Hi,

Last Sunday I took the car out for a long hard drive, it was a really hot day, about 30 deg. I noticed as I was nearing the end of my run that the oil pressure had dropped significantly, it normal sits abot 30psi idle and 70psi when revving, it was down to about 30psi when revving and 5 - 10 psi idle. I immediatly suspected it must have developed a leak, I looked around the engine and there were a few drips coming from the around the presure sensor, I assumed this to be the problem.
Last night I started the car again and the oil pressure was back to normal and there were no leaks, as it warmed up, drips appeared from the sensor and the pressure dropped a little. I am now thinking this is all temperature related, as the oil heats up, it thins and I am loosing pressure because of that, the thinner oil is then causing a small leak, but maybee this isn't such a problem (I have never had to top up the oil), unfortunatly I don't have a oil temp gauge, I will be looking to fit one over winter but can anyone confirm similar behaviour and is it still safe to drive with the decreased oil pressure ?? FYI the engine is a ZX9r.

Thanks,

John

"low' oil pressure at idle is common in bike engines - usually not a problem.

The ZX9r oil pressure spec is 17 to 26 psi at 4000rpm with oil temperature at 90 deg C.

The first thing to sort out is the leak, so a drop of thread sealer should do the trick.

My V6 has the same trend regarding oil pressure and I also put it down to oil temps. The fact that my car is a middy means that there is less air getting into the engine bay. I found that a slightly different oil grade improved things a little, changing from 5W30 to Halfords finest 5W50 Trackday oil. Oil pressure at tickover when hot is better but I think an oil cooler would improve things a little more - so this might be the answer in your case.

Most oil pumps are gear pumps, which work better with thicker fluids. As the oil warms, it thins, so the pump is less efficient, so the flow rate goes down (you get more internal leakage around the gears with thin oil).
Less flow will result in less pressure which is what you see.
I don't think the small leak you're seeing is the cause of the pressure drop but should still be fixed.

[Edited on 16/7/10 by richmars]

Sound normal and within the range sugested above. Might be worth trying a thicker oil. Do Kwak' list one for higher ambient temp? (ie. Yamaha list 5'c and higher 20/40 and 15'c and lower 10/30)

My R1 runs 35-85psi when cold but idle presure drops to 6psi when hot and also lower at revs. Again that matches the Yamaha figures.

adrian

Oil pressure dosn't really matter much oil temperature and oil flow rate do.

Only reason oil pressure is monitored is to watch for a change in oil pressure under a given set of conditions.

Oils that are too thick when cold are the real killer of engines. To improve the oil pressure when hot check the viscosity range of the oil you are using against the manufacturers specs, then look for a suitable oil that has wider viscosity range.

An even better cure is a oil cooler ---- but you should also fit an oil thermostat.