I’ve mentioned on here recently about the oil pressure issues we thought we might have been having, and had been optimistic that it was behind us following the mid-season rebuild of the engine.  However, that was not the case…

After refitting the engine, and having it running late last week, we found that the reported oil pressure (confirmed with a mechanical gauge to ensure the electronic version didn’t have a problem) was lower than usual at idle, and massively high when revving a bit. When I say massively high, I mean about 150psi – almost twice what we want!

So over the weekend, the engine was removed from the car again, and partially stripped to see what we’d done wrong. And we found nothing. The bearings were nice, with good clearance, and all in the right way around. All the oilways were clear, and the head gasket was on the correct way – all very basic things, but when you’re struggling you check everything.  The oil pump was taken off and checked, with nothing apparently wrong. Whilst we were there, we thought we’d make a quick drawing off the pressure relief valve system (as it is different from the TOM’S pumps we have in stock), and that is when we found the problem.

Have a look at the attached picture (drawing) – it shows a plunger, energised by a spring (with an adjustable platform to control the relief pressure) acting on a small steel insert. The small insert (as the pump casting is aluminium, and hence not tough enough to withstand the plunger acting on it without wearing quickly) had come adrift, and possibly on the skew.

This would allow oil to bypass the valve, causing lower than expected pressure at idle. And because the insert had moved, it effectively added preload to the spring, keeping the valve closed at much higher pressures than it should, and therefore giving excessive pressure readings (as the bypass flow would be irrelevant at those oil flow rates).  Now the insert is back in place, we hope that the valve will return to normal operation, and give the correct oil pressure behaviour.

Plus, we’ve backed off the valve a bit anyway to reduce maximum pressure, reducing the work done by the pump, and hence making it consume less engine power – which means more to the wheels. Only probably a few tenths of a horsepower, but every little helps!

The plan is to refit the engine tonight (Tuesday) and get it running – if it’s all okay, then we’re nearly set for Mallory; if not, then we have a few more long nights ahead of us. Great!