Series 3 Oil pressure light comes on when engine hot.

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hfletcher

hfletcher

Member
Posts
19
Location
West Berkshire, UK
Hello all, happy new year! I hope you all had good festivities.
Back in 2019, I made the mistake of connecting the oil pressure switch up to the light on the dash board, and it was on solid all the time. I've since fixed the issue by doing a complete oil change and filter replacement, now the engine pressure light is out most of the time, and I am also using the grade 20w50, as specified in the Haynes workshop manual for the vehicle.

However, of course it is not so easy. After all, why would it be?

The light is now off for most of the time, but after a while of driving, when the engine gets hot and the revs drop back down, the light comes back on. If driving hot for a very long time, the light starts to come on even when driving and revs are high.

Have done some searching and testing to find out what could be causing this:

  1. Wrong oil grade?
    20w50 should be fine, it is specified in the manual
  2. Blown gasket or worn piston rings?
    Removed oil filler cap to see if any smoke came from anywhere when running. No smoke seen.
  3. Possible dodgy switch being affected at high temperatures?
    it is britpart after all.
If anyone has any other suggestions or things I could try to see what the problem is, that would be helpful.

1973 Series 3 SWB with a Series 2a engine.

thanks
 
Two possibilities.

Dodgy electrics. Sender/ gauge /wiring.
Or
Actual low pressure.

If changing the filter fixed it the last time, then either the filter was hugely blocked. Or the oil pump wasn't very good. Or both.

Hot oil is thinner, and thinner is harder to pump.

Can you get a mechanical oil pressure gauge and fit it?
 
My light started coming on at the beginning of a weeks shift so I ordered a new pump before actually getting to look at the reason.

Worked out that the bulb holder was shorting out. A quick widdle and all was well. If you need a pump, give me a shout. :oops:

It’s no good sat in my shed :(
 
Wildefalcon said:
Two possibilities.

Dodgy electrics. Sender/ gauge /wiring.
Or
Actual low pressure.

If changing the filter fixed it the last time, then either the filter was hugely blocked. Or the oil pump wasn't very good. Or both.

Hot oil is thinner, and thinner is harder to pump.

Can you get a mechanical oil pressure gauge and fit it?
Click to expand...

I have put one of these on order now, and will test it out tomorrow.
 
Bobsticle said:
My light started coming on at the beginning of a weeks shift so I ordered a new pump before actually getting to look at the reason.

Worked out that the bulb holder was shorting out. A quick widdle and all was well. If you need a pump, give me a shout. :oops:

It’s no good sat in my shed :(
Click to expand...

I have ordered a mechanical gauge to measure the oil pressure. I will let you know if the pressure drops, because that would indicate a dodgy pump rather than electrical fault.
 
Worn big end bearings ,crank bearings ,rocker shaft and failed head gasket.
Does it use much oil?
Any sign of oil emulsified in the coolant?
 
4 psi or 7 psi switch? It does sound like low oil pressure but if its only low at idle and hot and goes out immediately you rev that's ok for an old worn engine. You could change the pump but if you have the sump off change the big end and main bearing shells in situe.
 
Make sure you have the right amount of oil in the engine. Old dipsticks can have problems with the " bell stop" slipping up the stick, result not enough oil in motor and the light coming on.
 
Wildefalcon is right, it is either genuinely low oil pressure or faulty electrics, most likely a duff oil pressure switch. You need to check the oil pressure with a gauge to rule one or the other out. When the engine is hot, as others have said, the oil gets thin and the oil pressure drops, this is especially noticeable at idle when you will see the oil light flicker or come on. Cheapest answer is to try a new oil switch, if that doesn't cure it do a pressure test. If you do have low oil pressure, it is most likely to be caused by worn crank shells; not very likely to be the oil pump, sorry guys who thought it might be but much more likely to be the shells.
 
If you actually want to "know" what is going on then get yourself an oil pressure gauge.

Could be a multitude of issues but without being able to actually see what the real time pressure is you could well be chasing your tail and changing senders, chasing wires and lamps for ages or you could be destroying your engine by running it with terminally low oil pressure.

I wrote a piece in my blog about my oil pressure gauge, volt meter and boost gauge fitting on my 200Tdi.

http://projectrusty1992.blogspot.com/2018/08/fitting-auxiliary-gauges.html

If you're lucky it's dodgy senders, if not then get ready for a rebuild of your engine.
 
Be aware that the oil pressure switch on the paper filter holder fitted to the Series Engines has a Whitworth thread. I seem to remember that it is 3/8" x 16TPI Whitworth but am not 100% certain.

A modern oil pressure gauge will likely not include a suitable sized fitting to connect it to the engine in place of the pressure switch.
 
Last edited:
Theses are cheap and do the job for testing
It could also be the oil pressure relief valve worn spring and plunger , that’s a sump off job but you also get to pull the pump out and check the gears
 

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