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.