Hello, Everyone,

I've done a number of searches, but haven't found any posts that address my strange problem.

THE PROBLEM: After I've driven my 1976 300D for about 15 or 20 minutes, the oil pressure will drop quickly, lower and lower, until it is at 0 regardless of RPM.

Within a minute or so of losing pressure, the engine quietly and smoothly shuts off, as though I turned it off with the key. If I sit for 10 minutes, the engine starts with no problem or noises, and I can drive about 1/2 mile with normal oil pressure readings, until it zeroes out again and shuts itself off.

Oil level does not change. Water temp remains at 175F or below throughout. Oil cooler is cold throughout. Engine is not obviously affected in any way.

THE HISTORY: I bought this lovely two-tone green 1976 300D a few months ago. Drove it pretty much every day after purchase, racking up 500 miles of town, scenic twisty and highway driving. No problematic behaviors. I parked it in my garage to replace the leaky radiator. While it was parked, I replaced a leaky front crank seal

FIX ATTEMPT: The first pressure loss/shut off happened during my test drive after replacing the radiator and crank seal. I pulled the oil filter and oil gauge/line. I found there was no plastic main filter, but I added one along with a new fine filter. I blew out the oil gauge line and blew some dust out of the gauge gears, but the gauge seems to be working fine. I saw no metal/shiny-ness in the old oil. I test drove it afterwards and everything seemed fixed, but the next day, on the way home from the dentist, it happened again.

So, I'm baffled. I can sort of imagine that maybe the oil pressure loss is something to do with a problem with the oil cooler thermostat, since it seems temperature related?

But, I do not understand (a) why that would result in a loss of pressure (b) how the engine is shutting itself off and (c) why the engine is not squealing or banging or seizing in protest.

Any thoughts?

Thanks,

Jesse.

Sounds as though the engine shuts off due to pistons getting hot and expanding from lack of oil. When it cools it restarts.

I'd drop the oil pump and examine it or replace it.

As as a thermostat, I don't agree as it should be bypassed if not open.

Did you pull the oil filter can apart and check it? Does it have a primary/plastic screen and secondary oil filter?

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Are you sure of the oil type and viscosity in it? Not sure that would have a huge difference, even a really low viscosity fluid should let the engine run in these conditions (wear aside, etc).

First and foremost I’d drop the pan and see what’s going on. Seems like low/no flow as the oils viscosity drops and the engine warms up.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Look to the oil thermostat and/or a line to/from the oil cooler for blockage.
When the oil thermostat opens flow to the cooler, it simultaneously cuts off flow to the cooler bypass circuit. The bypass circuit is what supplies the engine prior to the thermostat opening. If there is blockage in the cooler circuit, when the thermostat opens all oil flow to the engine is cut off.

@Sugar Bear - But, why would the pump only fail when the oil gets up to temp? I'm also surprised that piston drag would shut the engine down so promptly and cleanly. It doesn't seem to lose power down to off, but just shut off. For the first loss of pressure, there was no screen filter; for the second loss of pressure, I installed both filters, new.

@JHZR2 - For the first loss of pressure, the oil was 15W40, for the second it was 5W40.

@Frank Reiner - I will pull the oil filter housing and take a look. This explanation sure would fit my impression that this problem is directly tied to oil temperature.

Frank Reiner wins. I lose.

After reading Frank's response, I was suddenly almost certain what had happened, so I walked out to my garage and removed the upper oil cooler line. Sure enough, the plug I had stuck in the line to keep it from dripping on me was still there. How stupid is that.

Thanks to everyone for your suggestions.

Jesse.

This issue start right after the last oil change or not. If it did drain the oil into somethiing you can save it in and put 15-40 back in. There also is a chance that too much blowby pressure. in the crankcase is shutting the injection pump off. How does the oil pressure drop? Gradually over some time or just about instantly. Does the gauge normally read full scale at anything above an idle.

A device that compensates for changing viscosity is the pressure relief valve in the oil pump by the way. Could this be non functional and the engine has wear? So when the oil thins it in effect by passes it all. It is a probability to me at least. Open all the time would be strong on my list. Or a weak or broken spring. It is where I would ook if the answers to the questions resonate with it. Like many things there are always flies in what I think. I feel the oil is not cooling enough to restore viscosity. So you have oil pressure again. When you restart. Do you get full gauge deflection then above idle or not?

.I was wondering if the 5-40 was frothing or something. All synthetics claim to protect like a top rated grade. That does not mean they hold the same viscosity I supsect.

How many miles per quart was this car using? Under or over a quart a thousand? Now if the car was doing okay on the 5-40 for some time it may not be involved at all. I even wonder if you got a mislabled oil container with a 0-20 in it.

Your problem is a little strange if anything. The guess on the oil cooler was a good one. Incidentally how many quarts of oil did you put in the engine? Personally I am a fan of looking at anything one has last changed or repaired. If there is a problem afterwards. You changed something. lots of questions I know. Yet sometimes in the answers you notice something not mentioned.

Also us really old ancient members had a rule years ago. No less than 15-40 in older diesel engines. That was all of them back then. You want that oil cushion to deal with the heavier loadings inside a diesel engine.

There is no intent to make this an oil thread by me. I would not expect this problem running a 5-40 oil. Yet I suspect it may not be the best for the engine lasting. Originally these engines where 40W oil as there where no multigrades and I can assure you they had more viscosity than a 15-40 oil has.

The oil cooler system blockage makes the most sense. An idea for testing it would be to run without a coolant thermostat to see if the problem does not occur at all or at a longer interval if the oil gets hot enough.

If it is in the oil cooler system it will be interesting to see the root cause.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Good possible test for the oil cooler might be. As soon as the oil pressure drops off check the temperature of the cooler? Although do not run the engine with no oil pressure. Give it a minute to give the hot oil time to heat up the oil cooler as well. Again if this is rapid decline of the oil pressure or not is not known on site.

There is perhaps some evidence that the oil cooler is not in play other than in highway driving. As almost all failures of the hoses seem to occur on the highways.

I did a "Quick Reply" last night, but it doesn't appear to have posted, yet. Frank Reiner's post caused a flash of realization. I went right out to my garage, pulled the upper oil cooler line, and there was the rubber plug I had inserted to keep it from dripping on my face, still jammed in there. So stupid of me.

This morning I did some test driving, and the oil pressure was normal and afterwards the oil cooler was once again warm. A bit later I drove out to Malibu and back with no problems. The engine shows absolutely no ill effects from the two shut offs, which makes me think they were not piston-drag based.

I agree with everyone who is questioning such a thin oil. I used it because it's what Pelican put in their oil filter kit, even though it seemed strange for an old engine in temperate Los Angeles. The parts stores around me are all out of heavyweight diesel oil, but I will go back to 15W40 when I find some.

Thanks to everyone for your help. I'm happy to be back on the road.

Jesse.

Thanks for closing the loop

Curious though, if not piston drag, what other mechanism is there for stopping the engine?
Certainly none built in that I know of…

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1981 240D 4sp manual. Ivory White.

Excellent news!

IMO, If it wasn't piston drag it would likely be crank bearings. If it was crank bearings it probably wouldn't run ok now, it would knock and/or have low oil pressure.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Im wondering if the oil pump was dead heading when the bypass closed. The drag stalled the engine. Who knows.

Interesting theory about the oil pump...hmm.

Is there anything in the injection pump that would require oil pressure to operate? They do have that small black plastic line going to them.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer