Low oil pressure on 420G

I did 350 miles on Friday, and the same distance a month ago. The gauge reads 10-15 at idle and 30 at 3000. I have another sender which was on this engine a few years ago. I noted that it read 10 and 40 at the time, and off the scale when cold, if I remember correctly.
The pressure reading on the current gauge has been falling over time. The engine was built by VME in Wales, about 7 yrs ago.
I need to mount a mechanical instrument to verify, but want to prepare, mentally, for some work…
Can the pump be exchanged with the engine in place and sump off?
I read somewhere about a bad lower chain tensioner causing low pressure, but dont understand how. The valve guides are a bit worn, but has nothing to do with op, afaik.
Trying to borrow another sender which is the first step, and easier to install than the manual instrument. I’ll report back in a few weeks as I will be away for 2 weeks

Recent oil change, 20-50 Gulf Classic, but level deliberately kept in lower half of the hatched area on the dipstick

Think so. A bearing cap has to come off but the pan would be the worst part.

The tensioner is fed oil pressure via the main gallery!

Nothing at all.

Start with the gauge, and although unlikely since it’s not been a sudden change, have a look at the pressure relief valve in the oil filter housing. Better yet, pinch the hose (beware, it’s a lot of oil under pressure when the bypass is open!)


Thanks David. You mean: start with the sender (and not the gauge/instrument)!?
Don´t get the pinch the hose bit…l?

I think you should start by verifying the sender is working as they are known to fail, best with a mechanical gauge,

and I believe that it is likely not the pressure relief valve but it is worth checking; it is where the oil filter housing is and on the cars I know there is some length of hose where the pressure is bled back into the sump. Pinch that hose tight while it’s running and you may or may not see a rise in pressure if the valve is open a little, or its spring weak, etc.

I would (start & hopefully finish) with dispensing entirely with the unreliable oil pressure measuring system

A mechanical gauge is most original looking, but a bit of work, and expense

I have installed an electronic gauge & sender , as I consider an accurate reading of OP to be absolutely essential, probably cost me about $40 or so

All the old rubbish is in a box if someone ever wants to re-install it, a box of senders and gauges

I suspect the relief valve and or spring.
I had a bit of metal (end of a split pin perhaps. About 4mm long) in a XK150 engine, causing a very low idle op. About 5-10 on the XKs very accurate op gauge.
On another XK, I had too high pressure. Changed the spring and it was normal.
Looking at the installation on the 420G, the best would be to undo the bent oil return pipe’s mount on the sump, open the short hose’s forward clamp, and remove hose with pipe. Then undo the access to the relief valve and spring. There is a ”sp9der and pin” to be aware of also.
Best done from underneath, I reckon. Might order a new relief valve and spring before I start the work.
With 2 different op senders both giving 10 psi at idle with hot engine, I suspect the relief valve area…

I had a significant bead of silicone in my (later style) relief valve and the oil pressure went down to a maximum of 20, ~5-10 at idle.

Beware of the pressure involved, should you block off the relief, oil pressure is more or less unlimited besides what squeezes past the bearings, a very powerful hydraulic pump. When cold, the flow back to the sump is enormous.

The original sender/ system might be unreliable (it worked for decades?) at times but I would want to keep things original (at least put them in a box)

I dont get the ”beware the pressure” bit.
With a cold engine, not running of course, there is no pressure to wirry about when I dismantle the relief valve and other small parts around it.

No, not then, but unless you’re dead set on dismantling the entire apparatus you could see if clamping the return makes a difference. If the pressure stays low on a known good gauge you can begin suspecting bearings, the pump, tensioner or something like that.
Doesn’t matter when the engine is off obviously. Nothing to worry about then.

Ah, stupid of me. Sorry. But the hose is too short, between the metal ends of filter house and 90deg angled pipe. I’d say just 1”, and the pipe is quite stiff. Hard to clamp it, I think.
Will borrow a manual/mechanical op testing kit from my nearby garage

Good news😊
The manual op instrument shows 35-40 at 750 idle with hot engine, and >60 at 3000.
So my 2 senders are crap. Might buy a new one and hope it is fairly accurate. Just dont like looking at those low readings when driving


after cancelling at least 2 outings cause my OP read 0, that is when it was all over between me and Jaguar senders ( I subsequently found OP was unacceptably low and replaced engine).

I simply could not have made that call with a Jag sender, I would have 10 “spares”

Ok, so dont waste money on a new sender. Keep the old one but disregard it, as I now know that I have vry good op.

The repro sender units are a typical example of the sub standard parts we are expected to put up with these days. The same goes for fuel senders.
They are misleading and a constant source of anxiety. A waste of money. Fit a proper mechanical one. You can often remove the face and fit a Jaguar type face instead to make it look the same.

I do not know where the replacement senders are coming from but here in the USA but if it were me I would contact Nisonger Instruments www.nisonger.com and ask about a known good sender with a guarantee. They are the long standing Smiths instruments agents in North America.

If an original look was my No1 priority, I would fit the mechanical Smiths

You can buy the older units on Ebay, and there is, a “new” kit

Oil Pressure - Mechanical (gauge-shop.com.au)

My priority is to have accurate OP reading, especially at low RPM, that is why I installed the digital

Fairly sure an inaccurate or malfunctioning OP gauge system would mean a plane is grounded

I do intend to fit the mechanical gauge setup at some point in the future