Gregory Bender

Oil pressure

Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models


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Here is some information I wrote in reply to a question asked by Mike Jones on the Yahoo! MGconvert news group. Mike was inquiring about the normality of his observed oil pressure after adding an oil pressure gauge.

I've have done considerable fiddling with oil pressure gauges and modifying different components to get the best oil pressure possible. Here is what I've learned with my machines (2000 Quota 1100 and 1971 Ambassador).

  1. Keep in mind that the original oil pressure switch is set to ground out at 3.6 PSI.
  2. Oil pressure with cold oil is MUCH different than oil pressure hot oil, especially at low RPMs. It takes me about 10 miles to get the oil hot (and that is living in Florida or Arizona). With cold oil, I can easily pump 70 - 75 PSI revving the engine a bit. With hot oil, the pressure will max out no higher than my pressure relief valve setting, and can get 10 - 15 PSI lower than that setting when running in hot weather. A couple examples:
    1. Cool morning out here in the desert, temps between 50° - 60° F, riding at speed I'll easily obtain near max pressure (down 2 - 4 PSI from where I've set on my oil pressure relief valve). Same day, at idle, I might get 7 - 12 PSI, depending on how hard I've been pushing the engine.
    2. Hot day in Phoenix, temps between 110° - 115° F, riding at speed I might be down 10 - 15 PSI from my max pressure (again, from where I've set the pressure on my relief valve). Do a lot of stop and go traffic, I might get down 20 - 25 PSI. Same day, at idle, I might get 4 PSI.
  3. Oil pressure is bled off at many locations. Some it is bled off as designed, other pressure it bled off with component wear. Without going into a lot of detail, let me focus on a few locations that are critical:
    1. Oil pan gasket - There are numerous passageways that transmit pressured oil to the front and rear mains, etc. This gasket must be in perfect condition, especially around where these passageways meet. I used to reuse this gasket if it looked okay. I don't any longer because a slight imperfection can lead to a loss of oil pressure.
    2. Oil filter - If it is loose, it will certainly bleed off massive pressure. But, I had a pan on which the oil filter surface was not milled, only a rough casting that looked pretty smooth. I never thought about it until I was chasing lost pressure and found leaks there. I had the surface milled by my local machinist and that took care of that problem.
    3. Oil pressure relief valve - This is a huge culprit. I built a tester specifically so that I could determine the pressure it was set to bleed off at. I've dealt with probably half a dozen of these relief valves so far and many?/most?/all? have massive wear and don't seal well when fully closed. Invariably I find myself using valve grinding compound to make a good seal. Then, I adjust the pressure at which it blows using my tester and my air compressor. While the manuals provide a value of about 60 PSI as the blow-off point (or even a range of 45 - 60 on early models), I use 65 PSI (or a bit higher) on my personal machines. I've not experienced any difficulty with blown seals using these higher values. I know a racer who always set his blow-off point a lot higher without any blown seals running really high revs.

    There are many other locations where oil pressure can bleed off, but most of them require significant engine disassembly to examine.

  4. Gauge accuracy is always a concern, especially at the extremes of a given gauge, as my understanding is that gauges are most inaccurate at either end of their extremes. This tends not to bother us at the high extreme because we aren't bothered if we see 55 PSI vs 60 PSI. But, at the low end, we get really concerned when we see 2 PSI (below the factory limit of 3.6 PSI) and maybe the real value is 4 PSI (just above the factory limit of 3.6 PSI). I've not conducted gauge accuracy tests, so I really can't say how accurate gauges are across brands or within lots from the same manufacturer, etc. I can say that the inexpensive Sun Pro gauge I used once was just as accurate (from observed readings) as my better quality Murphy Switchgage. I fit the Murphy Switchgage because of it's the ability to illuminate my dash indicator light at a given pressure. My personal gut feeling is that most gauges sold today are pretty accurate.
  5. Pressure and Flow - both are necessary but one does not tell you about another. Yes, we quickly infer that we have sufficient flow because our pressure is high. But, we are very quick to believe that we have insufficient flow when our pressure drops. That is not necessarily the case at all. I've never even tried to figure out what kind of flow is achieved by any pump. It would be fun to have a flow meter hooked up, though :> When the engine is at idle, it is working a lot less hard and it needs less flow and less pressure. Raise the RPMs and the engine needs more...and the oil pump supplies more. So, don't get worried about seeing wild swings in pressure depending on revs.

    I (more than I should) watch my pressure gauge at idle waiting on a light to change green, but I largely forget it when I am in motion. As long as the pressure increases with revs and doesn't completely plummet at idle, I shouldn't give it a second thought.

In sum, I think what you are seeing is relatively normal, though I'm a bit concerned with only 2 PSI with hot oil at idle - and your weather is still pretty cool, yes? The next time you change oil, I would check the things I mentioned above (especially the oil pressure relief valve). Other than that, ride the thing. There is a reason almost all vehicles do not come equipped with gauges - people freak out when they see a gauge do something and don't understand what is happening. I'm not saying that gauges can't be very useful tools, just that they can easily lead people to take actions that are not warranted.