the quality of some oil pressure sender units on the 4.2l is poor if I remember well.
I must have been lucky with mine over the years, as I dont remember the readings ever being a concern. My memory could fail me.
Now, I just took the Coupe out of its slumber (new 20W50 oil, near top of the dipstick mark) after many years of just pettering around the neighborhood at modest suburban speeds. Today, I had an exhilarating run on the motorway, cruising at 2,800 rpm, 70 mph, oil pressure at 40 psi, did not miss a beat, big wide grin on my face all the way, for 25-30 miles :)
After the fun, slowing down, idling in traffic, the oil pressure reading dropped to 25 psi. Should I be worried?
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Gosh, no, Eric - it would be exceedingly odd if the oil pressure did not drop in idle…
You may not have noticed the difference before - with pottering around at moderate speeds. The difference in pressure between driving/idling and cold/hot sure is there - but less pronounced…
The racing handbook suggests that OP be at or above 40psi @ 3000rpm hot. You are good for the upper end for an XK6. My Mk2 at 66,000 miles runs just over 20psi at 600 rpm idle. Your car sounds good to me. Frank and others will know - I believe that SIII engines ran a different pump and greater pressures? Paul
They had a larger pump, I think so. My XJ6 has ~45 PSI cold or above maybe 1200rpm, hot idle is 20. The other 4.2 I know is from am E-type and shows 60+ at speed down to like 40-50 hot. Never below 25 in idle as far as I can remember.
If oil pressure should ever significantly drop at all speeds or the warning light went on you should turn off and worry (and first suspect the pressure relief valve in the filter housing; I did so rightfully). If your oil is reasonably clean I see no reason why something like that would ever happen.
Interesting side conversation re the oil pump. The engine will soon come out for a gearbox replacement and a light refresh. Would it be a good thing to consider fitting a S3 oil pump?
My MK2 had a old 3.4 engine used for racing in it , one of the reasons I changed it was low oil pressure , from cold it was fine a good 40lb , but on a long run when hot at traffic lights or the like it dropped down to just below 20lb , engine ran fine , but as the rear main oil seal had a bad leak and slight timing chain rattle I decided it was time to do something about it , over the past winter
I found a 3.4 XJ6 engine 1975 , had a full rebuild , new pistons and so on , oil pressure is 50lb cold , and 50lb very hot !!
With engine off - 50psi!
The relief valve would open at 50. If the pump delivers lots and lots of oil flow and the bearings are all tight why not? As long as oil splashes around the cams at hot idle nothing should be wrong, but if… oh my.
Assuming the gauge works and the bearings are still cooled by the oil.
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The relief valve should not open as low as 50 psi, David…
And if you read 50 psi with the engine standing it’s all wrong - oil pressure should drop to zero as soon as the engine stops…
‘The higher the better’ is not rue for oil pressure, it’s not ‘good’ - which is why the relief valve was put in. In cold weather the oil pressure would be very high, and one would expect to read relief set pressure. When the valve ‘opens’ oil is diverted back to the sump - and contributes nothing to bearing lubrication…
50 psi was a guess. It’s not wrong to open at 50, though (shimming or changing the springs can of course be tried if it opened way too early).
50 at standstill were ironical.
If it were at 50 all day thinner oil for a normally driven car is what I’d do. But it doesn’t matter, 50 is great and Eric has the same values as everyone else: fine.
Hi Eric,
I had a Series 2/3 relief system apart recently and found that it’s designed to be fully open in the low 20’s of PSI. This is only apparent on the gauge when the oil viscosity low enough to allow the bypass to flow freely, as it has it’s own restriction, even when the bypass piston fully depressed, and pump (engine) speed is low enough not to overload the bypass capacity. Higher pressure readings are due to the fact that the bypass path with the valve fully open is sufficiently restrictive for either the more viscous (cold) oil, or the excess oil flow at higher revs, or the two combined, are able maintain system pressure above the nominal pressure setting of the valve. At these times the bypass is fully open.
Cheers,
Simon
So if I reached peak pressure at 1500 rpm and there would be no rise, not even at 5000 rpm, my gauge would be broken? Or where would the oil go? The pump pumps faster, not much more different at high rpm?
Oil can only go through the bypass and clearances in the engine. Are you observing no rise after 1500 or this this a hypothetical? If the latter, I would expect to see a rise with RPM, but less so with lower viscosity oil (temperature, grade).
Both, but I always assumed that it meant my bypass opened at ~45-50 psi and dumped all the remaining oil.
The 1500 is hypothetical, 1000 hot are probably sufficient. Back when the bypass stuck open behavior did change to 20 on the highway of course. Viscosity helps of course, but if the bypass were completely open the rise should be linear at some point.
Where does the oil go?
Did you mean the bypass started to open at 20? That would explain it. It’s a crude device, isn’t it…
