Gradually falling oil pressure

[JOHN COOKSON]

From the "Long Shot" Department:
You have a mechanical fuel pump. When the diaphragms in these pumps begin to fail, they leak fuel (by design) into the engine oil. If this were happening, the presence of the gasoline would surely kill the viscosity of the oil. The longer the engine runs under these conditions, the worse the effect.
Two quick checks:

1. Sniff the oil dipstick for the presence of gasoline.
2. Check the oil level for anything suspicious. In severe cases, the level might actually rise with the addition of the fuel.

Other, wiser members will amplify or correct as needed, please.
Bill (Admin and Forum Executive Vice-President in charge of long shots)

I'll see your bet and raise.
Using heaver weight oil might give a satisfactory gauge reading while the root problem remains.
I would be more in favor of trying to find the root problem and then use heavier oil as a last resort. There have been several suggestions made. What has been the result after trying them? Don't remember seeing any result reported (I didn't reread the thread)
My A4 was placed in service in 1980. Oil pressure is still perfect using straight 30 weight oil.............


TRUE GRIT

[Administrator]

I think you may have confused my comments with someone else's.

I made no mention of using a heavier weight oil. I was simply suggesting a possible root cause (leaking diaphragm) which could become more suspect or discounted altogether by means of a couple of 15 second tests.

The degradation over time was, I thought, an important observation, possibly explained by a significant degradation in viscosity, which might in turn be explained by the introduction of gasoline.

Bill

[JOHN COOKSON]

I was speaking in general that it would be better to identify the cause of the wobbly oil pressure. Quoting you didn't help make that point clear. Sorry 'bout the confusion.

TRUE GRIT

[clark800]

Yeah, I would like to figure out the root cause before resorting to multi-weight oil.

I smelled the dipstick and there was a non-oil odor, but I'm not sure if it was a gasoline odor, it smelled more like the odor the engine gives off when it's running, which my neighbor said was normal for new engines. He said it was probably the paint off-gassing due to the heat of running.

I have not noticed any increase in the oil level on the dipstick. Also, my oil pressure returns to high levels after each cold start and follows the same gradual fall each time. If gasoline was leaking into the oil, would the gasoline evaporate out over time or stay in the oil? If it stays in the oil and accumulates, then I can probably rule it out as the issue; if it evaporates out, then I would have to test immediately after running for a while.

I just ordered the Indigo Oil filter, which will allow me to eliminate all factors related to the oil pressure regulating valve from the experiment. I'll report the findings next week.

[clark800]

I installed the Indigo oil filter and a mechanical pressure gauge; it turned out that the (brand new) electrical gauge was off by as much as a factor of 2 from the mechanical gauge, but the oil pressure problem persisted.

The oil pressure was steady for a few minutes around 35PSI, but after running for an hour, the pressure reading on the mechanical gauge had dropped to 16PSI at 2000RPM, 14PSI at 1800RPM, 12PSI at 1600RPM, 10PSI at 1400RPM, and 5PSI at 1000RPM, with a water temperature of around 175F. All the readings on the electrical gauge were about half of these. Tightening the adjusting screw on the Indigo oil filter did not seem to have any effect on the oil pressure at the end of the test, though it did at the beginning. At the end, the pressure seemed to be holding steady at these levels.

It seems like it might be an issue that requires taking the engine apart to fix. I guess I will try the multi-weight oil now.

[JOHN COOKSON]

Have you tried he oil pressure adjusting dohicky seat? Post #12.

TRUE GRIT

[joe_db]

I installed the Indigo oil filter and a mechanical pressure gauge; it turned out that the (brand new) electrical gauge was off by as much as a factor of 2 from the mechanical gauge, but the oil pressure problem persisted.

The oil pressure was steady for a few minutes around 35PSI, but after running for an hour, the pressure reading on the mechanical gauge had dropped to 16PSI at 2000RPM, 14PSI at 1800RPM, 12PSI at 1600RPM, 10PSI at 1400RPM, and 5PSI at 1000RPM, with a water temperature of around 175F. All the readings on the electrical gauge were about half of these. Tightening the adjusting screw on the Indigo oil filter did not seem to have any effect on the oil pressure at the end of the test, though it did at the beginning. At the end, the pressure seemed to be holding steady at these levels.

It seems like it might be an issue that requires taking the engine apart to fix. I guess I will try the multi-weight oil now.

What did you do with the original oil pressure adjuster?
Also note a mismatched sender/gauge combo can have effects you described on the electrical gauge, i.e. a 60 PSI sender on a 120 PSI gauge or some other mismatch.
Is it possible the oil pump itself needs work? I think it can have an issue with excessive clearance that would drop the pressure as it warms up for sure.

[sastanley]

Have you tried he oil pressure adjusting dohicky seat? Post #12.

TRUE GRIT

John,
If he followed the Indigo installation instructions, there are supposed to be parts included in the filter kit to "bottom-out" that adjustment, so that you can use the more accessible adjustment assembly in the kit. I mounted mine on a bulkhead along with the filter housing.

[clark800]

Well, the Shell Rotella 15W40 oil did not help. After running for 40 minutes, the oil pressure was 14PSI at 2000RPM, 12PSI at 1800RPM, 11 PSI at 1600RPM, 10PSI at 1400RPM and 5 PSI at 1000RPM, according to the mechanical gauge (basically the same as before). Of course I only put in about 3 quarts because that's all I could pull out of the dipstick tube, but it seems unlikely that more oil changes will improve the issue.

I also noticed that a lot of smoke starts coming out of the oil fill cap as the engine warms up. The smoke tests positive for carbon monoxide. I'm not sure if this is related.

As a side note, the electrical oil pressure sender and gauge are both 0-80PSI and I'm pretty sure they are compatible; they are both Sierra brand. The electrical and mechanical oil pressure gauges match nearly exactly for the first 15-20 minutes, but seem to diverge as the temperature warms up.

Regarding the Indigo Oil filter kit, it comes with a plug to replace the stock oil pressure relief valve. I removed the stock spring and inserted the plug which consists of a sphere (like the new-style relief valves), a cylindrical spacer, and a bolt to tighten in place. I have it closed off very tightly with the bolt.

[sastanley]

clark800, Referring to the old regulator, did you get the OEM ball out that is also in the stock pressure regulator? That is the piece that the spring would be pushing on in a late model block. Just want to make sure you don't have two spheres in there!

[clark800]

I had the old-style cone-shaped regulator in there, but yes it came out.

[sastanley]

Nice. Keep that thing handy...I suspect they are not made any longer and you might be able to sell it to another forum member!! Or, maybe send it back to Moyer for reconditioning and re-sell to someone that needs it.

[clark800]

Update

I pulled the engine out and sent it back to the mechanic I bought it from. He didn't find anything obvious, but replaced the oil pump and a bunch of internal parts. He ran a two hour test without load and said it ran perfectly. I reinstalled it and after all that ended up with the exact same problem.

So the new thing I noticed is that it does seem to hold oil pressure in neutral. In neutral at ~2000RPM, it starts out at 60PSI for 20 min, then falls to 40PSI over the next 20 min. After an hour it was at about 39PSI. Then I shifted to forward and within one second the oil pressure fell from 39PSI to 20PSI. From there the oil pressure gradually fell to about 15PSI over the next 20 minutes until I stopped the test. Readings were fairly consistent between the electrical and mechanical oil pressure gauges.

Also, the temperature was holding at 155F after an hour in neutral, but rose to 180F after shifting into forward. The mechanic said that the engine should never go above about 140F since the thermostat is disabled to protect the engine from salt water.

Is it possible that the engine is over-loaded? Would overload cause an immediate 50% drop in oil pressure? I can turn the shaft by hand when in neutral, both before and after running the test. The propeller I have is 17x12in whereas the manual calls for a 16x11in (this is for the 2:1 reduction gear). The mechanic didn't seem to think this was a big enough difference to cause such an issue. Is it worth buying a smaller propeller to test?

[joe_db]

I see a few potential problems here and we will start off with the prop. You most certainly can have issues with a too-large prop. Can we find out the manifold pressure at a few RPM points, say 2000, 2500, and 3000? If you can't even get to 3,000 with the reduction gear the prop is definitely too big IMHO.
Second off is the cooling. High temps can very well result in low oil pressure. You should acid/vinegar flush the engine and then set it up with no thermostat, a bypass valve in the closed position, and see what you get.
Lastly is the oil itself. Single weight oil basically gets thinner when hotter and thicker when colder. With the notable exception of old 2-stroke Detroit Diesels, the day when single-weight oils are desirable is well in the past. I use Shell 15w40 Rotella that acts like 15 weight when cold, thus easier starting, and 40 weight when hot, thus better oil pressure when hot. I have tried a number of oils over the years and found this oil to hold pressure better over a wide range of temperatures compared to others I have tried.
If you give all this a try I am pretty sure we can figure something out.

[ndutton]

The propeller I have is 17x12in whereas the manual calls for a 16x11in (this is for the 2:1 reduction gear). The mechanic didn't seem to think this was a big enough difference to cause such an issue. Is it worth buying a smaller propeller to test?

You are definitely over propped. I can't say whether or not it's contributing to your immediate symptoms but you are not doing this engine any favors with a prop this big.

The mechanic said that the engine should never go above about 140F since the thermostat is disabled to protect the engine from salt water.

Do you have any further detail on exactly how the thermostat was disabled? Was it removed? Did he block or restrict the bypass? If not, disabling the thermostat also disabled the cooling system and this elevated temp symptom suggests that's exactly what is going on.