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Oil Pressure Anomaly


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#1 colin3511

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Posted 11 May 2018 - 03:01 PM

Hi,

 

Very weird experience this afternoon. First time I have ever driven my 1972 CP, complete restoration car. Oil pressure started off at 80+ psi and dropped away as expected as it warmed up. However, as I continued on a 6 mile trip, the oil pressure dropped further until zero and the oil warning light came on. Stopped and checked oil level - at the lower end of the accepted scale but the oil had been circulating around the engine and I didn't wait for it to drain down. As I was only 3 miles from home I decided to try and get home. Started it up and the oil pressure returned to 60 psi at 2000 rpm. No issues on the way home.

 

Really confused as the gauge and warning light operate separately. Blocked gallery near their measuring points that cleared?

 

Welcome any thoughts.

 

Thanks,

 

Colin


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#2 Rem18

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Posted 11 May 2018 - 03:05 PM

What is this national Oil pressure day?

Your right Gauge is mechanical  and light of sender.

Just like the other guy sounds like some sort of blockage?

Filter? Dont know enough about new rebuilds but sounds like a coincidence if not?


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#3 peejay4A

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Posted 11 May 2018 - 03:14 PM

I would unscrew the relief valve from the block, give it a good clean and examine the valve face for blemishes. Chances are it was a piece of muck jamming it open that has now moved on but worth a look.
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Pete 

#4 Mike C

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Posted 11 May 2018 - 10:18 PM

Might have been an oil  passage blocked by assembly grease.


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#5 barkerwilliams

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Posted 12 May 2018 - 05:37 AM

Do you have a new oil cooler? 

 

If you also have an oil thermostat the cooler would have been closed and dry until the oil was hot enough for oil thermostat to open and the oil was diverted to fill the cooler and the air within the cooler purged through the pump as the cooler filled. That may account for a short temporary dip in oil pressure - but only for a few seconds.

 

 

Alan


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#6 boxofbits

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Posted 12 May 2018 - 08:27 AM

More than likely Pete's suggestion of some swarf or dirt holding the relief valve open.

Kevin
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#7 astontr6

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Posted 12 May 2018 - 08:57 AM

I would unscrew the relief valve from the block, give it a good clean and examine the valve face for blemishes. Chances are it was a piece of muck jamming it open that has now moved on but worth a look.

Peejay could be quite right! The oil gallery above the release valve collects sludge when you rebuild a 6 engine it is very important that you take out the blanking plugs and use a push through from one end to the other. You will be surprised at how much crud comes out which can indicate the lack of oil changes during its life.

 

Bruce


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#8 colin3511

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Posted 12 May 2018 - 09:24 AM

Thanks for all your replies.

No oil cooler. I have removed PRV and cleaned it, although it was OK. I did use plenty of assembly lube and this was the very first trip out. My first ever strip and engine rebuild so I am very cautious/nervous of any potential mistakes I may have made!

Looks dry again today so I'll take it out and keep an eye on the gauge.

Thanks,
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#9 colin3511

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Posted 12 May 2018 - 06:14 PM

Started up earlier and about 15 psi oil pressure from cold. I had an old spring and poppet from a previous car. The spring was slightly shorter but this doesn't necessarily mean less force to open. However zero pressure with that. Put 2 thin washers behind original poppet and 100+ psi on start up. Removed one washer and pressure dropped to 45 psi. Amazed at the difference with such small changes.

As my original set up was OK, am I correct in assuming the spring has lost its spring? It feels OK. Are the replacements OK?
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#10 boxofbits

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Posted 12 May 2018 - 08:54 PM

Started up earlier and about 15 psi oil pressure from cold. I had an old spring and poppet from a previous car. The spring was slightly shorter but this doesn't necessarily mean less force to open. However zero pressure with that. Put 2 thin washers behind original poppet and 100+ psi on start up. Removed one washer and pressure dropped to 45 psi. Amazed at the difference with such small changes.

As my original set up was OK, am I correct in assuming the spring has lost its spring? It feels OK. Are the replacements OK?

45psi is slightly low if oil is cold, meaning not run for 24 hours. But 45psi is fine for running, it is only a guideline and as long as you have pressure you should be okay. As a precaution I would change the valve and the spring together . Many cars including BMC Minors etc only have an oil light switch which comes on much lower than that, so watching a guage fluctuating can be a needless distraction.

We used to lap the valve in with grinding paste on an unwashed block, but youre too late for that , so best replace both as you dont know if the face of the valve is true either.

Kevin

Edited by boxofbits, 12 May 2018 - 08:57 PM.

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#11 Waldi

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Posted 13 May 2018 - 10:02 AM

My spring was worn on one side from rubbing against the block, so part of the wire diameter was reduced (flattebed). This affects springrate hence force. It was also 1-2 mm shorter than the new one. No oil pressure data yet, still rebuilding.
I would replace spring and valve, and check if the valve is moving without sticking.
Waldi
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#12 colin3511

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Posted 13 May 2018 - 12:37 PM

Hi Waldi,

You may have cracked it. My poppet had a definate step on the diameter that houses the spring. I have cleaned it up and 100 psi on cold start and needle moving nicely as the rpm change with oil warm. I'll monitor it and see if this is the cure or not. Note no extra washer fitted to achieve this.

Thanks,

Colin
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#13 Waldi

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Posted 13 May 2018 - 02:30 PM

Great Colin.
Waldi
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