8V71 auto shut off

[Coops6]

Hey Guys,
I fired up my 76 MCI 8 with an 8V71 for the first time in a few months.  She started right up, no smoke and aired right up.  I drove it about 4 miles to a car wash and clean her up.  I left it running during that time.  Then I headed out of town for a short trip.  1 mile out she shut off.  It would crank, but not smart.  After a couple of tries with the rear starter switch she fired up again and sounded great.  Then after a couple of minutes she shut right off.  It is definitely an air powered shut off switch of some type.  The oil pressure was 10lbs -15lbs and temp was 150 - 160.  I restarted her about 4 times.  Same thing.  It might run for 30 secs and might run for 5 min.  I am looking for ideas as to where to start. 
Thanks,

[luvrbus]

I had that problem with  the MCI8 I owned and it was the skinner valve disconnect your air line from the skinner valve going to the shut off and see if that helps there shouldn't be any air coming out of the line till the switch is off  good  luck

[FloridaCliff]

If it fired up and ran until pressure built, then shut off, I would say it is tripping the shut off.

Have you checked your fuel filters lately?

I remember once when I first got mine, it would idle fine, but under load would lose almost all power, ended up being filters.

Anyway, just a thought.....

Cliff

[jhaggerty]

A quick way to find out if it is an air pressure controlled problem is as follows. As soon as the engine shuts off, pump your foot break till you reduce your air pressure down towards zero.Probably below 30 lbs. If it starts right up then, it is probably the skinner valve as mentioned in the other post.

Jim

[Stan]

The first question: Since you were standing at the rear control when it stops, does the shut down cylinder push the stop lever on top of the governor? That is the normal shut down when you turn off the master switch.

If that is happening, then it is getting air from the shut down solenoid valve (Skinner valve) which is a normally open valve (it passes air when no voltage applied to the coil). Since it is intermittent, we can assume that the valve is not stuck closed. It could have a faulty coil. but more likely it is losing voltage to the coil. Go into  the rear  junction panel and check to see if the shut down relay is switching when it stops.

If it doesn't switch then the problem is after the relay. Get out the wiring diagram and voltmeter (or test light) and start checking back from the shut down relay in the rear junction panel to the Skinner valve. You are checking for bad or broken connection so wiggle all the wires when you check them.

If the relay is switching, then the problem is likely a sensor that may be bad or a bad wire to a sensor. I don't know what sensors you have but maybe low oil pressure, low water level or high water temperature. These sensors are different than the ones that send signals to the dash. A quick check of the sensors it to disconnect them and see if it solves the problem.

If the shut down cylinder is not moving then it is an entirely different problem. It could be fuel related or mechanical problem in the  governor. If a diesel engine is running, it will continue to run until the fuel is interrupted, either by being shut off or lack thereof.

[Coops6]

Thanks for the information.  I had suspected fuel filters at first, but when I am standing at the engine when it happens I can hear an air valve release.  Then when I hit the starter switch I hear the air valve again.  One time I had a hold of the throttle linkage to increase the rpms when it shut down.  It basically ripped it out of my hand. I now suspect it is the skinner valve, bad connection or a faulty sensor.  Of course I didn't know what a skinner valve was yesterday!  I'll try to get on it tonight and report what I find.  Nothing like getting an education on the side of the road.    I admit that I don't know anything about these beasts.  I appreciate your collective knowledge

[buswarrior]

Hello Coops6

Skinner is a brand name, and there are more of them in your coach than that one.

Best we all use some description of the beast, not the brand name.

As for your electro-pneumatic engine shut down system, parts of which may be built by Skinner, there is the little piston mounted on the governor that shuts off the fuel, and then up the hose from there you get an electrically controlled pnuematic solenoid valve. Its job is to send air pressure to the piston to move the fuel lever, starving the engine of fuel, and shutting the engine off.

No air, the engine can't be shut off with this system.

My books are not at hand, who knows the functioning of that valve? Energized to open, energized to close, energized both ways?  Functionality, darn, I can't remember...

This is a strange situation you find yourself in, either the innards of that valve are failing in a most inconsistently fascinating way, or, where is the electricity coming from, or missing from, to activate the solenoid? 

What were you spraying with the carwash hose?

happy coaching!
buswarrior

[Stan]

but when I am standing at the engine when it happens I can hear an air valve release.  Then when I hit the starter switch I hear the air valve again.

The starter circuit over rides the safety shutdown system so that indicates a safety shutdown has occurred. It is possible that you do in fact have a safety shutdown. Either your oil pressure gauge or temperature gauge may not be reading correct (check the mechanical gauges in the engine compartment) and the safety sensors are doing their job. The other and more likely possibility is a bad shut down sensor.

The oil shut down sensor is usualy mounted on a rail on the driver's side of the engine (along with the
electric pressure sender and a mechanical gauge). On the thermostat housings you will find anywhere from two to four sensors. There is always one electric sender and one shut down sensor but there may be one of each on each housing. There should also be a metal tube going to a mechanical temperature gauge in the rear control panel.

All of these sensors can be disconnected (mark your wires) to see if the engine will run. Putting them back on, one at a time will find the bad one. The sensors function by making a connection to ground so any sensor wire that has a bare spot touching ground will shut down the engine.

[buswarrior]

Darn, and drat...

Stan is onto it!

You still have all the auto shut down stuff active in your '76 coach?

This is another good reason a busnut with a pre-computer engine may consider scrapping the ancient defence against dumb hired drivers who will keep driving with a warning light on.

Yup, memory is really going south this AM, best to put me on the watch....

happy coaching!
buswarrior

[Coops6]

All,
Yes, I do have the dummy shut down system still in place. 
I was watching the engine compartment mechanical gauges at the time of the later shut downs.  And feel comfortable that they are reading correctly.  I do know where the oil & temp sending units are.  Is there also a low water level shut down?  I want to know more about scrapping the shutdown system, but I need more reliable gauges in the dash before I feel comfortable doing that. (another subject for another day) 
I did some cleaning in the engine compartment, but nothing up very high or on the oil sending units.  I also had the engine running while I was doing this.  I'll start disconnecting sensor wires and see where I can get. 
Thanks again,
Chris

[Stan]

The temperature sending unit probably looks identical to the high temperature (shut down) sender, just some different small numbers stamped somewhere on the head. The temp sender will read resistance with an ohmeter. The shut down sender should read open until it reaches the shut down temperature.

The oil pressure sender is also very similar to the Low Oil Pressure shut down sender. The sender also reads a resistance on the ohmeter and the shut down reads 0 ohms (closed switch) with a stopped engine and opens with oil pressure.

MCI used two different systems (option when built) and some have a 10 PSI switch that only turns on the light at the dash and then a 3 PSI switch that shut down the engine. Impossible to know what your's had and if it is original.

If it has a low water sender it will be in the  coolant head tank beside the sight glass. It is also possible that you don't even have a sight glass. but a tube hanging down with a valve (if you open the valve and coolant runs out, it is OK)  If you have a Low Water sender it will have a wire coming of it.

[RickB]

Is 10 lbs. of oil pressure low enough to engage the low oil pressure shutoff? My 8V71 idles with closer to 25-30 lbs.
I put a mechanical guage back in the engine compartment to keep an eye on things. Mechanical temp gauge as well.
Just thinking out loud...

[Fredward]

Chris,

My wife wants to know if your wife was along when the engine quit. If so she wanted me to ask you how she's likin' the bus now........
Wives are sometimes our harshest critics.

(I pulled my engine, replaced the clutch, stuffed the engine back in and then took it for a spin and she wasn't even impressed)

Seriously, I went through the Skinners and Schraeders on my 871 and might have an idea. Call me if you want 612-801-4826
Fred

[Stan]

RickB: 25 PSI oil pressure at idle on an 8V71 when hot is abnormally high. Many engines won't have more than 5 PSI at idle. I think the standard shut down switch is 3 PSI. That was on all three MCIs that I had and was according to the book. Some engines are running with 30 weight oil which drops the oil pressure even more.

[Fredward]

My understanding is the standard low oil switch is 2 or 3 psi.
Fred