Hello chaps. I am seeking insights into the high oil consumption of my A10.
She has been using about a pint per 200 miles for the last 1000 miles. Full engine rebuild including rebore to +80 about 1500 miles ago. At the same time the head was done up, with new guides where necessary and valves. After a few hundred miles of topping up the oil and occasional white smoke from the exhausts (more from RHC), I stripped off the top end again, but found nothing at all amiss, except witness of a possible oil path from pushrod tunnel to cylinder on the head gasket. I have checked compression which is good at around 130 psi. (8:1 nominal pistons), and she starts and runs well, pulling like the proverbial train, despite the oil burning. Plugs are black, but Boyer ignition must help here, with the much higher available HT voltage. No significant oil leaks anywhere by the way.
Now, I have an oil filter fitted in the toolbox, the 2CV type. Oil is decent quality 20W50. I suspect that the filter has a considerable bearing on the oil usage, but I cannot understand why. My suspicion is that the engine is running with excess oil in the cases, i.e. wet sumping. Suspicion is that crankcase oil level is high, so excessive oil on the bores inevitably getting past rings into cylinders. No oil from breather though. Return to tank looks reasonably strong but perhaps not as much included air as I would expect. Focussing in on the oil system, I first checked the oil pressure feed. Maximum cold pressure about 60 psi (SRM regulator valve, with two washers removed, else about 75 psi), with typical hot running pressure 40 psi, which I consider pretty good. Then move gauge to return line, monitoring at rear engine connection. Cold pressure around 20 psi, falling to just off the zero stop, perhaps 2-4 psi after just a few miles. With the filter temporarily replaced with a plain ¼ BSP male-male adaptor, working pressure fell by perhaps 1 psi, just perceptible on the 100 psi guage. But smoke in the exhaust on static revving was definitely less, and oil return to tank appears slightly greater.
Last evening took her on a 50 mile run with filter out of line. No sign of the occasional white smoke. Plugs looking cleaner, and oil level not dropped (but may be too soon to tell). So it looks like a partial result. But why should a pound or two of back pressure make such a difference? A gear type pump is considered to be positive displacement, more or less constant flow at a given speed, so that pressure is a function of flow. I guess I will have to check the pump again, but I did have it apart at the rebuild and it seemed to be in fine fettle then.
Problem is that I really want to have a filter in-line. Perhaps I will try a Vokes WDB40 filter I have on the shelf (picked up a new element for it at Netley Marsh last week for £2). Placed above the gearbox with much shorter total length of line hopefully this will cope better with any small back pressure. No idea of how efficient a filter this small element is compared with the paper canister one, but better than none).
Sorry this is so long, but it really is headache for me. Any suggestions and insights very welcome please. Any other measurements to take? There are other case reported anecdotally where filter in return line has caused problems, including on this forum, but no real explanation as to why, as far as I am aware. On the other hand there are many machines with filters fitted who are getting the benefits of cleaner oil, without apparent hassle.
Please HELP.
Mike