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Strange Oil Pressure


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Relatively newly built engine (1200 miles). Oil pressure is 100psi cold and 75psi when hot (@ 2000 rpm). The strange thing is that I quite regularly see a blip in the oil pressure as it drops to 40-50psi for a second or two. I fitted an additional electronic pressure gauge which replicated the mechanical gauge. As the PRV is only limiting the maximum pressure it can’t be that. Any ideas?

 Thanks 

Colin

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Do you have an oil cooler? I've found you can sometimes "see" thermostat opening and closing as a transient drop in pressure. Particulary obvious in cold weather 

Tim

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Alan, I have replaced the filter but it still does it. Anyway, doesn’t effect oil pressure as it’s just to allow oil to the bearings if the filter becomes blocked. The gauge reading shouldn’t change. I think that’s correct.

Colin

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I didn't Neil, but I didn't rebuild the engine. Can't see how this could effect it anyway. When up to temperature the PRV will be completely shut. It's there to limit the maximum pressure, not the running/hot pressure. I guess for protection on cold start or an oilway blockage.

Colin

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1 hour ago, colin3511 said:

I didn't Neil, but I didn't rebuild the engine. Can't see how this could effect it anyway. When up to temperature the PRV will be completely shut. It's there to limit the maximum pressure, not the running/hot pressure. I guess for protection on cold start or an oilway blockage.

Colin

It opens and shuts depending on pump output pressure- which depends on oil viscosity and engine speed. It could be opening at high RPM and then failing to close as the speed drops due to sticking of the piston  or,  a slight chance, of  the spring. 

With an new engine there's also the  possibility of some swarf in the valve body.

I'd at least pull the PRV apart and give it a clean and inspection.

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With that high oil pressure do you know if it exceeds the 100 as the gauge only goes that high? I would also check that there is just the PRV plunger and spring and not as I have found before an extra washer under the spring.

Stuart.

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I can be cruising at 2000 rpm at a steady 75psi then it suddenly drops to 40psi for a couple of seconds. The PRV will be closed all of this time as it’s setting will be more like 90 psi, as it reaches 100psi on a cold start. It would be a highly inefficient system if the PRV was open all the time. It’s there, I assume, to limit oil pressure in the engine and protect the pump if a dead head was to occur.

The electronic gauge goes up to 150 psi so happy max is 100psi. I have removed and cleaned the PRV as well as swapping it with two others I have. Also swapped springs. No washers to increase spring force.

 Thanks 

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Really? The pressure is the resistance to flow back to the sump. Therefore are the bearing clearances too small? Alternatively the pump displacement is higher than standard, or the volumetric efficiency of the pump is better than standard. Can I complain that the engine is too well put together?

Concerning about the oil pump not being pinned. Could it fail completely, or just continue to slip?

Thanks,

Colin

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“Concerning about the oil pump not being pinned. Could it fail completely, or just continue to slip?”

Yes...it could fail completely if slipping because it isn’t securely fixed to the shaft. Or it could continue for an indeterminate distance/ time before the picked up metal on the shaft which has been sheared off the surface, eventually shears completely and the rotor spins like a top...complete loss of oil pressure. Trouble is the many times it falters and gives low pressure could start an internal engine failure somewhere because of lack of flow and rupture of the oil miniscus. 

Stop running it and drain the sump and remove and inspect the oil pump rotor ASAP. Then you can decide upon what you see, also gives a chance for mains and big ends to be removed and bearings checked, and end float checked also. More we talk the more it sounds a sensible way forward, easy to do and cheap.

Mick Richards

Edited by Motorsport Mickey
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As I said speak to the builder he should know what has been done, do you have good oil delivery to the rocker shaft? Even a pinned shaft can brake as Stan on here found out my concern is the pressure you could also have partial blockage in the oil gallery’s 

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47 minutes ago, Peter Cobbold said:

Picking up air  due to low oil level in sump. Saloon dipstick in a TR engine can cause under-filling. Peter

Particularly if the oil pump pick up is not properly adjusted so as to pick up from the the bottom of the sump.

Occasionally the dipstick tube can get bent and enter more vertically and lead to under filling (and the reverse if set too horizontal.

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7 hours ago, stuart said:

With that high oil pressure do you know if it exceeds the 100 as the gauge only goes that high? I would also check that there is just the PRV plunger and spring and not as I have found before an extra washer under the spring.

Stuart.

yes check for a washer under the spring, it was a way some used to raise the oil pressure.

I had this a couple of years ago and after Forum advice [ Waldi ] i purchased a new plunger and spring . Problem solved for me ,

Roy

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Is the drop in oil pressure related to cornering? I had the same issue and it was caused by the oil pump pick up pipe falling off into the sump.  I tracked it down by over filling the sump which temporarily cured the issue and I then knew it was an oil pick up or incorrect dip stick problem. 

 

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2 hours ago, colin3511 said:

It was chemically cleaned and galleries cleaned out with rods. Good oil delivery to rockers. Looks like sump off to make sure everything is okay.

 Thanks 

If you still have any sort of warranty I'd get the engine builder to do this. As commented above, if the oil pressure fails and you don't immediately shut down the engine you'll have another engine rebuild on your hands and if you try to fix this problem without advising the original builder the new build will be at your cost.

I lost oil pressure totally when a filter blew off a new defective spin on adapter- over 100 psi oil pressure, cold testing the assembly at over 4000 rpm. Because I was testing in the garage I was able to immediately shut down the engine when the oil pressure collapsed. But you can't drive the car around watching the oil gauge.

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Hi Colin,

what grade of oil is used?

After checking dipstick length (see above) and confirm oil level,
 I would start with the PRV; I installed a new valve (mushroom) and spring and the valve had very minimal clearance in the bore. I smoothed both surfaces with fine sand paper until it moved smoothly. Check spring length and (coil) thickness and compare with a new one from respected supplier.

If that does not resolve the issue, it’s a sump off I’m afraid.

Waldi

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Use 20/50 Waldi. Hear your comments but I’m pretty sure the PRV is closed all the time this event happens. It wouldn’t suddenly open and lower the pressure. I could understand if it was stuck open and I suddenly gained pressure. I’ve tried 3 different sets of PRV parts as that was my first thought.

Engine was rebuilt by previous owner so not much I can do but investigate.

Thanks

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