Any Idea What Causes This Engine Quirk?

[Larry B]

My engine will often do the same thing. I came to the conclusion that my engine is starting faster than the engine oil pump could establish oil pressure at the low oil pressure switch to the engine shut down system. On the engine cradle rail directly beside the oil filter is a small oil manifold with two oil sending units and manual pressure guage on it. Large sender is for dash guage and small sender is for low oil pressure switch to engine shut down . As a test disconnect wire on small unit and I doute if you will be able to make your engine shut itself down again. Of course you do not want to run with this wire permanently disconnected. To overcome the auto shutdown thing hold the starter button in (even after engine has started) until the low oil pressure light on dash goes out. Hope this helps. You know your engine shut down system oil pressure wise is working. I have to hold my start button in most time when I start my bus once it is air up.
           Larry B 

[Debo]

Very interesting Larry. Thanks for your comments. When it warms up a little I'll probably give this a try. Good stuff.

[luvrbus]

Aa 8v71 is always going to have oil pressure at cranking the oil galleys stay full unless the dump or relief valve inside the pan is bad and leaking and dumping the oil into the pan

[gumpy]

Rick is mostly correct.

The shutdown you are experiencing is caused by the safety shutdown system and it's being activated by low oil pressure.

Here's why.

When the bus has been sitting, you have no air pressure to activate the shutoff cylinder. So when you start it the first time, it fires right up and does not shut down.
But you would not be able to shut the bus off until the air pressure reaches 60 psi or so. I always start my bus from the rear panel when it's been sitting, just in case
I need to shut it down before air pressure comes up.

When you shut it off after air has built up, and then restart it, it dies if you let off the start button before the oil pressure comes up because the oil pressure sensor 
activates the safety shutdown system. Since you now have air pressure, the shutdown cylinder can be activated, and so the engine shuts off.

The proper starting sequence is to hold the start button until the oil pressure light goes out on the dash. Then release the button. Voila, the engine continues to run.

For this to work properly, though, you need to have a working fuel pressure switch on your engine, that is connected to the starter solenoid. When you get
fuel pressure, it cuts out the starter, so by holding the button while waiting for oil pressure, the starter will not continue to turn while the engine is running.


craig

[luvrbus]

Those systems on the buses were design by Murphy but MCI made theirs a little more complicated using the fuel pressure switch

[gumpy]

Yes, and that fuel pressure switch is the weak link in the whole system. I've replaced mine at least 4 times, and have another one
that I have to put in this spring.

How did Eagle handle the starter cutout issue for low oil pressure during startup?

[Debo]

And there's my complete answer. It makes perfect sense after it's explained. Thanks Craig (and others) for doing a little head scratching and solving this. Next time I fire it up after it's been running I'll hold the start button until the oil pressure light goes out. I do have a fuel pressure sensor plumbed into the filter housing, so I'll start it from the rear and make sure it's disengaging the starter. Thanks guys, It's finally nice to have a definitive answer to this.

[luvrbus]

Craig,the Eagle just use a monetary toggle you hold in the on position it overrides the shut down cylinder through a relay to the skinner valve fwiw you never see a Eagle anymore with a override switch unless it is a DDEC     

[bevans6]

Did you try it and see that the engine shutdown system was causing it to not start?  Mine doesn't have an oil pressure shut down system, or a fuel pressure sensor, so mine starts every time.  Should I worry?      

Brian