Old engine: 30 weight or 40?

[Wotname]

Originally Posted by sailorboy1

So far I am the only one questioning why/how the oil pressure would go down after hour plus. It isn't like after that time the engine or oil pump clearances got bigger after that.

Yes, this is a good question and one I silently asked myself. I have a theory and it might even be correct .

Assuming the engine is really a 3QM30 (as stated) or a 3QM30H and not a 3QM30F or 3QM30Y then we must remember these engines (3QM30 & 3QM30H) are raw water cooled and thus the temperature of the water side of the cooling is significantly lower than a coolant cooled engine.

One significant effect of a lower operating temperate is the oil temperature takes much longer to stabilise. Of course it depends on the load applied to the engine but if running at low loads, the oil temperature will remain very low. For instance my 2GM20 (raw water cooled) would take hours for the oil temperature to get proper hot. At 80% plus power load, it would take about 1 hour for the oil temperature to reach max.

I suspect the OP's oil pressure is dropping off after a few hours as it is directly related to the oil temperature. He could shoot the oil temperature at the oil filter to confirm the oil temperature relationship to observed oil pressure.

At the end of the day, the QM engines are heavily overbuilt slow(ish) revving basic diesel engines. They were designed for workboats. Keep a good clean diesel rated oil in it and it will still go for thousands of hours at a reduced oil pressure. The internals of the engine are wearing (bearing etc). The fancy upmarket oils will not rebuild the bearings.

Perhaps the best thing for the OP is to log the oil pressures / engine hours / oil temperature (at the oil filter) /rpm and keep an eye on the numbers. If the pressure starts going south for the same temperature and RPM, then a rebuild might be order.

Unfortunately the oil pump is buried deep inside the engine and it isn't practicable to replace it without tearing the engine right apart but the problem will most likely to bearing wear, not pump wear.

[Bowdrie]

Interesting thread.
I went thru the same thing years ago, thought the oil pressure on the Lugger was too low, so contacted Alaska Diesel/Northern Lights.
With engine and oil fully warmed-up, the oil pressure at idle was ~15lbs, and at normal cruising ~1,500 ~18>20.
His answer, (paraphrasing,) "Required oil pressure is a function of bearing surface area(s) and RPM".
The upshot: small industrial engines with large bearings that operate at relatively low bearing loads don't require high pressures, the VOLUME of flow was more important.
He told me not to worry about it.
Interestingly enough, Chevy says that the "old" small blocks, (350 ci.,) should have 5lbs plus 10lbs additional for each 1,000 rpm, so cruising along at ~2,000 RPM, the minimum would be ~25lbs.
Edit: The Lugger, with its external/remote filter holds ~3-1/2 gallons, takes a long time to warm up.

[Milko Berben]

I'm sorry to say your engine has reached its time replace or overhaul is what you are looking for unless you want to run it until it dies remember it will be at an inconvenient moment.

[Dieseldude]

There are a lot of varied manufacturer and user claims about engine oils. Best you can do is to chose a multigrade diesel rated oil from a reputable manufacturer. In my older Cummins, I've been using Shell Rotella T6 for years. Get good cold weather starts without extended cranking. The pressure gauge stays steady. Synthetic can be pushed beyond run hours of conventional oil, so I save money, or at least break even on the extra cost of synthetic.


A decrease in oil pressure after several hours running can be caused by many things. Is it happening on new oil, or oil with a lot of running time, or both? Could be that the filter bypass valve is opening, so back pressure is decreasing. In an older worn engine, oil might get sludgy and cause bypass to open. But if this is the cause, it should not happen on new oil unless there is a lot of residual sludge left in the engine between oil changes. You might consider doing an engine flush on your next oil change to eliminate any sludge build up. There are lots of flush compounds on the market. Who knows which ones are better? Though there is no guarantee that a flush will be thorough. Before doing anything, it could be helpful to have a sample tested after a couple hundred hours run time on the oil. This will give clues on what sort of wear might be causing the issue.



Not sure what filter you are using, but if not a high quality one, this could be causing the pressure issue. You can chose the engine manufacturer's brand. But any high quality filter by a reputable manufacturer should be fine. There are brands to avoid though. I'll never use Fram. Like many components, engine manufacturers do not make their own filters. So called "economy" filters are not economical in the long term. Choosing a good filter is a whole discussion of its own. The best online reviews are by people who have dismantled some new filters and shown the internal construction. It can be hard to get up to date reviews though, as designs can change.



Beyond this, it would be prudent to inspect the oil pump for wear, or just replace it. It's an old engine, and likely the pump is original.


Since nothing was said about engine operating temperature, I'll assume that it is not running too hot. Would be good to confirm that coolant temperature is right with a good IR gun though.

[EngNate]

I'd go with 15W40. Rotella T3 is my brand. And a rule of thumb for minimum acceptable pressure of 10psi per 1000 rpm. And the detergent thing - Don't switch to detergent oil with an engine that's seen long service on non detergent. The detergent oil will dissolve and pick up years of deposits that will clog oil filters repeatedly.

[garyfdl]

I have an old 2GM (albeit new-to-me). I'm running 15w-40. Shell (Rotella) and Mobil (Delvac) make it.

[wrapper]

I'm sure this has been asked before, what about use of synthetic Rotella T5 on an old Yanmar (3GMD)

[garyfdl]

Originally Posted by wrapper

I'm sure this has been asked before, what about use of synthetic Rotella T5 on an old Yanmar (3GMD)

Looking at the manufacturer's spec's I don't see any problem/that's what I just put in my 40+ y/o 2GM. (Full disclosure - I just started using it in a new-to-me boat.)