Fluctuating Oil Pressure/Power Loss 8V71

[tyreman]

Need some help diagnosing low power on MC9 8V71. After using the bus on multiple trips I have finally started to notice some inconsistencies in power issues. Last week on a trip through the mountains of WV and VA my oil pressure would fluctuate from 26-45 psi on the dash gauge with a corresponding increase in power and ability to accelerate on inclines when the upper end of oil pressure was higher. If the pressure was low it would barely maintain speed while flat or slight incline. I just had the blower drive replaced and it is smoking a little more than before (black) I checked the recently replaced air filter (snow white) and screen on top of the blower to confirm no obstructions. Is not using oil to speak of, over 1100 miles last week and didn't have to add any. What am I missing?

[neoneddy]

What oil are you running?    Are you hitting the top RPM range?

http://www.tejascoach.com/tips.html   A good write up here, you could have a tired engine, try some 50w?  Or add some Lucas to thicken it up some.

Black smoke is more fuel than it can burn.   I don't know enough about the 8v71's having a 6v92 TA myself, only way black smoke would happen AND have low power is intake restriction or blower inefficiencies I'd think.

[tyreman]

Last oil change the shop put in 15w40 without my consent, I "assumed" they knew to use straight 40. Changing oil today to correct weight, will see if that helps. There doesn't seem to be any correlation of RPM to pressure levels.

[neoneddy]

I'd add some Lucas in there to counter the residual 15w40 in there.   

By any chance do you get better performance when it's cold vs hot?  I know my oil pressure sits at 40-50 at nearly any rpm cold, after a few hundred miles it drops to 20-30 at high idle and then only back up to 50 when the rpms increase again.

[tyreman]

Can't say for sure on the cold weather running, I haven't had the bus quite a year yet and am still getting familiarized with the whole 2 stroke with automatic thing. First several months the transmission was not working correctly and had probably been broken for over 10 years. The PO warned me he never got the Jakes to work and it would take off down hills so top hills in lower gear,  I didn't realize it right away not having anything previous to compare it to. I have driven over 1.5 million miles in all kinds of 4 strokes with manuals but these are completely different. After installing a rebuilt tranny and fixing the Jakes she'll glide downhill just fine.

[neoneddy]

I can't imagine rolling down a hill  hoping I slow down on the upcoming up hill.  Nice to know my HT740 is working well.

Let us know how things go  once you've got a proper oil change.

[Utahclaimjumper]

The magic word in all this is MOUNTAINS & GRADES,,8V71 engines  don't do mountains well when the elevations get tall the air gets thin and they don't breath well.. Oil fluctuations will occur with heat changes that change with grade climbing...
You can drop down to normal levels during the same day and watch it go back to normal readings.
Black smoke is also a normal condition of climbing,, your foot is in it,, the air is thinner,, and the engine is in an overfueled condition,, all normal.>>>Dan

[richard5933]

Mine drops oil pressure to about 25 lbs at the bottom of the power curve. Usually a really big clue that I need to downshift to continue climbing. You're not going to crest too many steep hills with an 8V71 in top gear.

[DoubleEagle]

When the oil pressure dropped, was the engine temperature higher as well? The oil will be thinner as the temperature increases, and when you have 15-40, it will be a little thinner to begin with. Going over 200 degrees in a 2 cycle starts to get dicey, regardless of the oil grade. As has been said, you can't expect too much from a 8V71 non-turbo. 

[tyreman]

Got the oil changed yesterday, planning on a short fuel run this morning, a station down the street has diesel for 2.43, I'll be topping off ALL tanks! Thankfully I have never seen the temp over 180, I'll advise how the new oil performs later today.

Just back from the short run for fuel and it actually went down to 2.40 per gal WOO HOO! Oil pressure rose slightly to just under 50psi at speed and seemed to have a little more power to the wheels. I'm anxious to take it out again for a longer trip and get a better feel for the overall performance. 

[buswarrior]

The oil pressure variable is just a symptom, not a cause of a power loss.

I'd be wondering about full travel of the accelerator linkage, and full travel of the fuel rack under loaded conditions.

I had a weakened floor cross member under the driver that the accelerator linkage was attached to... it flexed once the bell crank went far enough, effectively lengthening the throttle linkage, and the engine didn't go to full fuel. These sort of hang ups don't always behave consistently, loose bits, angle of your foot on the pedal, etc...

If for nothing better than preventive maintenance, inspect and grease the entire length of linkages, with a partner manipulating the throttle so you can confirm solid and free moving.

I hate digging into the more complicated stuff before the easy stuff.

Any busnuts nearby, you could run head to head and see how much difference in performance?

happy coaching!
buswarrior

[bevans6]

Actually oil pressure is directly related to engine RPM at any pressure below the pressure limiter cut off, it's a matter of volume pumped vs total restriction in the system (unless something is quite broken, which it doesn't sound like).  If it's coincident with low power, that has to be low rpm as well,  as others have noted.  Run the rack, set the no-load governed high idle speed, and run it on the governor.  The power curve of these things is straight up and to the right, they make more power the faster you turn them, right up to the governor.

[tyreman]

Sorry for the delayed update but I was distracted by the hurricane last week. We did a quick overnight trip over the long Labor
day weekend and after changing to the correct 40wt oil the bus had a noticeable power increase with a drop in smoke out the pipe. The most exciting part was I got over 7 MPG for the first time!!! Was running just under 5 mpg when I picked it up last November. The increase was not just the oil but multiple other maintenance items we have done this year. Fixed Jakes, set fuel racks, replaced air tube from filter to blower, blower drive, rebuilt transmission, exhaust leak....there were other PM tasks that the previous two owners neglected as well but we're making progress!!   
Thanks for all the tips and hints to keep this thing going down the highway!

[Van]

Great report!