MCI MC9/8v71 loss of speed/power issue.

[TheHeavenlyChillbillies]

Hi there.
I have owned my 84 MC9 for about 4 years. I live and work in it...so it gets her fair share of use.  Over the last year and a half I have been running in to an intermittent issue where she seems to lose a lot of engine power.  Any slight incline and it quickly slows to 40 MPH...the top speed drops to 50-55 MPH on a straight away.  Even going down hill the bus won't do much over 60.  The whole bus feels like the engine is struggling... Possibly starving for air? When I hit the slightest incline it feels like I'm hitting quick sand...the drop in speed is pretty dramatic.  In comparison when the bus is running properly... I'm doing 55/60 uphills... 70+ on flat ground.

On a 450 mile drive today...the first two hours was a struggle...very slow/no power.  About 2 or 3 hours in it just started to run fine and I did 70 for the rest of the trip. There was no drastic change in topography.  Western NY to Michigan... So no really big hills one way or the other.  And the change is instantly noticeable.  You can feel the engine open up... It feels like I was dragging something and it dropped off.

It seems to be an all or nothing issue.  It's either running great or struggling.

After a lot of reading and talking to people my two thoughts are that either a brake is dragging or there's an air supply issue... A few people mentioned maybe I have a hole in my air box.  That being said...I don't have enough knowledge to confirm either of these.  So...I'm reaching out for some insight as to how I can trouble shoot what might be causing the problem.

I'm not sure what other information I can/need to provide so I'll gladly answer any questions to help figure it out.

It's an 8v71.  It has used straight SAE 40 for it's whole life. I have changed fuel pump, fuel filters, and air filter since I have owned it.  The bus does have a vegetable oil conversion on it... And I have the same issues regardless of whether it's running on diesel or vegetable oil.

Any help/insight would be greatly appreciated.

[richard5933]

How long since the last fuel filter change? They can get clogged very quickly if you run into a bad batch of fuel. I have no experience with veg oil conversion, but only can imagine that it could potentially have a part to play here.

[TheHeavenlyChillbillies]

The diesel fuel filters were changed 5000 miles ago... When they were changed it did not seem to fix the power issue.  The two systems are totally in dependent of each other until 6 inches before the fuel pump.  Seperate tanks, send/return lines, filters.

To be clear... This wasn't a gradual issue.  It just started happening... And it's either running poorly or it's not.

[Bill Gerrie]

Buy an temp gun at Harbor Freight to check the brake drums if you might think it is a dragging brake. They are cheap. Is the exhaust black when it drops power? Check the air filter for damage. Put a pressure gauge after the fuel pump to see if the pressure is dropping when the problem occurs. Run a line to the drivers area so you can read it under way. Best bet is an electric gauge. 

[chessie4905]

 Does exhaust smoke increase when it loses power?

[TheHeavenlyChillbillies]

There is no smoke. Had a follow car observe.

[chessie4905]

Using a pressure gage to monitor fuel pressure while driving when it acts up will eliminate fuel supply. Off hand, I would suspect something randomly blocking fuel pickup in tank as being one of the prime suspect, but you said the systems are independent of each other. Is there a selector valve? Debris at valve?

[buswarrior]

If it changes back and forth...

I would be looking for something loose that is able to block, and then coincidentally drop out of the way and not block, until the next time...

So, intake air pathway, the fuel tank pick-up and any fuel tubing, exhaust and muffler...

A rag in the fuel tank... delaminating pipe or tube... collapsing baffle...

Filters don't clear themselves without divine intervention...

If you are dragging a brake that badly, your report would be about setting fire to a tire and burning the bus down... but check the drum temps to be sure... coast to a stop as best you can, so not to screw up the heat reading.

Intermittent trouble is the worst...

Happy coaching!
Buswarrior

[bevans6]

You'd smell a dragging brake way before it could slow you down that much.  The fuel pressure/volume of delivery is actually quite separate from the fuel pressure that runs the engine.  The vast majority of fuel that the pump sends into the engine is returned to the tank, and only a small fraction is injected into the engine.  The fuel pressure that runs the engine is developed by the injectors internally.  I've had the engine run for a second with residual fuel left in the heads, zero pressure (no fuel filters installed, in fact).  Scared the heck out of me, I was testing a starter motor and damn if it didn't start...

It's a mechanical engine and I presume no air throttle.  I suspect something in the governor to injector train of mechanical items.  If it was air related there would be smoke, but even so I would check the shut down flap if it has one, the air filters, etc.  If oil bath filters still, have they been serviced recently?  Is the air intake from outside to the filter clogged?  Does the throttle linkage/cable open the throttle lever all the way?  If it's an automatic, could the modulator cable be hanging up and not letting the throttle open fully?