14 replies to this topic

[M111OWW]

Hi,

My oil pressure is hunting when the engine is hot and idling.
The pressure it's self is very good, engine is tip top and I use millers ctv 20/50..

When cold the idle pressure is 75psi, cold engine and oil so quite normal.

Cruising at anything over 2K rpm it's 75psi even after a good work out!!

Hot pressure at idle has always hunted between 50 and 60 psi in a very rhythmic fashion.. although blipping the throttle settles it down for a few seconds the hunting quickly returns...

Any suggestions??

[minidaves]

50psi oil pressure i would not worry, i assume it hunts on tickover.

[Spider]

What sort of Oil Pressure Relief Valve are you running?

[M111OWW]

Using the standard bullet type, tried the ball bearing type with correct spring and it stuck open while sat in traffic when the engine was new, the pressure just disappeared!!
Cost me a set of mains n big end bearings!!

Changed back to the bullet and it's been fine except for it hunting when hot..

[Rorf]

75 psi at idle when cold is far too high - what oil pump are you using. 50 psi when hot and running above 2000 rpm for a average performance engine is OK. High oil pressures actually contribute to more friction and bearing wash out and therefor loss in HP.

 

A ball bearing pressure release getting stuck in the open position then definitely some horrible bits and pieces being sucked up by the oil pump

[nicklouse]

having a pulsing oil pressure at tick over is not uncommon and I read a topic about what causes it somewhere but I forget where and the outcome.

 

which, to me, means that I felt is was of little importance.

[Dusky]

75 psi at idle when cold is far too high -

Factory manual states 70 psi as the pressure for the relief valve to be open. 75 psi isn't anything abnormal imho. All engines I had had that as cold oil pressure.

[Spider]

What type of Main Bearings does it have?

Grooved and plain or both grooved?

 

Dusty, the factory manuals I have state 60 PSI, not that I'd be too worried by 70.
 

[M111OWW]

They are grooved and plain ACL +10"...
Oil pump is from Swiftune, got it in the kit of parts with the SW5 vernier and tappets....
Engine is in rude health and pulls from 1500rpm (SPI John cooper Si)....

[Magneto]

Mine did that for a while, I was a quart low on oil......topping it up seems to have cured it. Never saw it do that before or since.......

[carbon]

 

75 psi at idle when cold is far too high -

Factory manual states 70 psi as the pressure for the relief valve to be open. 75 psi isn't anything abnormal imho. All engines I had had that as cold oil pressure.

 

Agree with Dusky, would go further to say that you should see 75 psi at idle with a cold engine (1275) using good quality 20W50 oil.

 

Mine starts off indicating about 90psi from a cold morning start, will drop to about 85 psi after a mile or so. Takes 5+ miles before getting down to 60psi. Might drop below 50psi when properly hot.

 

If cold idle right after start is over 100psi that's too high. I did get to about 125psi when I was running ball bearing instead of standard thimble relief valve - moments later the oil gallery plug at flywheel end popped. Engine out job to replace, and I have never used ball bearing since.

[Cooperman]

It all sounds fine to me.

[Spider]

They are grooved and plain ACL +10"...
Oil pump is from Swiftune, got it in the kit of parts with the SW5 vernier and tappets....
Engine is in rude health and pulls from 1500rpm (SPI John cooper Si)....

 

Apart from running a High Volume Oil Pump, all else sounds fine then.

 

I would suggest swapping back to a stock pump though, as the High Volume one only serves to heat the Oil.

[Midas Mk1]

Swiftune standard and competition one should be fine, it's not a 'turbo' pump. Just fitted the swiftly CNC'd one to mine and gives nice pressure.

[tiger99]

I have a suggestion as to the cause of the problem, which I suspect is harmless but slightly wasteful of energy. The oil pump flow rate is too high for now (it may be useful later when the bearings are a bit worn) so the pressure relief valve is intermittently opening. But the flow rate is too high to the upper parts of the engine too, so oil is pumped up to the valvegear quicker than it drains down. (As it builds up in the head, the downflow will increase until equilibrium is reached.) So too much oil high up, some churned up and going all over the place in the gearbox, leaving the pump momentarily sucking a mixture of air and oil, which removes less oil from the sump, so the level rises again until the pump is sucking only oil, pressure rises again....

 

Make sense? Or just a wild guess?

 

But I do think that your bearings are getting plenty of oil.

 

There is too much hype surrounding fancy mechanical oil pumps. They are rarely needed except on turbo engines, and if not needed, just wast energy which appears as extra heat in the oil, where you don't want it. The ultimate is a controlled electric pump which will run as fast or slow as needed to maintain the pressure at its design value without the pressure relief valve opening. It would need an interlock to shut the engine down if it failed, but it could remove the biggest single cause of bearing wear by pressurising the bearings before engine start, and maintaining turbocharger (if any) oil pressure till it stops spinning.