Low oil pressure - on the gauge

[KKson]

Right, I've been concerned for a bit of time re the engine oil pressure so had a fiddle today. I had a brand new oil pressure transmitter from David Gerald so hooked it up to the air compressor and temporarily wired it into the dashboard pressure gauge. Pumped it up to 50psi, confirmed with a local pressure gauge, and dash gauge read 20psi. Tried it at several different pressures and dash gauge consistently read less than half the true reading. I'm therefore assuming the gauge needs re-calibrating. Any recommendations as to who does this sort of thing? Thanks.

[adam quantrill]

Is it a vdo gauge?

[KKson]

I think so. Looks like this:

[Wedg1e]

VDO gauges need VDO senders. If it is a VDO sender then something's not quite right.

[KKson]

David Gerald states that it is a VDO compatible unit so that's why I'm questioning the gauge.It's also reading low with currently installed sender, but has been ok previously?

[adam quantrill]

Yes you definitely need the right sender, DG usually know their stuff but there's always a chance that the wrong spec has been sent or put into the sender.

So - is it now worse than the original sender? Can you still test the old one?

Even if you have VDO the maximum pressure of the sender must be matched to the gauge - this might be your problem. Common maximum pressures are: 30, 80, 100, 150psi.

I would guess we need 80psi ones with a resistance range of 10-180 ohms.

You can now get them on the flea for about £12, except they might be 2-terminal ones in which case ground one terminal onto the engine block.

[adam quantrill]

This thread makes an interesting read.

Oil+pressure+question...

And recommends the 80 psi ebay sender.

[Henry Harris]

In my experience, my engine is a 450, the original oil pressure gauge showed 20 - 25 psi when warm at 2,500 - 3,000 rpm. There is the old saying that as long as the oil pressure light is out, then that is sufficient. However, like you I have been curious. So at JE Engineering, they checked out the oil pressure with a line gauge and it was about 30 psi. Since the engine rebuild I have had an line gauge fitted. Now oil pressure is 70 - 75 psi at normal running speeds, dropping to 50 - 60 psi at idle.

Hope this helps.

[Henry Harris]

In my experience, my engine is a 450, the original oil pressure gauge showed 20 - 25 psi when warm at 2,500 - 3,000 rpm. There is the old saying that as long as the oil pressure light is out, then that is sufficient. However, like you I have been curious. So at JE Engineering, they checked out the oil pressure with a line gauge and it was about 30 psi. Since the engine rebuild I have had an line gauge fitted. Now oil pressure is 70 - 75 psi at normal running speeds, dropping to 50 - 60 psi at idle.

Hope this helps.

[mrzigazaga]

There is a saying.."Low oil pressure is better than no oil pressure"..I had a similar problem when trying to match up temperature sender on Delilah..My temp gauge used to say 280 degrees F which used to scare the pants off me but after loads of testing with laser pointer and probe thermometers it was actually reading normal..It was the resistance in the new sender...More testing needed mate i thinks...Ziga

[adam quantrill]

Henry Harris said:

Now oil pressure is 70 - 75 psi at normal running speeds, dropping to 50 - 60 psi at idle.

Hi Henry, that's a bit on the high side - you might consider a lower rate spring in the relief valve. High oil pressure wears the cam/dizzy drive gears, so you might notice it needing the timing advancing maybe a little more frequently than usual on a service. At least when it fails, it fails safe because the dizzy stops rotating which cuts the ignition (all unless you have a trigger wheel...)

[Number 7]

adam quantrill said:

Henry Harris said:

Now oil pressure is 70 - 75 psi at normal running speeds, dropping to 50 - 60 psi at idle.

Hi Henry, that's a bit on the high side - you might consider a lower rate spring in the relief valve. High oil pressure wears the cam/dizzy drive gears, so you might notice it needing the timing advancing maybe a little more frequently than usual on a service. At least when it fails, it fails safe because the dizzy stops rotating which cuts the ignition (all unless you have a trigger wheel...)

Unless it's a Serp engine. Des Hamill's book suggests 40 psi at 5k rpm as a good pressure, but with an absolute maximum of 60 psi for the pre-serp engine, beyond which, as you say, fast drive gear wear takes places.

[KKson]

adam quantrill said:

So - is it now worse than the original sender? Can you still test the old one?

Okay I've pulled the old sender unit from the car and tested that also. No matter how hard I pumped it up it read zero on the dash gauge? It was tricky holding the hoses, foot pump etc in place so I guess that it's the built in delay that stopped me getting a pretty instant reading. It typically takes 20 to 30 seconds to come to life once the engine is started, even though the red oil light goes off instantly.

I did take the sender unit off and connected an open plastic hose to the outlet and started the engine up and thick gooey oil did pump rapidly up the pipe and into an old plastic jug. Certainly there was definitely plenty of flow and pressure.

I then tried putting both the old and new sender units back in and running the engine up and both now read around 25psi at 1500rpm and no matter how hard I rev it the gauge does not increase one psi in pressure.

I've order a proper oil test pressure kit with plumbed in gauge so before I mess about any more I'll plumb the gauge in to see exactly what the real engine pressure is and if it does indeed vary with engine revs, which it certainly did do until a few weeks ago.

How many days to the BBWF?

[mrzigazaga]

KKson said:

then tried putting both the old and new sender units back in and running the engine up and both now read around 25psi at 1500rpm and no matter how hard I rev it the gauge does not increase one psi in pressure.

Mine reads that unless I'm going 60-70mph then it goes to what looks like 45-50psi??

[adam quantrill]

We as you have previously calibrated the new sender+gauge combination as reading low, it sounds like your actual oil pressure is just fine.

[KKson]

adam quantrill said:

We as you have previously calibrated the new sender+gauge combination as reading low, it sounds like your actual oil pressure is just fine.

Fingers crossed. It has been fine so I'm assuming it a gremlin, and the red warning light goes out instantly and never flickers ever at low revs with the cooling fan running. Is there a voltage stabiliser on the gauges that could affect it? I suppose the dreaded "poor earth" fault would also impact. The fuel gauge does have a mind of it's own on occasions so wondering if it is electrical and linked.

[ElvisWedgely]

I wouldn't worry about it too much. Now you know it reads half the true reading, just multiply the reading by two. If the red light goes off and stays off at idle, and the engine is not tappety or noisy at idle, and blue smoke isn't pouring out the back, then all is good. Also, with oil pressure, it's not just how high it reads that matters, it's how quickly it goes up when you rev the engine.

Tony. TCB.

[Wedg1e]

KKson said:

Is there a voltage stabiliser on the gauges that could affect it?

No... that would add weight

[adam quantrill]

... also try running the cockpit fan and/or indicators. On mine the temp gauge wibbles up and down maybe 10 degrees. Probably a small effect for the oil too (although that changes on its own so harder to tell). Could be caused in part by the dodgy earthing for the instruments.

[jimburr]

Wedge electrics are great, put the lights on mine all the gauges go up just a tiny bit, so fisrt time was :- oil pressure ok, petrol oh that's better, temp oh but fan just kicked in as normal. Not worried to much. But do have plans to fit an old school oil pressure gauge under the bonnet just to confirm what's going on.