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Hey y'all this is my first post on the forum as Facebook groups have been rather... inconclusive so far.
I recently bought a 1978 Toyota Corona Luxury Edition station wagon that had been sitting for about 25 years. The engine had unfortunately rusted itself together, and needed to be replaced. I sourced an engine from a 1980 Corona, did some light modifications (swapped oil pans and pickups as they were different) to the new engine, inspected it for rod play and scoring in the cylinder walls (Only slight side-to-side movement along the crank, no up and down on the rods, and factory crosshatching still present on the walls in all 4 cylinders), and have gotten it reinstalled into the car. The engine turns over great and seems to run well for the 10-15 minute intervals I've run the car (it can't move yet), but has a really concerning issue... delayed oil pressure?
The engine idles at around 15PSI cold and drops to around 9 when hot, which apparently is within spec according to Toyota's manual (Anything above 4.7 is apparently good). At about 3000ish RPM, it seems that I'm around 30-40PSI on my harbor freight oil pressure test kit, which again seems just fine. I don't have a tach, but my admittedly inexperienced ear says that's the RPM the engine was at. I have an exhaust leak and a slight tapping sound, but nothing that sounds like a rod or main or anything. Weber 32/36 carb isn't tuned very well but it does idle and rev fine enough for now. No emissions stuff of any kind, not even PCV at the moment. I am running Pennzoil High Mileage 10W30 conventional motor oil, which I am aware is too thin, but shouldn't cause the issue I've been encountering. I'm reluctant to throw out this oil as it was rather pricey and I know it's good stuff.
The catch though is that the oil pressure takes about 20-30 seconds to mount from both a cold AND a warm start, and it's been a bit baffling to myself and those around me. Both the HF gauge and the stock idiot light agree, so I don't think it's a false reading (It stays on for about 20 seconds, which aligns with the gauge) From what I've read online, it could be the oil pump, pickup tube, relief valve, or just old engine things, but none of them really seem to make much sense for these issues. If it was the oil pump, I would assume I would have other symptoms, like nonlinear oil pressure readings, grinding noises, or just no oil pressure at all. If it was something wrong with the pickup and it's sucking air, I would think it would mount pressure much quicker when warm as the oil is thinner and thus easier to suck up into the pump. I'm not super sure on the regulator... I can't quite wrap my head around how it works, but it seems like it could be the most likely culprit of them all? Maybe it's sticking open and not allowing the pump to easily pressurize? And finally, while this engine likely has over 200K miles on it, the presence of the factory crosshatching on the cylinder walls combined with no detectable play in the bottom end or scoring in the top end indicates to me that this engine still has a ton of life left in it and shouldn't be on its way out. There's also no audible change in the engine when it does mount pressure. I would think that it would be a lot more clacky and noisy if it was lacking oil pressure, especially on the timing chain, which apparently is tensioned by oil pressure as well. It starts up and immediately shoots to ~900RPM and sounds identical to how it does when the oil pressure finally mounts up to normal levels.
Is this just something I should ignore and carry on with, or is it something serious that I need to address (I think it is at the moment)? Has anyone else had this issue before? This is my first automotive project and I do not have the time or the resources to do a rebuild, so I want to do all I can to prolong the life of this engine. Any help on the matter is appreciated. I'm terrified of bricking this engine because if I do, the car goes on to someone else with more resources to engine swap or do whatever with. I'm a college student with not a lot of disposable income or time to spare.
It's possible I suppose, though I'm not sure if even if the system was completely dry if it would take that long to refill a small oil filter. I'm running a Mobil 1 Extended protection filter at the moment, and I don't really have much of a reason to doubt its quality. If it was a FRAM though I would absolutely doubt the quality hah. Any other ideas?
12-20-2021, 07:51 PM
78 Corona Delayed Oil Pressure
