Master Warning Light truble.

The engine oil pressure switch only activates at one pressure setting. If the oil pressure is adequate at idle to keep the oil pressure warning light off, but then the switch activates when you rev up the engine, that means the oil pressure is actually dropping at higher speeds. Confirm this by hooking up an oil pressure gauge.

Once you have confirmed this, then the question is: what would cause oil pressure to actually decrease as engine RPM increases. That is a mystery to me.

What is the history of this car: how long have you owned it, and how often was the engine oil and oil filter changed?

The engine oil pump is located at the front of the engine (the side facing the passenger-side fender). The entire housing at the front of the engine constitutes the oil pump (the engine valve cover, engine coolant pump and the crankshaft pulley must be removed for access.) I am wondering if there is an oil pressure relief valve within the pump which is releasing sooner than it should.

 

You don't know how to do what: hook up an oil pressure gauge? If so, you need to purchase a gauge along with the appropriate fittings. Then you need to unscrew the oil pressure sender switch using a 24 mm deep socket. Then screw the oil pressure gauge into the hole.

When replacing the sender switch, you need to use sealant on the threads and wait an hour for the sealant to set up. Clean as much of the engine oil off the threads as you can, before applying the sealant.

If you are asking about how to replace the oil pump, that is a pretty big job and you should subscribe to techinfo.toyota.com to see what is involved.

 

I would get an oil change. This issue could be caused by a clogged filter. When they change the oil ask them to look for metal bits in the oil. Your engine could already be in bad shape. If you drive with that light on the engine will get ruined.

 

Yeah, that's a strange one. Sounds like a faulty switch to me. But it could be that as the pressure increases, for some reason the contact closes again. Is it possible that the switch is both a low and high (overpressure) switch, and that the relief valve is sticking?

I think it would be easier to simply put a new one in and see what happens. After all, if you have to go out and buy an oil pressure gauge and the appropriate fittings, and the oil pressure turns out to be okay, you still have to buy a new pressure switch. Just put in a new one.

However, there are another couple of "unthinkable" scenarios that could apply...

1. The Oil Pump pickup is clogged with something (sludge???). This seems extremely unlikely, but if it was, it might explain the behaviour. At low RPM, there would (must) be sufficient oil flow for the pump to maintain normal (sufficient) oil flow to keep the pressure switch off, but at high RPM, the pump would begin to starve and cavitate, so the oil pressure drops to the point where the oil switch turns on - given that at high RPM, the oil demand would be higher due to centrifugal action, bearing displacements, etc, and the pump is turning faster so is trying to get more oil through the same restriction. This could result in oil pump cavitation. I have seen this happen with hydraulic pumps.

2. This is possibly much more less than (1) and I don't know anything about the arrangement of the Prius G2 Oil Pump and pickup etc anyway. However, I know this has been a major problem on the Nissan Navara 4-cylinder diesel engine. An O-ring that seals the oil pump pickup tube to the bottom of the engine block breaks at around 100,000km. Not sure why, but I guess it's unretained. Then, the oil pump starts to suck air along with the oil. The first sign of this problem is usually when #4 Big End bearing fails. Apparently, the oil light does not typically go on abnormally. Very helpful! I guess that means that #4 gets most of the air for some reason.

But this would not result in the observed symptoms in this case, unless the oil level is "just" above the leak, so there is no problem at steady speeds but under acceleration, the oil sloshes to the back of the sump and exposes the air leak. A lot of if's and maybe's...

3. It could also be an electrical fault - maybe the Oil Pressure signal wire is chafed or has the insulation damaged and is shorting when the engine rocks under acceleration. Take a close look at the oil sender signal wire. Follow it back into the main loom. It is even plugged on to the oil sender switch? Or is the switch itself damaged? May have taken a hit when changing the oil filter, or some such...

 

Another thought.

What grade of oil (viscosity) e.g. 5W30, or 20W50 or similar, did you use?

Maybe Toyota anticipated uneducated mechanics and DIYers refilling with standard grade Engine Oil. Maybe the Oil is the wrong grade, and the Oil Pressure switch IS an under and over pressure type.

 

How much oil did it take?

It should have taken about 3.5 to 3.7 liters. Can you confirm how much oil you added.

 

At this point, for sake of the process of elimination, I would replace or test the oil pressure switch. It might be the cheapest thing to replace at this point (vs. the oil pump, etc..) and I have had what KiwiAl said happen to me before a couple of times.

Noteworthy is if you were actually having a loss of oil pressure when accelerating, you'd likely be hearing some engine noise (noisy valve train, etc) and you'd be complaining of that as well as the TOD (Triangle Of Death). Mehopes this alone points to a faulty oil pressure switch.

 

I believe that the oil pressure switch only registers abnormally low pressure. It certainly is easy and quick to change it.

However I believe the OP will find that the oil pump will have to be replaced, which is a pretty major project.