Oil pressure at idle suddenly higher than it used to be

[Elanman99]

After recently replacing the water pump (no other relevant work done) the car has done a trouble free 700 mile round trip to Classic Le Mans. Before the return trip oil pressure was about 10/15 at idle and 35/40 at say 3000rpm as it has been for the last 20 years or so. In the last 100 miles before I got home I noticed that the pressure at idle had increased significantly. Now at idle it never drops below 30psi.

This is not a gauge error so I'm wondering what the cause is and whether I should worry?

Oil and filter were replaced before the trip and all seemed (was) normal until the last part of my journey, the only thing I found when I got back was that the oil level on dipstick had dropped well below the 'Fill' mark (a bit embarrassing!).

I am baffled as to how running with low oil level could increase pressure, searches of the forum have not produced any useful results.

Ian

[richardcox_lotus]

Are you sure you?ve lost oil ?

Blockage somewhere in an internal passage preventing drain back to the sump?

[oldelanman]

I suspect the pressure relief valve plunger in your oil pump may be sticking.
I had a similar problem where the plunger stuck in the closed position and the resultant extreme pressure wrecked my newly refurbished gauge and blew the seal out of the oil filter, luckily it happened in the garage and not out on the road. On stripping the pump I found the plunger was corroded.

[Chancer]

Not the cause of high(er) oil pressure at idle on a hot engine that previously idled at 15psi.

Can the OP expand on why he considers it not to be a guage/sender error?

[Elanman99]

Not the cause of high(er) oil pressure at idle on a hot engine that previously idled at 15psi.

Can the OP expand on why he considers it not to be a guage/sender error?

The pressure indicator is a non standard setup that I made using a precision transducer, my electronics, and an analogue meter (ex helicopter). I still have the original mechanical gauge and temporarily connected it because it was more expedient than putting the calibration rig I used originally.

Ian

[rgh0]

It would be unusual for a stuck closed pressure relief valve to cause high pressure at idle as the relief valve is normally closed at idle anyway. It suggests a blockage in the system that is causing the pressure to rise until the relief valve opens at idle. Potentially the oil filter has had an internal collapse

cheers
Rohan

[Chancer]

Agreed Rohan.

Thanks for the explanation re the guage, sorry for doubting you

In fact the partial blockage wherever it is makes even more sense on reflection, as the revs rise to the PRV setting then the excess pressure is diverted back to the sump so the high(er) revs pressure would appear to be normal but the engine could be starved of oil. It might just be the cylinder head being starved.

[oldelanman]

Chancer, Rohan,
You are quite right ... I didn't think it through

[MarkDa]

Is the oil pressure is 30 psi at idle and speed?
If so, the advice so far is a blockage somewhere causing the relief valve to be permanently open.
Changing the filter again would at least rule that out as being an issue.

[Chancer]

I fitted one of the first pre-production 1800 Zetecs to a Caterham, the pressure relief valve was modified for production versions, mine stuck(closed) a couple of times and each time it blew out the O ring sealing the oil filter and given that its contained in a groove between flat mating faces a lot of force was needed yet the revs were never high, less than 3K although the zetec has a very efficient large diameter oil pump running at crankshaft speed.

One time was on the starting grid at Lydden Hill took a lot of cement to clear up

[gibbon]

How did you get on with this in the end? Did you resolve it?

I have the same issue. Pressure gauge always seemed pretty sensitive to rev changes 10-15 ish i guess at idle up to 30 at good revs.

I changed the oil around 300 miles ago. The pressure gauge then recently fell to zero whilst driving. I switched off straight away. Checked oil level, all fine, no leaks.

I started again, the gauge then after maybe a minute of running moved up to 30 ish at idle, but to me, seemed to move up a little slowly. It now stays at idle on start up and at any revs pretty much at 30 ish, with very slight fluctuation at higher revs to maybe 32 or so. Slow moving needle, almost like it feels damped.

I changed the oil filter yesterday, exactly the same behaviour from the gauge.

Engine otherwise feels and sounds great.

Should i be concerned? Should i not drive the car?

What should my next steps be and a brief idea of how to do said suggested steps would be fantastic and much appreciated.

Many thanks

Gibbon

[gibbon]

How did you get on with this in the end? Did you resolve it?

I have the same issue. Pressure gauge always seemed pretty sensitive to rev changes 10-15 ish i guess at idle up to 30 at good revs.

I changed the oil around 300 miles ago. The pressure gauge then recently fell to zero whilst driving. I switched off straight away. Checked oil level, all fine, no leaks.

I started again, the gauge then after maybe a minute of running moved up to 30 ish at idle, but to me, seemed to move up a little slowly. It now stays at idle on start up and at any revs pretty much at 30 ish, with very slight fluctuation at higher revs to maybe 32 or so. Slow moving needle, almost like it feels damped.

I changed the oil filter yesterday, exactly the same behaviour from the gauge.

Engine otherwise feels and sounds great.

Its a '68 plus 2, with a believe what is an electrical pressure gauge / sender, but i am not 100% sure on this.

Should i be concerned? Should i not drive the car?

What should my next steps be and a brief idea of how to do said suggested steps would be fantastic and much appreciated.

Many thanks

Gibbon

[gibbon]

Anyone?

[MarkDa]

If it's electric then it's probably the sender or gauge.
Which isn't much help I know but it may prompt research into how they work and how to check them.

[Craven]

You need to know is the indicated pressure real or just an indicator fault.
First thing you need to establish is the type of pressure sensor system you have, the gauge itself is a clue. Only a general comment, mechanical system gauge will have a ~ 270 deg scale centrally mounted needle an electrical system usually has a top mounted needle swinging over about 120 degs. If you can, post a picture of your gauge.
Mechanical system is very robust and reliable, electrical has far more components that can fail. Most likely the transducer screwed in the block, the gauge itself can have just a sticky needle, or the voltage regulator may have a poor earth connection giving unstable 10 volts to the gauge.
May help or prime someone else.