Author Topic: Ok, so talk to me about these engines, the DDs in particular...  (Read 13014 times)

Offline Geom

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The 6v71, 8v71, 6v92 to be very specific (along with other connected essential systems -tranny, cooling, brakes/air, steering, etc).

Are they really as "fragile" as some of the posts would seem to indicate?

It seems that there is a very ritualistic approach to keeping these babies running and these rituals seem pretty important. There is also a lot of paying attention to some very important things, regularly. It also seems like there will be a sizable maintenance component involved. And even with that it seems the risk of catastrophic failure, either due to $*** happens or user error, is seemingly high.

I apologize if this has been asked before, but one of the things that drew me to a bus was the reliability aspect. So I'm conflicted.

I completely understand the need for regular and proper maintenance.
So in these buses is that a few hundred dollars a year, a couple thousand a year, or many thousands a year? Average about 5-10k miles per year.

I completely understand (and relish) the need to be attentive. And I'm more than willing to listen to everything and all that, etc.
But how much of that is "art" and how much is science?
Is it just a gut feel something doesn't sound right and a process of elimination as a result of constant "looking things over" or is it paying close attention to a few key things and look for occasional out of place things?

How much of an oh-$#** bucket should one plan for and based on what criteria?
Am I expecting "the big one" any time, all the time, just over the next hill; or -oh look that thing broke, every once in a while?

Overall. Realistically as possible. From your own experience or direct observation.


1966 GM 4107
6v92 Turbo

Offline wg4t50

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #1 on: June 23, 2013, 02:06:09 AM »
I see the issue as some folks have a feel/understanding about mechanical things, and seems most do not.
The aware folks, who keep an eye on the overall for signs & sounds.
Then there are the folks who only know something is amiss when it stops running.
Me, I am a nut case, when I purchased my MC7, it was with the understanding it needed everything, so by the time I am thru with it, it is a 100% rebuild bumper to bumper, spent much $$$$$$ on it, loved every minute of it, and it never gave me anyting but joy, always brought me home smiling, yes I put over 200K miles on it, every state east of the Miss including Canada from Detroit east.
SO I guess it is what it is.
I learned the DD engines, was lucky having played the 12V-71 game too for 4 years.
Dave M
MCI7 20+ Yrs
Foretravel w/ISM500
WG4T CW for ever.
Central Virginia

Offline Debo

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #2 on: June 23, 2013, 05:24:37 AM »
My take on all of this is that because of a recent high-profile engine failure, there are a lot of engine horror stories circulating. In reality, the DD 2-stroke is at least as reliable as any other mechanical device, and a LOT more reliable than some given proper, regular, reasonable attention. I think there are two key points to be made:

a. Unless you got a zero-time rebuilt engine, there's no way to know the exact history of it. I'm OK with that the same way I'm OK with buying a used car. I did my research and found what I was comfortable living with. As time goes on and I log more miles, I'll become even more comfortable with it the way I hardly think about the fact that my daily driver is a used car now. Yes, when something catastrophic happens, it has the capability to be expensive to fix, but that comes with the territory, and that's why I put money in my "oh poo" fund. Without proper preventive maintenance and attention to detail, any mechanical thing is always one step away from, uh, soiling itself. Keeping up with it is no more difficult than getting the proper manuals and educating yourself about its workings. People tend to fear what they don't understand, and with a better understanding comes an increased confidence.

b. The key to mitigating disaster is detecting problems early. It doesn't take long for small problems to become big ones. The good news is that if you catch it early, you have a good chance of preventing a meltdown. Early detection means checking things early and often in the driveway, and on the road. Every time I stop I walk around my bus with an I/R thermometer and shoot the tires, hubs, radiators, cylinder heads, etc. I'm looking for any problem that could become a big one later. This may seem excessive to some, but I don't prefer sitting on the side of the road broken down. I also think it's important to have proper warning devices on my bus. Low water, high temp, oil pressure, etc. All of these are early indicators of problems and a LOT cheaper to install than a new engine. Airplanes have tons of engine monitoring sensors because pilots need to know the status of things early if there's a problem. Our bus engines are so far back there and out-of-sight that it's harder for us to catch things until it's too late.

Sorry if this was too basic or didn't answer your question. It's just been my experience that I build a certain trust and relationship with mechanical things that I depend on by educating myself and keeping a watchful eye. Of course there's always the random, unforeseeable, wacky thing that happens, but even then I can at least feel a little better knowing I did absolutely everything humanly possible to avoid it.
« Last Edit: June 23, 2013, 05:34:53 AM by Debo »
1981 MCI MC9
Detroit 8V-71N
Spicer 4-Speed Manual
Outer Banks, NC (Nags Head)

Offline Mex-Busnut

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #3 on: June 23, 2013, 05:42:55 AM »

Whenever I buy a used vehicle (including our buses), I begin going over it with a fine-toothed comb. In our coach, we changed out all airlines, water hoses, brakes, wheel bearings, clutch and put in a new radiator. We also rebuilt the air compressor (It was full of oil.) and generator. The engine received new pistons, sleeves and oil pump. The heads were also rebuilt. I have the huge advantage of living in Mexico, where labor is much cheaper than North of the border. My engine cost me the equivalent of about U. S. $4,000. My brand new core for the radiator, with installation, less than $600.

My thinking is (from my online research) that with proper maintenance this should last me 500,000 to 600,000 miles, so in that context, not that expensive.

You might also take into consideration that a huge number of the buses on this board are 30, 40, 50 years old and a couple even older than that. Many a bus purchased by a typical bus nut has over 1,000,000 miles on it or far more... and still running great!

Many a commercial semi runs 100,000 a year for years, using some of the same engines in our buses.
Dr. Steve, San Juan del Río, Querétaro, Mexico, North America, Planet Earth, Milky Way.
1981 Dina Olímpico (Flxible Flxliner clone), 6V92TA Detroit Diesel
Rockwell model RM135A 9-speed manual tranny.
Jake brakes
100 miles North West of Mexico City, Mexico. 6,800 feet altitude.

Offline harpold700 3

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #4 on: June 23, 2013, 06:12:02 AM »
Absolutely critical to keep them cool.

Offline sledhead

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #5 on: June 23, 2013, 06:20:35 AM »
The only negative I see is the age, condition of the bus you choose . I hate to say it but right now if you can find a bus that has been redone by a bus nut ( new tires , rebuilt eng. all air stuff , brakes  gone over and all the regular things we bus nuts have done . Or for the size you want I would go with a 1990 to a 1995 bus 40' long with a newer 4 stroke and a auto trans. Now you would still have to have the eng. rebuilt at some time in the future as usually the reason they are for sale is they will need work to stay in the commercial world .The older busses are great but will usually need a lot of work and money to make them reliable .  

Now said all this my bus has had all the updates by me for maybe $ 25,000 and is still a money pit but out of choice now .Yearly maint. is only $1000. or so . change , oil , filters 2 on trans. 1 on eng. 2 fuel filters 1 air filter , grease , adj. small stuff.     Would I sell it yes but I would buy another and do it again    bus nut  thing                  

dave , karen
1990 mci 102c  6v92 ta ht740  kit,living room slide .... sold
2000 featherlite vogue vantare 550 hp cat
 home base huntsville ontario canada

Offline TexasBorderDude

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #6 on: June 23, 2013, 06:34:32 AM »
They just aren't reliable at all >:(  (sarchasm) The V71 series was introduced in the 1930's and is still under contract with the army until 2025 in self propelled guns. No electronics, it it's got air and fuel... it runs.  High heat is it's achilles. Anything mechanical is subject to failure.  Pound for pound they're probably the same or less to repair than any other modern Detroit, Cummins or other.
A curmudgeon's reputation for malevolence is undeserved. They're neither warped nor evil at heart. They don't hate mankind, just mankind's absurdities. They're just as sensitive and soft-hearted as the next guy, but they hide their vulnerability beneath a crust of misanthropy.

Online luvrbus

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #7 on: June 23, 2013, 06:35:46 AM »
Don't ever buy a bus thinking they run the same mileage as truck engine not going to happen buses are tough on engines the 60 series may last 700,000 miles in one top expect about 1/2 the life of any engine in a bus Cat,Cummins or Detroit 

Of the 3 you mention the 6v71 would be my least desire to own good little engines in their right application and a highway coach is not it as Harold said keep a diesel engine cool or pay the price
« Last Edit: June 23, 2013, 07:12:01 AM by luvrbus »
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Offline PCC

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #8 on: June 23, 2013, 12:44:01 PM »
I am running 8V92, and it runs perfect, but when the temp outside gets over 100F, she gets over 200 in the engine.

Never a whisper of trouble, though I do get concerned at that high temp.

Thinking of changing to a larger rad. Good idea? (question to those in the know)
For some, patience is a virtue.
Dealing with me, it is required.
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Offline Dave5Cs

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #9 on: June 23, 2013, 07:01:19 PM »
I have the 6V-71 but also a Saudi bus with larger cooling system and 2 larger Radiators. It runs about 160 -170-180 if it is really hot out.

 1998 Jeep TJ (PFTOED)

Offline bebackbus

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #10 on: June 23, 2013, 07:34:22 PM »
wow!  If I was a new prospective bus owner I would be scared to death of the big bad detroit engines.  When I was looking for a bus this site ran the praises of these wonderful engines.  My first coach was a GM4106 with 8v71.  I knew nothing about it other than what friends on this site and other bus nuts taught me.  I am not a mechanic  and didn't do all the rebuild stuff.  I drove that bus with no major problems.  From Alaska to Arizona many times and across the US coast to coast.  I now have a newer bus, 1975.  When I first got it it had some engine problems from previous owners hotrodding it and a bad rebuild.  Since it was rebuilt no major problems.  Been to Alaska and back many times.  i am not afraid of running it anywhere. If it will start it will run.  One of the most dependable motors IMHA.  15 years and mostly a fulltimer.  Sit back relax and enjoy that beautiful sound of a Detroit purring.

Offline bebackbus

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #11 on: June 23, 2013, 07:50:41 PM »
Sorry, Didn't answer question.   4106 no $$$ other than oil changes, filters $300.   Mci once I got the rebuild done proper. No engine problems.  Broken Altenater bolt.  Regular maintenance.  $300.

Offline HB of CJ

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Excellent Question
« Reply #12 on: June 23, 2013, 08:11:55 PM »
The 2-stroke Detroit Diesels were first designed in the late 1930s.  In a Grey Hound type Bus Conversion, the engine is usually in the rear with the radiators(s) on the rear sides.  Some older Crown Supercoach school buses had the engine in the middle under the floor, but the radiator was still on the side.  Even when things were perfect, the cooling system layout left something to be desired.  Heavy trucks have the radiator right in the front where it receives the full air blast.  Bus overheating concerns were never completely overcome.

Buses are heavy kinda squarish vehicles designed to make money hauling people around.  While constructed well enough to work fine as designed, they were most not over designed simply for that notion alone.  They were just good enough to do the job and not much historically was ever added to let them do the job any bit better.  Having said that, some buses did show excellent engineering forethought and construction techniques.  They are complicated machines mostly from an earlier era of history.

They are also maintenance intensive; examples would also include airplanes; pleasure power boats, sail boats, heavy machinery, race horses, fast beautiful women and big alligators.  They require time and $money$ just sitting still in your driveway or large shop garage.  The reason why I sold my old Crown Supercoach, (vin 37317) was because I could no longer afford to MAINTAIN ONLY the planned Bus Conversion project.  Small, much cheaper, stick and staples RVs also are expensive to own, but not as much.

hope this helps some.  HB of CJ (old coot)

Offline TomC

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #13 on: June 23, 2013, 08:51:07 PM »
When Greyhound ordered their buses with 2 stroke engines, they were detuned way down with using 55 or 60 injectors. That's because most bus drivers use the gas pedal full floored or no pedal-very little in between. When any of the 71 engines are detuned to 55 injectors (6-71 would have 195hp and 510lb/ft torque; 8V-71 would have 257hp and 677lb/ft torque) they would just about run forever-and it could just about take forever to get up the hill. As contrast, my 8V-71 turbo puts out 375hp and 1125lb/ft torque. The last 8V-71TA that Detroit made put out 400hp and 1200lb/ft torque with 7G80 injectors. But-I also do not run with my foot to the floor. I'm very gentle on the gas pedal (mainly to try and get better fuel mileage) and to keep heat down.

As said-heat is the killer for 2 stroke engines-mainly because they never relax. Each stroke is a power stroke-compared to a 4 stroke engine that has the extra 2 strokes to cool down in between. Good engines, but they do need attention. The new 4 stroke engines with common rail fuel injection, exhaust gas recirculation, catalytic converter, Diesel particulate filter, Selective Catalyst Reduction (Urea or Diesel Exhaust Fluid) are far superior to anything ever made. We have 80,000lb rigs pushing the 9mpg range. That's efficiency! Good luck, TomC
Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.

Offline Geom

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Re: Ok, so talk to me about these engines, the DDs in particular...
« Reply #14 on: June 23, 2013, 10:17:57 PM »

Thanks for the responses, great info!

Ok, so if I gather from bits and pieces from the responses, the general consensus seems that,

Yes they can be a bit of maintenance, but mostly a labor of love and manageable.

Paying attention is important, as small things can become big things quickly.
(I love that I/R thermo idea Debo)

The occasional walking around poking at things ritual is important.

Heat, and reduction thereof, is key. (Which makes sense with a rear mounted engine with minimal air flow).

Regular, turn key -> vroom, maintenance is about $300 to $1000 (I've heard as high as $2500) yearly.

Short of the exciting story that everybody hears about that thing that happened somewhere to some people, they're otherwise reliable machines.
1966 GM 4107
6v92 Turbo