Constipated -- 2 Stroke Detroit 671

[john connell]

Not exactly a classic truck but pretty close. This question concerns a '54 GMC coach originally owned by Greyhound Bus Company. It's powered by a classic 2 stroke, 671 Detroit Diesel engine that I'm hoping some of you guys are familiar with. I love the engine and have great respect for it's operation but it seems that I've "done her wrong". This old girl has sat for the majority of the previous 18 years with only minimal exercise while operating right in my own back yard. To celebrate my wife's 60th. she asked if we could take the bus, it was repurposed into an RV back in the 70's, to the lake with the entire family. For the week prior to the journey, I checked it over quite thoroughly assuring that she was road worthy. The evening before we left the problem raised her head and all plans had to be canceled. The starter motor no longer has the ability to rotate the engine. I can only bar the engine by hand because of extreme resistance. The part that is very interesting to me is that I've found that throughout the 360 degree rotation of the crankshaft, by counting the number of teeth on the ring gear, I find that the resistance is at intervals and those intervals repeat themselves each revolution. For the life of me I cant think what would influence the engine rotation in this manner. From the time I began checking her over until the moment she refused t run further I had driven about 30 miles. Thinking back about it, I was having trouble starting and thought my problem was an old battery but instead it was that the starter was working much harder as the miles passed. Any thoughts on this are certainly appreciated as I'm at a loss, especially because of this 2 stroke operation continues to confuse me.

 

[Tom in Mo.]

Quote----I can only bar the engine by hand because of extreme resistance. The part that is very interesting to me is that I've found that throughout the 360 degree rotation of the crankshaft, by counting the number of teeth on the ring gear, I find that the resistance is at intervals and those intervals repeat themselves each revolution. For the life of me I cant think what would influence the engine rotation in this manner.

And what is the count of the intervals of resistance per revolution?

 

[john connell]

Thanks very much for your interest, Tom. Perhaps your input will spark the thoughts of others to take a stab at this also. While barring the engine it came to me that I dont know the location of the pistons while the crankshaft moves. I know the firing order is 156342 and I found TDC after removing all the injectors to assure there is not any compression influencing my efforts. At TDC, I painted a corresponding crankshaft tooth and counted all on the ring gear coming up with 116 total teeth. Then I marked each 10th tooth, so I'd know where the rotation effort changes. What I find is that starting from zero, the engine turns easily until I get to 10. It is then quite tight until the 35th tooth. It turns freely until around 45 and then stiff again until 75. During this area the effort increases significantly. The rest of the rotation back to TDC it turns easily. I've created a diagram showing the location of the pistons as they each achieve the top of their stroke each 19 teeth. I decided to add lubricant to each as they begin to come up on compression stroke but haven't done this just yet. I realize that the lubricant will be lost as soon as the piston lowers sufficiently but perhaps the rings and cylinder walls will benefit. In the beginning I thought my starter was bad and spent $400 on a new one which didn't make things better at all. I think that the problem must be internal engine because of the exact replication of the effort each revolution. I'm certainly open to any suggestions. I've run out of really good ideas already and haven't been able to find any remaining gurus with knowledge on these ancient beauties. Sure hope I fix it.

 

[Tom in Mo.]

I don't have a GM diesel but love them from afar. They make a perfect marine engine.
Resistance three times in a revolution of a six cylinder relates to cam shaft interest? Twice per revolution could point to a rod bearing. Grasping at straws here but possible.
The tractor guys on the Oliver tractor discussion board will probably jump on this if you toss them a bone. Type in GM 671 Problem and they will help. Also, consider a google search of a GM diesel discussion board. I know there is a lot of garbage info out there but also there are guys that really know their stuff.
This board just doesn't get much traffic.

 

[john connell]

Thanks again, Tom and I'll take your advice on searching elsewhere. I appreciate your input and encouragement as this is quite perplexing to me. I am learning a lot but am entirely in the dark as to what's going on. Best wishes, John

 

[Nordic]

I don't know if it's easy or hard on your RV to access the air box covers? You will be able to see if everything is OK in that aspect. I won't take the time here but I discovered a most peculiar abnormality on one of my 471's.

 

[buickanddeere]

Using what spec of engine oil ?

 

[Mule Meat]

If it sat for 18 years. Is it possible that a couple of cylinders could have gotten moisture in them and formed a small amount of rust on the walls?

 

[tom upton]

Thanks very much for your interest, Tom. Perhaps your input will spark the thoughts of others to take a stab at this also. While barring the engine it came to me that I dont know the location of the pistons while the crankshaft moves. I know the firing order is 156342 and I found TDC after removing all the injectors to assure there is not any compression influencing my efforts. At TDC, I painted a corresponding crankshaft tooth and counted all on the ring gear coming up with 116 total teeth. Then I marked each 10th tooth, so I'd know where the rotation effort changes. What I find is that starting from zero, the engine turns easily until I get to 10. It is then quite tight until the 35th tooth. It turns freely until around 45 and then stiff again until 75. During this area the effort increases significantly. The rest of the rotation back to TDC it turns easily. I've created a diagram showing the location of the pistons as they each achieve the top of their stroke each 19 teeth. I decided to add lubricant to each as they begin to come up on compression stroke but haven't done this just yet. I realize that the lubricant will be lost as soon as the piston lowers sufficiently but perhaps the rings and cylinder walls will benefit. In the beginning I thought my starter was bad and spent $400 on a new one which didn't make things better at all. I think that the problem must be internal engine because of the exact replication of the effort each revolution. I'm certainly open to any suggestions. I've run out of really good ideas already and haven't been able to find any remaining gurus with knowledge on these ancient beauties. Sure hope I fix it.

Just noticed you have the firing order mixed up a bit. Maybe just a typo, but just in case you are still working on this? 153624 is the firing order.