UK Car Repair: What is Oil Pressure, lotus super 7, pressure relief valve

Question
My question is related to your general expertise on gasoline (for me in Australia that's petrol) 4 stroke engine theory......and is

What exactly is 'oil pressure' ?

This is a phrase that we all use...but until the oil pressure on my car (Caterham/Lotus Super 7 with Ford 1600 Kent engine) dropped yesterday for no apparent reason I hadn't thought about where exactly the presure of the oil in the engine is being measured.....or how and why.

Thanks for your help with this.

And if you have any suggestions as to why it might have dropped over the last couple of days (there has been sufficient oil in the engine at all times)...then I would be grateful.

John Morgan
Adelaide Hills
Australia

Answer
Hi John,
Oil pressure in an engine is the pressure that is built up by the oil pump because the pump is pumping more oil than what can be pushed through the engine bearing, cam bearings, lifters and rockers. Most oil pumps have the capacity to pump up pressure exceeding 300 to 400 PSI but a "pressure relief valve" relieves (regulates) that pressure down to from 25 to 65 PSI in most Buick,Olds,Rover V-8s. I am not familure with the 1600 Kent Ford but I would still expect to see 45 to 80 PSI in most four cylinder engines at cruise speed. If a bearing is worn out (excessive clearance) the oil rushes through and not much pressure is built up. However, there are several other things that can cause oil pressure to drop. A common cause is the oil pressure relief valve sticks open or partially open. These releif valves are either mounted in the oil pump or in the engine block or even in the filter block.

Listen to the engine at idle and on a quick rev up from idle to see if it now makes any unusual noises that were not there before. Another possibility is the gauge itself. find a shop that can connect a mechanical (hydrolic) gauge directly to the engine block to see what the actual pressure is. Then look at the oil to see if it is thinner than normal (gasoline contaminated) or tan or brown color (coolent contaminated) Then change oil and filter and have the pressure relief valve checked for proper spring pressure and seating. If all that does not correct the problem (if the pressure is extreemly low) have a shop check bearing clearance. (1.5 to 2.5 thousandth of an inch on rod bearing and 1.5 to 3.5 on a main bearing)

I hope this gives you a guide to find your problem, Let us know what you find. good luck.
HMF