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Topic: Low power c8.3 Cummins (Read 3049 times) previous topic - next topic

Low power c8.3 Cummins

Diesel shop checked boost, 25lb, lift pump pressure, not sure but I think around 35 lb.
Unit does not smoke. It runs fine but is lacking power.

Re: Low power c8.3 Cummins

Reply #1 –
Boost is at very high end of scale, so OK.  Check air filter, and replace fuel filters as a start.  Do not depend on air filter restriction gauge to be accurate.  If air filter is more than three years old, install new.

Re: Low power c8.3 Cummins

Reply #2 –
P. S. - My c8.3 has two fuel filters and yours likely does also...  be sure to change BOTH

Re: Low power c8.3 Cummins

Reply #3 –
All fuel filters have been changed including air filter.

Re: Low power c8.3 Cummins

Reply #5 –
Are you saying this loss of power is something that has happened recently?

25psi is about the highest boost you are going to see on a C8.3 without modifying things.  35psi fuel pressure is quite high if you're using the factory lift pump and overflow valve, I'm guessing this is at idle, even then still higher than I've ever gotten with a stock lift pump.  Do you have a means to check fuel pressure under a load, preferably WOT on the road? 

Is it possible that one of your brakes is sticking?  Take a IR gun around after driving and see if any of the brakes is hotter than the others.

Re: Low power c8.3 Cummins

Reply #6 –
Quote from: soav –
It runs fine but is lacking power.
Compared to what?  What is your frame of reference?  Have you driven/owned other RV's of similar weight powered by the 8.3?  I'm not trying to be a wise guy - just want to understand your basis of comparison.  In my case, for instance, I had zero prior experience with diesel powered RVs, so I cannot judge the relative performance of our coach.
I think it runs good, but I don't really know for sure...

Re: Low power c8.3 Cummins

Reply #7 –
Thinking more, I am confused as to how the mechanic checked boost on your coach since it is purely mechanical (could not drive it with a computer hooked up to it).  Seems it could only be done if a boost gauge had been installed since boost is created only when DRIVEN - there will be none at idle no matter how high the engine is revved.  If you have any desire to purchase a boost gauge, BANKS makes one/a kit specifically for motorhomes.  If you have not already, would recommend you check all the boots (rubber hoses) connecting the turbo / charge air cooler "plumbing".  It is possible the hose clamps are no longer tight or have worn through causing a loss of boost.  Again, boost can only be created while a coach is DRIVEN. 

Re: Low power c8.3 Cummins

Reply #8 –
You can dyno it too and directly measure the HP at the wheels.  That's what I did after I purchased my coach.  There's a % for loss in the transmission.  I think it's 20%.

Re: Low power c8.3 Cummins

Reply #9 –
Air filter may be clogged and need replacement. Blowing it out with an air hose won't work. Boost pressure should be about 20-25 psi at the top end of each gear. Low boost pressure may be leak in the intercooler or connections. Check fuel pressure when under high load and RPM. Clogged fuel filters may give good pressure at idle but not pass enough fuel under load.

Pierce

Re: Low power c8.3 Cummins

Reply #10 –
Not clear what you mean by lacking power. Fuel will be restricted and engine will have low power if fuel shut-off solenoid is not opening completely. Pull up by hand and use a cable tie to keep it up to see how it runs with full open solenoid.

Re: Low power c8.3 Cummins

Reply #11 –
Quote from: cajk –
Compared to what?  What is your frame of reference?  Have you driven/owned other RV's of similar weight powered by the 8.3?  I'm not trying to be a wise guy - just want to understand your basis of comparison.  In my case, for instance, I had zero prior experience with diesel powered RVs, so I cannot judge the relative performance of our coach.
I think it runs good, but I don't really know for sure...
This is something that happened recently. From a dead stop it does not pick up speed as it used to and it's slower climbing hills.

Re: Low power c8.3 Cummins

Reply #12 –
The boost was checked while driving and so was the fuel pressure. The engine is a 1993 with no computer in a Beaver Motorhome.

Re: Low power c8.3 Cummins

Reply #13 –
Have you checked out the tranny?  Torque converter slippage will exhibit those symptoms.

Re: Low power c8.3 Cummins

Reply #14 –
Fuel solenoid...  Forgot about that one!  Definitely check it!  It happened to a friend with a c8.3 and will exhibit your described symptoms and is a simple & in-expensive fix.  The following link from Barry's superb and informative collection of information should help

Fuel Shut Off Solenoid Replacement

Re: Low power c8.3 Cummins

Reply #15 –
Thanks, I'll check it again, looked good last time I checked it.

Re: Low power c8.3 Cummins

Reply #17 –
I'm going to paste this as it's some of the best information I've ever run across on the 8.3. This guy knows his stuff.

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 ]

Re: Low power c8.3 Cummins

Reply #18 –
I would use an infrared thermometer and check the temperature of each exhaust runner where it bolts to the head. I have seen many a tech not notice a miss on a diesel and if you can't hear or feel that, temperature will be the key to diagnose that. Just my 2 cents..................Rick

Re: Low power c8.3 Cummins

Reply #19 –
Betting it's a simple, small problem. Just takes a while and a good mechanic that troubleshoots well to find it. Somewhere in the fuel supply would be my first guess.

Pierce