I recently bought a 621b that was supposed to be in excellent condition and have a newly rebuilt engine. I have had nothing but issues with it since- starting with the fact that they used a cummins out of a DAK semi. It ran perfectly and had no blow-by, but it had no power and overheated quickly. The power issue was due to the injection pump being set up for peak HP at 2800 RPM as apposed to 2200 like the original motor was built- so I sent the pump of and had it tuned. This fixed the power issue. The next hurdle was snapping the throttle arm on the pump due to the fact the correct pump had more travel at the gas peddle and therefore when I put my foot down the last inch was just stressing the metal. I welded and ground it then adjusted the peddle travel so that was solved. Then on to the overheating issue. First I used a thermal gun to realize the engine wasn't really that hot when the gauge was showing overheating. I then discovered the sender from the semi hadn't been changed so the correct ohm Case sender was installed. This only allowed me to see the engine took a bit longer to really overheat. I ended up having to re-core the radiator which almost solved the issue- now the fan has the incorrect hub and site too far from the rad. I am going to add an electric fan and look for the correct hub, but the machine only ran hot when really pushing it on a day that the temps were in the upper 90's. The next issue was the differentials both groaned and jumped when the machine was being turned. I checked the oil in both and it looked old but had no metal in it at all. I replaced it with oil that has additives for limited slip diffs but it still groans and jumps when I turn it. I now realize I am dumping about a half gallon an hour of oil from somewhere at the rear of the transmission inside the bell housing. Does anyone have an explanation for the differentials popping and jumping like I have described? Also, does anyone know of a stop leak additive that is okay to add to an ZR tranny but is compatible with the 15w40 engine oil it takes? I am leary of engine oil additives as they may contain friction modifiers. I would like to try something that will slow the leak down so I can finish the job I need the machine for. I assume the engine and tranny have to come apart for a rear seal change (if that's even where it is leaking) Thanks in advance guys. Shawn
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