I wanted to share this post I ran across on a Cummins performance forum. This guy knows his stuff and touches on a lot of good issues including the infamous banjo bolt fuel pressure issue. Also a remarkable explanation of what causes exhaust manifolds to crack and how to prevent it. Hope you find it useful and thanks to "Cobalt"
The base 8.3 is a workhorse engine. If i had a coach this is probably the engine i would have in it. The p7100 pump is as bullet proof as the engine if you keep water out of it. [ good fuel system maintenance ]
When it comes to timing I think you should leave it where the factory said to set it. Remember when they designed this engine they designed it to work hard and have a very long and trouble free service life. Unless you truly understand what your doing to cylinder pressure and fueling leave timing set where it is supposed to be. Now we are assuming the timing is set correctly to begin with ok.
Does your engine have an after cooler or is it charge air cooled? If you dont understand what im asking look for large diameter pipes that go from the turbocharger outlet to the front of the radiator or side if you have a side and not rear cooling system. If it is charge air cooled and its an older coach have someone check the charge air system for leaks. It should have no leaks. Leaks in that system cause low power and smoke and poor fuel mileage and hot pyro temps.
Next make sure the primary transfer pump which is a piston type pump and is bolted on the left lower side of the block [ standing at flywheel looking over top and twords the front of the engine your left hand is left side of block ] is putting 35 to 45 psi of fuel pressure into the p7100 when the engines rpm is around 1200rpm. If its not there is a check valve on the front head side of the p7100. This check valve which looks like a big nut has pump fuel drain line hooked to it going back to the fuel tank. The nut is actually a check valve that looks like a big short bolt when you screw it out of the pump. If the check valve spring is weak or it has an issue fuel transfer pump pressure will be too low. If your not sure if the problem [ if you have one ] with transfer pump fuel pressure is the transfer pump or the check valve pinch the fuel line coming off that nut shut for about 10 seconds with a pair of pliars. Dont bother doing this unless you have a fuel pressure gauge hooked between the transfer pump and the p7100.
and yes you have to usually drill into the cap nut that holds the fuel line in to the p7100 and tap it to 1/8 pipe to put in a gauge. The gage should go to 100 psi as a good lift pump can put out 80 to 95 psi when dead headed. If yoiu close off the return line and the pressure is still only 20 to 40 psi the lift pump has issues. By the way dont close off the return line to save money if the check valve on the pump is bad [200bucks for valve ] if you close off that line you will seize the p7100 and a rebuilt exchange is ball park 3500 bucks.
so now you have fuel pressure and you made sure your throttle linkage is pulling the throttle lever to absolute full throttle. If it is not you are not moving the internal rack to fuel fuel and losing horsepower. Short 1/4 inch of full throttle can be around 40 horsepower loss. Also make sure the pull off solenoid is pulling the off lever all the way to its stop. It is designed to do this geometrically so as long as the pull off linkage inside the boot is not worn out or broken your stop adjustment should be correct. Next make sure the turbo wastegate can on the turbo is not leaking air from a bad diaphram. If it is even a slow leak due to the signal air plumbing you may not be opening the afc in the 7100 all the way. Once in awhile you will find the afc air in fitting on the 7100 loose in the can it fits in and that causes power loss too.
You didn't mention the horse power this was but i have seen them at 350 in trucks and run for 800k miles.
The headgaskets are very good with really no problems. If you jack the timing up you may cause premature headgasket failure. Make sure the radiator is in good shape and charge air cooler if it is not aftercooled. Most of the failures i see on this engine in a coach are due to the following. cooling system failure [ plugged or salt rotted radiator causing overheating and premature headgasket failure and more if overheated enough.
Next I see cracked exhaust manifolds and or cracked warped manifolds and or turbine housings as well. This is due to leaks in the charge air system or the boost system what ever you want to call it. If the turbo boost is below specs do to a system failure like a cracked charge air cooler or hole in a charge air boot the exhaust system will take some serious heat because the fuel cant burn in the cylinder due to low oxygen to fuel ratio so the burn continues slower and the fuel is still burning coming out the exhaust port instead of being consumed in the cylinder. I have seen exhaust manifolds so hot they almost look transparent. This drastically shortens engine life. Not to mention fuel mileage is in the dumper.
So if it was mine:
*check air[boost] system
*I would pull out the nozzles and take them to a good bosch shop and have them flow tested and spray pattern checked. Many times a new tip on the nozzle and the nozzle pop off adjusted back to specs will do magic for mileage and smooth running and power. If you want to tweak the 7100 pull it and take it to a trusted bosch shop with the nozzles and ask them to set it up a few percent. You will know if you find a good pump man because he can tell you what percent to set up the pump to enhance pulling but not waste fuel or over fuel. I wont tell you what percent so dont ask.
*cooling system inspected and radiator repaired or recored if needed.
*oil sample analyzed. This only costs around 30 bucks and is well worth the money. IF you do an oil sample change your oil and filter and then run the unit until it is time to change the oil again and then take the sample at the oil change. Know the total mileage on the oil sample. know the brand and viscosity of the oil. know any additives you might think you need to put in the oil but really dont need to put in. know if you added any oil between the change and the sample. Tell the sample lab the above and you will get a good sample. The lab will explain the sample to you if you dont understand what your seeing on the report and dont feel bad unless your a chemist you wont. But the sample is a very good litmus of what is happening inside your engine. Last I would take it to someone who was recommended as a good mechanic and pay them to look it over and see if they see anything that needs addressed with just a visual inspection if you dont want to pay them for the diagnostics i talked about. No dowl pin issues no headgasket issues a really darn good engine.
You said you wanted a reliable engine your mom could drive trouble free. My response here is to that end. There are many who can tell you how to boost performance to almost magical ends out of this engine. But remember "the candle that burns twice as bright only burns half as long" [ bladerunner ]
ok i wrote too much already and this was just all my opinion but it is worth thinking about what i said ;-)