Low Oil pressure when hot

[Ra1994gh]

So after engine rebuild M20B20 from 87-89, new piston ring set, new bearing set for both rod and main bearings (brand Kolbenschmidt Standard sizes), the head was rebuilt at a professional shop and parts that needed changed was changed. Before starting the engine I primed the oil pump manually untill I flushed out the air. However after first startup everything good with 10w-40 oil and new Bosch oil filter. When the oil gets hot the oil pressure lamp will turn on at idle. When you rev it it turns off and pressure rises.
I used a manual gauge and I get lower than 0,2 bars at idle which makes sense that the sensor get triggered. And when I rev it not over 2500 rpm it will rise to around 1,5-2 bars max.
I changed the oil pump with an old pump I had on another working engine and the same with the oil pressure relief valve just to be sure, but I still have the same exact symptoms. I have inspected both pumps internally and both seems in good condition.
I am not sure what to do next now?
Change to 10w-60 oil and new oil filter would be my best guess?
I also got an advise that maybe there is not enough pressure if the two surfaces between the output oil pump and the bottom of the engine block and maybe I need to use some kind of sealant suitable?

[zaq123]

A lot of things can be an issue there. Starting with improper bearing selection (if gap wasn't verified) and ending with intermediate shaft bearing being shot in the block. OEM oil sensor is not a sensor, it's basically a switch that activates at 0.5bar. If the light stays longer than a second on the initial start up (after oil change etc), it basically tells one that the engine is done and needs to be rebuilt. Hot pressure idle on healthy engine should be around 1bar. If it's ever dips under 0.7-0.8 bar, I'd be starting to monitor it very very closely.

At 0.2bar - obviously a major issue that requires engine disassembly and close inspection.
Healthy M20 should be around 1 bar hot idle and around 3.5bar at 3500 rpm with that 40 oil.

M20 oil pump is pretty robust. Was the oil pressure relief valve put back during reassembly? when you were priming the engine and rotating it by hand at the same time, you should see the pressure gauge showing something as well as you should see steady stream from the rocker oiling bar holes (not pressurised spray but more of a light stream of oil).

Sorry for these bad news which have no easy solution. It sucks big time considering you just had it rebuilt.

[82eye]

m20 oil pump can be torn down and rebuilt. 20w50 is the oil recommended and will run a higher pressure than 10w40.
i never primed an m20 beyond turning it by hand a couple times before first start after a rebuild.

edit:

it won't help if the bearings are shot or out of spec. you can also shim the spring for higher pressure but it does nothing for idle pressure.

[Ra1994gh]

Thanks for quick reply.
To be honest I have not measured the crankshaft it self or the clearance in the rods as the old bearings looked ok and no major wear. So I bought completely standard bearings which I think would only be better for the engine. I mean normally low oil pressure could be due to wrong or cooked bearings so the clearance is so high that oil will not stay and lubricate but rather quickly escape and pump not being able to produce pressure which I figure could not be the case here?
The oil pressure relief valve was also changed with the one from my old engine just to be sure it was not that. But with no difference.
When priming the pump by hand/or drill the gauge were showing between 2-3 bars and I cannot remember how many rpm the drill was turning. I saw a steady stream of oil both down at the gear it self and there is also oil in the top of the head by only removing the oil cap.
What do you recommend to do next? Should I do some measuring or any other kind of test or just try with thicker oil 10w-60 and new oil filter? Or maybe delete the oil pressure relief valve and screw in a plug instead?

[Ra1994gh]

When I tore down both pumps they looked in good condition and everything looked very good actually but I changed it anyway just to rule it out.

[82eye]

20w50 is the standard oil for an m20.
you could tee in a proper gauge to see true pressure if you suspect the switch is failing. inspect and clean the oil pressure relief valve, they can get varnished and stuck.

edit: if the pressure remains low on the gauge you tee in with 20w50 and a good relief valve, then the engine has deeper issues. likely bearings as noted above.
i find it hard to believe a competent shop wouldn't check the bearings on a rebuild.

[TobyB]

I'd be looking for something major- a gross leak in the oil system.

.2 bar hot on an engine that isn't rattling like a box of rocks-
and that has such low pressure off- idle, too-
really seems like there's an unintended
oil vent somewhere.
Like when the front tube comes loose on an M10...

just a suspicion,
t
who lives in fear of not replacing the right Welch plug....

[Ra1994gh]

Thanks for the input. I will change the oil to 20w-50 I have just ordered that, and new oil filter and install the pressure gauge in the place of the oil pressure sensor/switch and tell you guys exactly what measures I get both cold and hot. The engine sounds surprisingly good and healthy so I also wonder myself!
The shop only refurbished and looked through the head and etc. The rest have been done by myself at our own little shop.

[digger]

how hot is the oil when it puts the light on at idle and what is the ambient air temp?
what is pressure immediately after starting at idle when the oil is cold after sitting overnight ?

As a data point i use 10W-40 atm, at 650 rpm my oil light flickers on if i accidentally lug the engine down to that speed when taking off from a stop when the oil is super hot in summer months and A/C on, its been that way all along but goes off at idle 900 rpm. At 2500 rpm it is more like 3b though and cold it still pegs at the bypass 75psi roughly

[zaq123]

For starters, there are 3 standard size bearings for crank mains and one standard size for con. rods. Then there are 2 more oversized ones for both mains and con.rods if the crank was machined smaller (which can be from the factory as still within Specs) . It's impossible to say what was used at the factory unless measured. The biggest clue could be a color dot (yellow, white or green paint) on the crank next to each journal but I'd still confirm as there are 3 sizes for each color. But again, that color mark will only limits the selection to 3 part numbers. Otherwise you might have to try 9 different part numbers/bearing sizes.

Again maybe not your issue but wrong bearing selection will affect oil pressure. Also 10w40 is completely ok oil for m20. 10w50 will not make huge difference. I use 10w40 (Driven HR5 oil) and have 1.4 bar 93C dle and 3.5-4 bar at 3000rpm with 93C oil temp. My M52 crank was marked yellow but I confirmed every journal with plastigauge because as I mentioned, there are 3 different sizes of yellow or white or green.

[zaq123]

Not the best picture but you get an idea. It's M52 crankshaft I have laying around. M20 marks can be more like brush strokes instead of dots.