxiously awaiting your next installment for my new entertainment.

 

I think this probably warrants it’s own thread. It would be a shame for all of this info to get buried under this random topic. If you do start a new one can you just link it here?

 

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LOL. I'm pretty much out of TV shows too. I get it.

There are two "stages" to the compensated gauge. One of those stages is he simple gauge we already talked about above. The other stage is the compensating or regulating stage. It's job is to compensate for changes in ambient temperature and varying system voltage.

The compensation stage consists of another heating coil wrapped around another bimetallic strip. This bimetallic strip forms an electrical switch such that when the strip heats up, it breaks contact. And when it cools down, it re-establishes contact. The trick is, that it makes and breaks contact to it's own power source and the power source for the simple gauge stage.

The result is that this second strip/heating coil combo will make and break the power source such that it will always achieve the same average temperature. Of course the temperature is rising and falling some, making and breaking connection, but the AVERAGE is always the same.

Colder day? Power to the compensation strip will have to be on longer to reach the temp that bends the strip to the point where it breaks the switch connection. Hot day? Just the opposite. Power will be on for a shorter time.

Low system voltage? Again, power will have to be connected longer in order to heat the strip to the desired temperature,. And conversely, if the voltage is higher, it'll take less time.

The result is that the compensation stage creates an ON/OFF/ON/OFF pulse train whose duty cycle and frequency will change depending on the ambient temperature and system voltage. This effectively creates a voltage source that will always supply a constant amount of POWER to the gauge system under all conditions.

Clear as mud? Connect that constant power source to the simple gauge from above, and you have this:
gaugetheory2.jpg

If you put your key in and turn it to "ON" you should see the gauges start to rise. Don't start the car. Just pick a needle and watch carefully. You'll see the needle start to rise, but probably before it reaches it's final position, it'll pause... Then start rising again. Then pause again. This will continue until it finally reaches it's final position. And even then, if you watch carefully, you'll see the needle actually wiggles a tiny bit.

This effect can most easily be seen with a needle that's moving well above minimum like a full tank of gas. That pausing and wiggling is the compensation stage opening and closing.

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Very good explanation, Captain. I agree that his should be a sticky.


Sent from my iPad using Tapatalk Pro

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