What causes a drop in oil pressure

[maxi77]

Are you absolutely sure all the electrical connections between the sender and the gauge are 100%. Poor connections can give all sorts of funny readings.

 

[Colvic Watson]

Thanks Peter, I'll mention that to the yard, they are planning a 'mechanical test' - any ideas what that means?

 

[DownWest]

Might be a mechanical gauge rather than an electrical one.
DW

 

[RobinBirch]

Thanks Peter, I'll mention that to the yard, they are planning a 'mechanical test' - any ideas what that means?

Don't think its anything they do on the engine, more an assessment of the capacity of your wallet

I think an earlier post from Paapaecko is very relevant. If you've got oil sloshing over the rocker gear at high revs but reduced pressure registering on the gauge you don't have an oil pressure problem.

Old engines do have their foibles. My BMC Captain has a few -you do learn to live with them though.

 

[pete]

Strange ! less pressure with higher rev's
I agree with Trundlebug is the oil level low ? and the pump is removing oil from the sump faster than it is running back in.

Once fitted a Cortina engine in an Mk 2 Escort, had to fit the escort sump and oil pick-up pipe because the dished sump was on the opposite end.
Struggled to get oil pressure initially but then it was ok until cornering fast.
Turned out to be they had different length dip sticks and we had fitted the long (wrong) one !

Pete

 

[Leonidas]

................and even too much oil; it can get whipped up into a froth by the crank and the effective viscosity drops.

Or possibly too little oil in the sump and the pump looses suction at high revs, cavitates and pressure drops. ?????

 

[Colvic Watson]

How big a job is replacing it?

 

[sailorman]

How big a job is replacing it?

drop the sump to do it

 

[Red Admiral]

One problem solved is another problem found at the moment. Now that the engine senders and control panel are fixed, there is according to the mechanic:

an oil pressure drop once the engine is put through a higher rev range

This is normal for any engine as soon as the oil is up to working temperature. On a well worn engine at tickover it can drop to virtually nothing. That's the trouble with gauges. They give you something extra to worry about!

Unless you have any untoward noises I shouldn't worry. Just make sure you have sufficient oil in the engine. An old Jaguar I had with the 6 cylinder XK engine ran for two years with never more than 20psi (should have been 60) on the gauge and absolutely nothing on idling. When I got rid of it the engine was still running without problem.

 

[tinkicker0]

................and even too much oil; it can get whipped up into a froth by the crank and the effective viscosity drops.

Certainly the first and easiest thing to check.

 

[gordmac]

Oil pressure is basically caused by the resistance to the flow of oil for a given flow and viscosity (thickness). Think about sucking up through a straw. Pumps are normally displacement type so the faster you turn it (more engine speed) the more flow you get, the pump has a relief valve to dump the extra flow when the pressure gets to a given level, the amount of flow through the engine is what is being controlled basically. The pressure should open the valve a little over tickover and the pressure will more or less remain at this value.
The relief valve should be fixed, the clearances and therefore flow resistance also fixed, as the oil heats it will get thinner so cold pressure should be a little over hot pressure.
If there is a partial blockage on the inlet to the pump stopping it getting all the oil it wants to pump at higher revs the flow and therefore pressure may be reduced at revs.

 

[Colvic Watson]

Following the ideas here and the yard mechanics' suggestions, on Monday they are going to try again, first with a 'manual' test, then after the oil is warm they will remove it and change filter, use a flush to clean through then new oil and filter. Then test again, that way they can eliminate cruddy oil and gauze as a cause. The test will hopefully show that at most there is a smallish drop and it's OK to live with it. Then they can fix the pinhole leak in the manifold, find and fix the hydraulic steering fluid leak and....

One day this will end and there will be no more jobs

I'll update the thread

 

[Colvic Watson]

Will do