Lotus Elan

Oil pressure at idle suddenly higher than it used to be

PostPost by: Elanman99 » Thu Jul 12, 2018 8:06 pm

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.

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PostPost by: richardcox_lotus » Thu Jul 12, 2018 9:15 pm

Are you sure you’ve lost oil ?

Blockage somewhere in an internal passage preventing drain back to the sump?
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PostPost by: oldelanman » Fri Jul 13, 2018 8:16 am

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.
oil-pump-relief-valve-003.jpg and
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PostPost by: Chancer » Fri Jul 13, 2018 8:52 am

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?
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PostPost by: Elanman99 » Fri Jul 13, 2018 9:02 am

Chancer wrote: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.

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PostPost by: rgh0 » Fri Jul 13, 2018 9:13 am

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

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PostPost by: Chancer » Fri Jul 13, 2018 9:34 am

Agreed Rohan.

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

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.
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PostPost by: oldelanman » Fri Jul 13, 2018 5:50 pm

Chancer, Rohan,
You are quite right ... I didn't think it through :roll: :oops:
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PostPost by: MarkDa » Fri Jul 13, 2018 5:54 pm

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.
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PostPost by: Chancer » Fri Jul 13, 2018 6:56 pm

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 :oops: took a lot of cement to clear up :oops:
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