Dumb acceleration and braking questions from a newbie

[Rufus Scout]

My 1985 Marathon with Detroit Diesel 8v92 and 165,000 miles blows a cloud of black smoke upon accelerating from a dead stop. The cloud dissipates within a few seconds of acceleration and it does not smoke while motoring along. Normal? Problem?

Also, the throttle pedal is quite stiff, it really takes some serious effort to press on it.

As for brakes, I realize that we're talking about a heavy vehicle. It seems to brake from speed quite well, but the last segment of braking, say 20mph to a stop seems to be quite leisurely. Normal? Problem? It does not seem dangerous, but it requires planning for sure.

While I am not a complete motorhome newbie, my experience has all been with gas powered 2x4/plastic motorhomes, so I am curious. Would someone care to help educate me? Thanks.

[Pete]

Rufus, I too had a 1985 Prevost....(American Conversion) mechanical engine. It is normal to exhaust black smoke for 3-5 seconds upon acceleration, and then stop. It will also smoke at altitude, ie in mountains ect.
As for the braking situation, do you have a jake brake on the coach? I think you do, so make sure it is operating properly. As you probably are aware, the jake will accelerate your braking. If your jake is a two stage, make sure it is in the proper setting (hi/lo).
Good luck with the coach, no engine has the sweet sound of an 8v92.

[Rufus Scout]

Pete,

Thanks so much for the quick reply. It sure is no monster "off the line" but once it's rolling it gets up to speed reasonably well.

I was wondering about the Jake brake. It has a two stage brake, I have no idea whether or not it's working correctly. When I get off the pedal the EGT drops very quickly and there is some felt deceleration but I have no experience to know whether it's correct or not.

How is the two stage switch utilized? There SEEMS to be no difference in retardation from say 60mph with the switch in either position.

[Pete]

Is your 8v92 a DDEC (electronic) or mechanical? There is a relay at the back (nearest you) when you open the engine bay doors on the in the center of the engine that controls the jake brake. It fails on occasion. Feel free to call me 979-627-7050 so I can understand your questions better.

[Rufus Scout]

Pete,

Mechanical engine. One of the things that endeared it to me. ;) I will check the relay, thanks!

[Ray Davis]

You will want to get that checked. You do not want to be going down a mountain grade without a jake. My first bus was an 8V92, and when I had a tune-up the mechanic forgot to hook up the wires back in the engine bay.

Although not as loud as a diesel truck, if you go through a tunnel, open your window and let off the throttle, you should definitely hear something. You should also feel a little deceleration, but it not huge at low RPM. The jake is most effective at high transmission speeds, so you would normally use a combination of downshifting and jake brake to control your downhill speeds.

[Coach Pro LLC]

Hello Mr. Scout.

Great bus. I was there when it was built....

For the Jake, high and low means all cylinders retarded or just 4 of 8. Low Jake will be so-so, but you should feel and hear high Jake for sure.

For the brakes, it is hard to say without driving and then offering an opinion based on other coaches. A through brake inspection from a qualified shop would not hurt though.

On the throttle, I think you are pre-DDEC 1 which means you will have a pneumatic actuator to the injector rack. You might check for air leaks in that area.

Overall it sounds like you are in great shape.

Safe travels and if you have any questions please ask.

Pat Sprenger
Coach Pro LLC
800-918-7172

[Rufus Scout]

Thank you, Mr. Pro; both for the explanation of the Jacobs brake and the throttle thoughts.

Pretty neat to hear that you were there when the coach was built!

[Coloradobus]

Rufus, we also have an 8V-92 on the 1992 Beaver XL 40 Prevost. If yours is like ours, you can goose the throttle sitting still, with your drivers window, open listen for the warbling growl as the jake slows engine RPM when you remove your foot from the accelerator pedal. If you don't hear the racket, then check the relay. If it still doesn't make the required noise upon decel, something is up.
Our Jake is OK to weak going down mountain grades with a 5700 lbs Lexus style Toyota Land Cruiser in tow. We sold it a week ago, and will be towing a C Class Mercedes stick shift wagon at only 3300 lbs. I hope to notice a difference decelerating with our jake.

[Pete]

Jim & Chris, glad to see your post. I recall sometime in the past, I posted the same response on the Jake brake to someone who had asked the same question and was quickly told by someone that you could not hear the warble sound if you were not in motion. I, like you had tested my Jake using the same method. It is good to know someone else did the same test with the same results.

[merle&louise]

Talking about Jake brakes, do any of you turn your Jake brake off when entering a town that has signs prohibiting the use of Jake brakes? I don't; I figure they want dump trucks with staight exhausts to turn off theirs because they are so loud! Ours don't seem to be as offensive.

Maybe I'm in the minority, but I just leave mine on all the time.

[Coloradobus]

Tuga, we don't turn ours off either. They want you to slow into these towns, and by golly if they want us to slow, then the Jake is going to be on, and it is always on HIGH.
Pete, That is one thing I noticed about the 8V's Jake versus the Series 60 Jake. The Series 60 will not warble growl when you take your foot of the gas sitting still, the 8V as you and I note does. Remembering our bus fleet 30 years ago with 8V-71's, the jake warbled sitting still. When we got the first Series 60 in 2001, much to my amazement, we could only hear it going downhill with the toll window open, coach rolling. Then with the Beaver was re-surprised again, the Jake acted ike our MCI MC5-B and MC-8 Yellowstone busses of the past.