1966 Jaguar 3.4 Mk2 - oil pressure anomalies

I am experiencing oil pressure anomalies on a totally reconditioned engine. On starting the engine the oil pressure climbs to around 70 psi then promptly drifts back down to zero after a few seconds (oil still cold). I have replaced the sender and then the gauge from a scrapped 1977 Mk2, both made no difference.

Next I replaced the whole filter head, as it occurred to me that the ‘blocked filter bypass’ was possibly coming into play - correct quality new filter and correct grade of oil is used - still the rise and immediate fall in pressure reading persists, regardless of the engine rpm. Throughout all the starting and stopping the engine sounds quiet and smooth, so it would seem that it is getting some oil.

Can anyone help please?

3JagsPete

Have you checked the pressure relief valve?

Yes. The replacement filter head had the relief valve in it, I assumed that they were unlikely to have the same problem… I hope I was right!

Thanks 3JagsPete

Did you install a new oil pump? One of the new upgraded ones? The pump suction tubing fittings are a little tricky. Might want to check all the correct o rings are in place. Seems you have confirmed you have very little oil flow. Might as we’ll check for rod and main bearing clearances while you are at it.

It may be a electrical problem , ie wiring or the like , I would fit a mechanical oil pressure gauge , by tapping in to a main oil way on the side of the engine block !

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The wiring should be OK as it was totally renewed during the rebuild. A new factory correct loom was used and everything appears to work well.

Thanks for your interest - 3JagsPete

The mains and conrod bearings are new on a reground and polished crankshaft so should be good oil pressure wise. The oil feed and pick up pipes were ‘smoothed’ to ensure a better and easier fit through new O rings. I am going to investigate the oil pump today, having lost 3 hours sleep in the middle of last night thinking about it.

Many thanks, 3JagsPete

My reply has gone under the next reply, I don’t know how I did that! sorry.

3JagsPete

Always a good idea before thinking the worst.

Just a thought and its from memory…
Can you check the +ve feed wire to the gauge is NOT from the voltage regulator but is the direct +12volts.

I will check your suggestion. I am currently changing the oil pump to an original H. E. pump which is in good condition ( although I have found nothing wrong with the pattern type pump). I have checked the feed to the main bearings all clear and obviously getting plenty of oil. So I am fairly happy with the mechanical side of things, your suggestion sounds like a good idea - will let you know.

Many thanks 3JagsPete

Obvious the direct reading type is the only way to eliminate the gauge reading/indication

With best regards

Hi Phil,

I called a friend who is more knowledgeable than I am with things electrical, showed him your email. He set to and carried out some tests with meters and things and found that you are absolutely correct about the fluctuating current from the regulator. Once the engine has run for a moment the voltage drops drastically on the oil pressure gauge feed wire.

Many thanks for your insight, much appreciated. 3JagsPete

Sounds like you need a new instrument voltage stabilizer. I found these on a Moss website for an MGB, Solid state ones in positive or negative ground. Not too expensive but I have no idea how well they work. Any forum members with experience? Here is the link.
https://mossmotors.com/mgb/dashboard/solid-state-voltage-stabilizers

No he doesn’t. The oil pressure gauge is not feed from the regulated side of the regulator it’s from the +12 volt connections