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Low oil pressure on rebuilt engine?


mike91

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Hello,
Just had my engine rebuilt and started it for the first time yesterday, but slightly worrying is the fact that oil pressure is exceptionally low... around 15-20psi at tick over when hot. If the engine speed drops to 500rpm the warning light comes on, which is really REALLY not what I want on a new engine...
It has had a new uprated oil pump, uprated relief spring, new big ends and shells, polished crank... the only thing I can think is it's perhaps the oil cooler I have fitted? It's 18 row, so quite a big one it is fitted with a thermostat, but I would really appreciate your input?
Thanks, Mike.

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Hi Mike

What oil pressure do you get when revving the engine (both hot and cold)?  I would expect a freshly rebuilt 4-cylinder engine to show the following pressures
cold idle: 40-50 PSI
cold revving: 100 PSI
hot idle: 20-30 PSI
hot revving: 50-70 PSI

Unless the lines to the oil cooler are really soft and inflating like balloons, then the oil-cooler will not be the cause for your low pressure.
As Clive says, whip out the prv and clean it - shine a torch into its hole and look at the seating - is there any dirt/swarf in there?

Also, do you have an external oil feed fitted to the rockershaft?  These are a fashionable accessory that some people fit in the mistaken belief that they are somehow improving their engine but are usually not needed.  A lot of the external feeds available are unrestricted, which means that they pass far too much oil to the rocker shaft and rob the main oil gallery of oil pressure - this is especially noticeable at tick-over.

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Steve_Curton wrote:
And don't forget to check the sender unit or gauge are not faulty!


Good point.  A failing electrical sender will cause the oil pressure light to come on at higher pressures but, since Mike mentions both a pressure reading (presumably from a gauge) and the oil light flickering on tickover, I think we can rule this out. The likelyhood of both the gauge and pressure sender being faulty in the same way is pretty slim.

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bodders1 wrote:


Good point.  A failing electrical sender will cause the oil pressure light to come on at higher pressures but, since Mike mentions both a pressure reading (presumably from a gauge) and the oil light flickering on tickover, I think we can rule this out. The likelyhood of both the gauge and pressure sender being faulty in the same way is pretty slim.


Generally agreed, but that sod has a law unto himself :)

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Causes of low oil pressure:

- Thin oil.  Should be 20/50 although my very well used 2L gets ok oil pressure with 10/40 (smokes alot though)
- Oil pump internal tolerances excessive.  Not uncommon on new ones these days.  You describe yours as "uprated" - how so?
- Oil pressure relief valve incorrectly seated or spring incorrect.  You describe the spring as "uprated".  How so? - I've always found the standard offering entirely adequate - you don't want more than 80 psi, ever.
- Bearing clearances excessive.  -010 shells on -020 regrind?  Plastigage?
- I suppose worn cam bores would lower oil pressure too, though not all that much maybe.

Would be helpful to know what pressures you get at 2000 rpm cold and then again at 2000 rpm hot (and I mean really hot, minimum 20 open road miles)

Nick

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Hello,
Thank you so much for your responses!
Okay, where to begin...
I am running Castrol 20w/50 oil.
I think the "uprated" on the oil pump refers to it being the later type, with angled oil pickup?
As for the pressure relief spring, when I bought the oil cooler, I was recommended to buy this"uprated" spring, to stop pressure drop over the oil cooler!?
The gauge and light are both new, electric senders so I would hope they would be accurate still...
I did consider the fact that maybe the engineering company ground my crank too far for the standard size shells that they gave me, but they are quite a reputable company so I would hope not!

Oil pressure is as follows-
Cold idle - 30-40psi
Cold rev - 50-55psi
Hot idle - 15-20psi
Hot rev - 30-40psi

I will check the prv is properly seated today, I think I kept my standard spring from pre-rebuild too, so I might put that in and see how it effects. I hope it is something that simple, oil pump or wrong crank bearings feels like taking 10 steps back on my project!  :-/

Cheers, Mike

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438 wrote:
...I did consider the fact that maybe the engineering company ground my crank too far for the standard size shells that they gave me, but they are quite a reputable company so I would hope not!



??) ... did you really mean standard size shells??

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438 wrote:

As for the pressure relief spring, when I bought the oil cooler, I was recommended to buy this"uprated" spring, to stop pressure drop over the oil cooler!?


Oh dear.  Another snake-oil salesman.  
Unless the oil cooler lines are inflating like balloons, or leaking oil from the joints, it will have no effect on oil pressure.
Oil pressure is dictated by the force which the oil pump can squeeze oil into the oil gallery compared to the rate which oil leaks out again via the various bearings.
A worn or scored oil pump, or one where the rotor clearance is larger than tolerance, will leak oil internally and, therefore, not pump with sufficient force.  Worn or inaccurately ground main or big-end bearings will allow too much oil to flow out, and cause a low oil pressure.

438 wrote:
Oil pressure is as follows-
Cold idle - 30-40psi
Cold rev - 50-55psi
Hot idle - 15-20psi
Hot rev - 30-40psi


This is low right across the board (see my expected pressures earlier in this thread).  It might be the prv, e.g. something preventing it from closing, but I suspect its more likely to be a bad new oil pump, or inaccuracy in the crank grind.

If you still have the old oil pump, you might try swapping it back (I believe you can do this by dropping the sump without tearing the engine apart - although I have never tried) to rule out the new pump as the cause.    If this shows the pump is not the cause of the problem, then I'm afraid it's time to take the crank out and measure the tolerances of the journals with a micrometer, then take it up withe the company that did your grind.

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Yes standard sized shells! I was told the crank was only polished, not re-ground... the same company who did it, also provided all the bearings, so they should be right.
I am so worried now though :(

Didn't get a chance to check the spring seating today, so will have to do it some point this weekend.
My old oil pump was equally knackered, I got near to no oil pressure before rebuild... can the engine stay in, if worst comes to worst, and the crank has to come back out?

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438 wrote:

Oil pressure is as follows-
Cold idle - 30-40psi
Cold rev - 50-55psi
Hot idle - 15-20psi
Hot rev - 30-40psi


Your pressures don't seem much different to my 13/60's pressures:
Cold idle = 60psi
Cold rev = 60psi (I guess this is where the PRV opens)
Hot idle = 15 - 20psi
Hot rev = 40psi

The hot figures seem low. They've been checked with two gauges so I accept them as accurate. 20/70 oil made a difference but seemed to sap power. The only reason I can think of for the low pressures is that when I pulled the old camshaft out the rear journal was scored, so I just fitted another camshaft. The rear cam gallery recess may have been scored too.

Stil, it's held together for over ten years and 60,000 miles, so high pressure ain't everything.

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