The 'E' Type Forum

I see this has been discussed before, but trying to read and learn, there seems to be different opinions...

Just had my engine started for the first time. It's totally rebuilt by a trusted mechanic (myself), so no worn bearings etc, new oil pump, sealing rings etc., should be ok

Took me several hours to get the SUs running well, now it has a nice 5-600 rpm idle. On initial start up there's of course a lot to check, get it running, no leaks, temp, charging (it doesn't, need dynamo/relay work), and of course oil pressure. So I only have three readings in my head:
-On initial start up, it ran about 2000 rpm to keep it running, and it was cold of course, gauge needle disappeared to the right
-At a quick glance later, during SU testing, the needle was about in the middle
-When now running perfectly at low rpm and hot (fan came on, but temp gauge read in the middle), oil pressure gauge showed only 5-10 PSI. Raising rpm a bit, the needle moved a bit to the right, but still below 20

I know that oil gets around in the engine, since it disappears from the oil stick when engine is running, and cams are wet with oil

So, how should I react to this:
1. Should I consider this normal?
2. Buy a mechanical test gauge?
3. Take off the oil filter attachment and look at OPRV? Replace?
4. Change oil viscosity, manual mentions all from 5-20 to 20-50, quite a gap I'd say
5. Look at voltage to gauge, 9V or 12-13V? Since my dynamo doesn't seem to work, does this lower voltage affect gauge readings?

Reading over my own post here, and pretty sure I rebuilt the engine right, I'm not too worried, but a low reading oil pressure gauge does not make me happy...

Try clamping off the bypass return hose from the filter to the sump. That'll show if you've got a leaking prv, worked a treat on mine.

It’s probably just the calibration of your gauge or sender. The only real test is to temporarily attach a capillary gauge (mechanical) to show real pressure.

If I attach mechanical gauge, what are minimum good numbers, hot cold @rpm?

I went the capillary guage route, got mine from SNG. Extremely reliable, whereas if i relied on the old sender I'd think the engine was knackered. My SNG one was tested against the workshop professional one, and readings were identical.
At VERY hot, the minimum oil pressure I get 25lbs, at normal hot 30-35.
At anything above 2000 revs, it's never below 50 even in the summer in Spain on a long run working hard.
I think acceptable numbers are lower than that, they are just my figures.
I SHOULD ADD - mine is a 4.2 engine.

Whether good or bad my engine has survived over 80,000 miles since it's last rebuild with 45-50psi at 2400 rpm (approx 60mph 100kph in top gear) and less than 20psi at idle.

Numbers are approximate but are consistent.

Mikael,
I concur with Malcolm and Chris.
You may have read my thread on the issue and the improvement in OP with the replacement of the original electrical sender by a mechanical gauge:
viewtopic.php?f=4&t=19209

With the mechanical gauge, my OP now reads about 20 psi at idle hot and about 50 psi at more than 2000 rpm hot.
In comparison, the OP read by the electrical sender was indeed dramatically low: 0 psi and less than 15 psi respectively.

Thanks, ordered mechanical oil pressure test kit. Not a gauge to install, I can live with crazy low OEM gauge, if I know what reality is.

Any input to oil viscocity, gut feeling would be 15-40 or 20-50 on an old, though unworn, engine?

"Any input to oil viscocity, gut feeling would be 15-40 or 20-50 on an old, though unworn, engine?"


My engine is old and worn.

I use 20W60 semi-synthetic. There are a couple of manufacturers making this multi grade specifically for older high performance engines -Jags,Astons, etc.

Viscosity is not the most important consideration. More important is clean fresh oil. And a clean filter. I change oil and filter at 3000 miles religiously, despite using a long life oil. Also important is that wear takes place on a cold start up more than any other time.

The standard recommendation which I always use is 20/50 mineral oil.

BTW, even if you know your guage is faulty, I wouldn't "live with it" as you suggest. You never know if one day it might REALLY be that low! You either need a new sender (if that is the problem) or a mechanical replacement.

So pre rebuild (by CMES in Bracknell -excellent job btw!) my oil pressure indication was maybe 30 PSI on start falling back to less than 20...electric gauge but I did test it using a compressor at sample pressures and it wasn't a million psi out. Post rebuild it leaps to 60 psi and there it remains...lowest I've seen was maybe 50 when idling after a good run using their running in oil. This was replaced at 500 miles with Miller's 20-50 sport and now it seems 60 psi is where it stays..if yours says less than 20 then start by suspecting the gauge...as others have said use a capillary mechanical one...if that gives similar readings then possibly the pressure relief valve is stuck open?

Yes, your readings are way too low.

Connect your new Mechanical Pressure Test Gauge Kit to the oil galley plug after the filter.
That will tell you actual oil pressure to the bearings.
You may need and adapter as I think those plugs are BSPP (Parallel) or perhaps BSPT (Taper). If the plugs have a sealing washer they are NOT Tapered Pipe thread.

The Elect Sending units are notoriously inaccurate. Is your's new or old?
If your real pressure is good try a new sending unit before the expensive of a gauge.

Got my mechanical test gauge today. And by chance I had a tee so I could monitor the electric gauge compared to the assumed much more accurate new mech test gauge.

-Starting cold and approx 1500 rpm, mech said 65, elec disappeared to the right
-With choker off and temp gauge at 70, 550 rpm, mech said 45, elec agreed exactly
-So hot that thermostat had opened but not fan on, temp gauge about 75-80, 550 rpm, mech said 38, elec 20
-Revving it briefly to 2500 rpm, mech said almost 60, I didn't wait for elec

So my conclusion is that my oil pressure is OK. And that while the elec gauge is pretty ok "in the middle", it shows too much at high pressure and too little at low pressure. Indicating a not very well calibrated sender (bought new from Jag parts dealer recently)

If I can get a better sender, I'll buy it? If not, I'll live with it for now, and not worry about low reading when idling hot.

Thanks for input, one less worry...

Good going

Your oil pressure is fine, but I still wouldn't live with a bad sender. What if your oil pressure really did drop drastically one day, while driving, and you didn't notice it because you can't believe the bad sender?
I say that from experience. The filter head came loose on my car on the motorway, and I only noticed because I saw the guage plummet from over 50 to 20. I believe you need to know what the real pressure is.
I'd go ahead and get the mechanical one fitted.

Noone knows of a sensor/sender that is calibrated better to our OEM gauge?

JDC used to sell a sender and guage as a matched pair..they may still do.....also read through this thread especially post number 10 it will give you a better undstanding of the senders viewtopic.php?f=4&t=18515&p=150089&hili ... er#p150089

I always wanted to understand the ins and outs of OP senders and gauges. The post linked on understanding senders whilst detailed and comprehensive left me quite confused especially since I don't know whether during restoration the original or a replacement were used.....

Danny....a detailed description of senders and guages is in the service manual with diagrams.. Steve