Oil Pressure - Load vs. No Load (non Rover related tech question)

[jp-]

Recently, (in another vehicle) I experienced an oil leak while under load (3-4k rpm) pulling a loaded trailer up hills. Over the course of a few hundred miles, I lost about 2.5 quarts. The oil was blown all over the lower half of the engine making it impossible to determine the direct area of the leak. Once the trailer was removed and the engine pressure washed, I ran the vehicle in the driveway at 3-4k rpm for a good minute or two. No leaks. I then drove the vehicle (no trailer) for a week with liberal use of the right foot. Again no leaks. My father then posed the question to me about oil pressure under load vs. oil pressure at no load. The question is, would the oil pressure be higher at the exact same rpm in a loaded engine versus an engine pulling no load (or minimal load)?

Mechanically, the oil pump is meant to push a certain volume of oil at a certain pressure at a certain rpm. Therefore, it seems to make no sense that the pressure would be any higher at identical rpms. However, I know that the oil leak was present under load and not when unloaded. This is further documented by the fact that during the same mileage, I lost no more than 1/4 quart unloaded.

Thoughts on this?

[BackInA88]

How steep were the hills?
Was the oil leaking out the front or the back of the engine?

[Tim Smith]

I wonder if your oil pressure increase was being caused by blow-by in the cylinders...

Not really sure about these things but it would make sense that if you were asking more of the engine -> more throttle -> more power per bang... that you would also be increasing your chances of blow-by in the cylinders -> all of sudden you are driving Exxon Valdez.
:confused:

[jp-]

The hills were not particularly steep; I was just trying to maintain a constant 55mph.

The oil must be coming from the front of the engine because there was oil at the front. I am assuming here that the oil wouldn't flow from the back of the engine to the front while driving forward.

Tim, you may have hit on something. Perhaps it was not the oil pressure that was raised, but the crankcase pressure (as in blow-by). This would have the net effect of increased oil pressure, causing more pressure on all the oil seals. I can accept this hypothesis.

Good call.

[jac04]

Now let's think about this a little. The crankcase is vented, and the vent is usually quite larrge (in the case of atmospheric vents) or it is routed back to an area of the intake tract that is under vacuum, essentially 'sucking' out the crankcase gases. There is no doubt that there would be more blow-by under heavy load, but I highly doubt that it would cause any significant increase in crankcase pressure. However, this is assuming that your crankcase ventilation system is operating properly. Maybe it isn't.

[rovertek]

what kind of vehicle is it? i would try raising the rear of the vehicle up with a jack and check with the engine running to see if its the front seal, or you can purchase a UV/ dye test kit at your auto parts or tool supplier, they're rather inexpensive and work great pinpointing leaks, just put a bottle of dye in with the oil and periodically check with the UV lite and yellow glasses and the leaking oil will show up florescent green, you might just find more than one leak.....

[Tim Smith]

Now let's think about this a little. The crankcase is vented, and the vent is usually quite larrge (in the case of atmospheric vents) or it is routed back to an area of the intake tract that is under vacuum, essentially 'sucking' out the crankcase gases. There is no doubt that there would be more blow-by under heavy load, but I highly doubt that it would cause any significant increase in crankcase pressure. However, this is assuming that your crankcase ventilation system is operating properly. Maybe it isn't.

OK, yes... I agree with your thoughts but this is what made me think of blow-by.

I've got an '85 Mercedes 300d with about 245,000 miles on the clock. I've recently had a loss of compression and subsequently power loss (which I blame on the Mobil 1 oil I ran at around 220,000 miles) so I started asking around about it. What I found out was the best, most intuitive and fun check I've heard to date.

<Sorry MercedesRover, I just gotta tell this one.>
---
Loosen the oil fill cap (at idle when the engine is warm) and let it sit in place of where the cap goes. Leave it unlatched but sitting over the fill hole.

If it blows off of the hole like a geyser, the motor is shot.
If it rattles like a tea kettle, the motor has considerable wear in it but probably still has some life in it.
If it sits there perfectly, then you've got an A+ motor worth the best praise and care.
---
I thought it was a joke till I tried it and when it rattled like a tea kettle with puffs of smoke, it all made sense to me. The motor had also started getting dirty with oil around the time I lost power. I guess the pressure was building up in the crank case and since the breather is designed for only so much, the oil gasses were finding the first way out. What a devious little opportunist, oil is.

Either way, it's just a guess. And the story above is talking about a specific motor with lots and lots of fans with their own testing methods. So take it with a grain of salt.

Good luck either way JP.

[jp-]

However, this is assuming that your crankcase ventilation system is operating properly. Maybe it isn't.

It isn't.


Rovertek,

The vehicle is a 91 Montero with 283,000 miles. The 3.0l V6 was known for having one of the worst PCV systems out there. The diameter of the valve is no larger than 1/4". A 5/8" valve would be closer to what it needs. I did modify the PCV system a little, but haven't checked it in quite a while (forgot about it is more like it). Tim's comment about the blow-by brought it back to memory. I have no doubt that it's slap clogged up again.

Thanks all.

[LaneRover]

My brother had a jeep that for some reason a previous owner had removed the pcv system - I think to put a shiney chrome airfilter cover on. thus the engine tended to blow a lot of oil out of all sorts of places. We ended up just leaving the oil filler cap off of the thing to fix the problem (we were in highschool at the time and thus no money to REALLY fix things).

So, I would say that if you drag the trailer up a hill with the oil cap off and didn't have the same problem with oil leakage then it is pressure in the crankcase. If it is only a problem when you tow then just tow with your oil cap loose. If you tow stuff A LOT then I would do a more permanent fix.

LaneRover

[jp-]

Just for an update:

I replaced the PCV valve but it was not completely clogged. I then did as Tim suggested and loosened the oil cap, and revved the motor. It lifted up at least a 1/4", so the PCV system is totally clogged, but not in the valve (either upstream or downstream of that). I made a 700 mile trip Saturday to pick up the TR4A that I just got. I ran the whole way with the oil cap off (thanks for the suggestion LaneRover) and only lost (burned) 1/2 quart of oil. The motor still pulls strong, even with the blow-by, so I guess I'll just drill a hole in my oil cap until I rebuild the engine again.

[Tim Smith]

LOL, cool! It's fixed then.

[rovertek]

try installing a road draft tube.....