Problem with engine stop circuit

windtrader · April 06, 2025, 08:07:12 PM

8V71n in a MCi MC8.

Symptoms
1) The bus will fire up but just stalls and dies, will not stay running.It is intermittent. After some random number of starts it will stay on. The stop plunger is pulled back off the stop lever in the governor.

2) When the bus is running it runs perfect with NO jakes turned on. If the Jake switch is flipped on (one or both banks) the engine stalls and dies. This happens when parked. When on the road and I turn jake on, then when swithed off it stalls out. Once stopped, put in neutral and hit start and it fires up.

3) Fast idle switch is on and the idle does no increase as it should.

I initially thought the problem was in the jake brakes but now since the problems all occurred in the past week, probably related to the engine stop relay circuit.

I chased the air line from the fast idle air cylinder to the relay on the back firewall in the engine compartment. The stop air cylinder goes to a different but same model relay.

From there a electrical wire pair runs from the relay into the rear wiring box. I'm chasing wires there now to figure out which replay they go to. I'm hoping they go to the engine stop replay, check it is operating properly or chase wires further up the circuit.

Clifford,
This should be a simple single fix since it all went bad so close in time. I'm in Barstow, I think not far from you. Might it be possible for you to take a quick look? Advise on a couple other issues greatly appreciated. don

Dave5Cs · April 07, 2025, 09:40:48 AM

Don sent you text

luvrbus · April 07, 2025, 10:24:11 AM

Don I will be here till Wednesday, I am 120 miles from you, sounds like you are losing the ground to skinner valve or the skinner valve is going bad or that crazy fuel pressure sender I have a MCI 8 coming in today from the rally in CA

windtrader · April 07, 2025, 08:35:08 PM

That ix too funny, I was there at Calico but decided to head home, overnight near Fresno, home tomorrow. I will look into the suggestions. It is intermittent.

windtrader · April 08, 2025, 08:54:45 AM

Clifford,
Where and what does the skinner look like?

update on jakes - yesterday, i tried the jakes again, this time it felt like they were not braking as much as usual, full or half. The other notable symptom is there is a lot of engine oil splattering the back access door and some splatter around the front of the engine. I am quite sure it was created by the jake spitting oil. Someone at the rally mentioned that jakes can spit oil.

This may also solve the problem where quite a bit of oil was consumed on one leg of the trip. I blamed the oil change thinking it was filled with multi-grade instead of straight 40w. I changed the oil with straight 40w and there was no oil usage.

Another observation is the oil leaking seems to come from the fan housing. I know that there is a leak in that fan gearbox. I filled it a lot more than usual and that may be causing excessive leaking. If that is so, then that is a different place where oil is leaking, obviously not related to the engine oil.

Dave5Cs · April 09, 2025, 07:54:37 AM

Don the thing you took the screw out of and moved over from the stop lever is a Skinner valve. Has an air hose on the back side of it.

The oil leak if coming from the fan gear box is common especially if you had multi grade on less than 40 wt in it. When pulling hills sometimes mine will spew some oil from there. You have to remove the whole fan box and fans to replace the 3 seals.

luvrbus · April 09, 2025, 08:33:33 AM

Quote from: lvmci on April 08, 2025, 09:06:01 PM
8V92 skinner valve

That's not the skinner valve Tom that is the Schrader cylinder like Dave said the skinner is the elenctic over air valve that controls that cylinder,your 2 skinner valves are on the backwall of the bulkhead

lvmci · April 09, 2025, 04:23:43 PM

OK thanks Clifford

windtrader · April 10, 2025, 06:34:10 PM

Hi Clifford,

The air lines connected to the stop cylinder goes to one of the silver skinner valves which have a electric wire going into the rear box.

The last time assessing the situation, I heard a sharp quick pssst sound and the stop cylinder retracted.

I will test the skinner valves and electrics to them. From there wires go into that scary of a mess that is the rear wiring box. There are numerous relays in there too.

dave - yeah, what seems to be going on with the fan box leak is if I fill past the midline of the level window, it skews out the excess. The last service filled it up and it's been splattering. I'll let it find its happy level and keep it there.

luvrbus · April 10, 2025, 08:36:52 PM

There is a 6 amp auto re set breaker under the drivers window the stop relay was a option you may or may not have the relay in the rear J box ,I can tell you the stud and wire number Friday if you don't have manual,MCI was terrible about changing the wiring on their buses with the serial numbers

windtrader · April 15, 2025, 12:08:54 AM

Thanks Clifford,
Will update once I continue chasing wires back from the relays on the bulkhead. And you are correct, even with the factory manual, some of the circuits like this one are hard to find.