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Fuel System Science Project

I have been recently gathering the materials to conduct a evaluation of my fuel system.  I have just completed the project, and will report my findings in this thread.  I will make a separate post for each step in the operation.  I hope this info will be of value to the Forum membership, especially those with the C-8.3L Cummins (mechanical) engine.  I am extremely pleased with the outcome of my project!

My 8.3L engine was pretty much all original when we purchased our U280SE last November.  It seems to run fine to me, but since I have zero experience with diesel powered vehicles, I have no basis for comparison.  Because my coach has 163,000 miles, and 3900 hours on the engine, I felt it likely that some of the original fuel system components were probably "getting tired".  The goal of my project was to ascertain the current condition of my fuel system, and if a problem was found, to hopefully correct it.

According to several online sources, one of the best fuel system diagnostic tools is a fuel pressure gauge that takes its reading somewhere between the lift pump (also called a transfer pump) and the fuel injection pump.  Also according to the experts, (for the C-8.3L with P7100 injection pump) at idle this pressure should read somewhere between 15 and 25 psig.  Anything below this range indicates a "problem" somewhere between the fuel tank and the injection pump.  So the first step in my project was to install a fuel pressure gauge.  The parts I used are listed below, along with a few photos.  The installation was pretty simple.  When I started the engine and went back to view the gauge, my fears were confirmed: fuel pressure at idle was only 3 psig!  Definitely not good, so on to step 2, and hopefully an improvement in this critical number.

Parts list:
TORK TEK CUMMINS 14MM X 1.5 PRESSURE GAUGE SNUBBER CBS010 - Tork Teknology
JEGS Performance Products 41057 JEGS Braided Stainless Steel Mechanical Gauge
Valley Instrument Grade A Back Mount 2 1/2in. Glycerin Filled Gauge — 0-30

Re: Fuel System Science Project

Reply #1 –
Chuck,

You are on the right path. The fuel pressure gauge should be installed between the secondary filter and the injection pump so it can detect any restriction in the filters as well as fuel pump pressure. A deluxe setup would have another gauge on the discharge side of the fuel pump so you could compare readings and verify your fuel pump as well as the secondary filter.

I installed a fuel pressure gauge on the discharge side of the secondary filter as well as one on the primary filter so I can check how well the hand pump is working and see if there is a air leak between the hand pump and the fuel tank.

Another gauge was installed on the discharge side of the secondary filter on the generator. It will tell if the electric pump is working as well as filter restrictions. The 4 psi is about normal from the electric pump with or without the engine running.

Our engine pressure are not the same as we have different engines. The 30 psi is residual pressure (engine off) and the idle pressure is normally about 60 psi.

Pierce

Re: Fuel System Science Project

Reply #2 –
What repairs, if any, have you made to the fuel system components? 

Is everything original?  New fuel lines?  Lift pump? 

Could you list the fuel pressure as you increase the engine speed while not in gear?

Re: Fuel System Science Project

Reply #3 –
Chuck,

Depending on where the pressure gauge is installed, the low pressure could be caused by a weak or dirty fuel bypass ball and spring on the return line out of the injector pump. That line could be clamped off and the pressure reading compared to before. Follow the link to a diagram of a typical Bosch mechanical injection pump. The part that can sometimes causes trouble is #200 in the photo at the link below. The other problem is that it is usually located on the back side of the pump next to the block and hard to get at if you have big hands. diesel bypass fuel injection pump valve - Google Search

You also need to see what your fuel pressure at the gauge is at full throttle. Either a sensor with a wire to a remote gauge, a video camera or someone in the compartment while you are going up a hill at full throttle. :D

Pierce

Re: Fuel System Science Project

Reply #4 –
Thanks for the replies - as you continue reading, you will see I have covered most of these points.

Step 2.  The lift pump (transfer pump) is one potential source of trouble, and low fuel pressure.  My lift pump was original.  It was seeping fuel from the "prime button", so was due for replacement.  That was done using the parts listed below.  It is a fairly simple task, except for the awkward working position.  I strongly recommend the use of studs rather than bolts...makes it into a "one-handed installation".  Unfortunately, when I started the engine after mounting the new pump, my pressure still read the same: 3 psig.  On to step 3!

Fuel Transfer Pump Alliant Power Stock Replacement Dodge Cummins 1994-1998 5.9L
Larryb - LarryB's EASY Stud Mounting Kit for Lift Transfer pump, 12 valve

Re: Fuel System Science Project

Reply #5 –
It's been years since I did a lot of Bosch inline pump work. I think we were looking at about 7 psi going into the injection pump. The fuel at 3 or 7 psi goes into the body of the pump and then the pistons take it and send it up to the injectors at somewhere around 3000 psi. With a restriction from a partially clogged filter or other reasons, it's easy for the injection pump to run away from it's supply of fuel with a resulting loss of power. Too low fuel pressure may cause cavitation where the fuel makes the transition from the low pressure to the high pressure part of the pump. These Bosch pumps have not changed that much in about 100 years. The pump in my previous link had the lift pump mounted on the side of the high pressure pump body. They almost never fail.

Pierce

Re: Fuel System Science Project

Reply #6 –
Here is an excellent link on the overflow/bypass valve with great photos. The guys here modify the valve for racing but you get the idea. New ones are pretty inexpensive. How to modify the overflow valve OFV - Competition Diesel.Com - Bringing The

You also get the idea how big hands don't work here. If changing or cleaning it, there is a a copper washer that is easy to drop and it needs the washer.

Pierce

Re: Fuel System Science Project

Reply #7 –
Step 3.  Another item that can cause low pressure at the injection pump is clogged filters.  The lift pump has to suck fuel through the primary filter, and push fuel through the secondary filter.  Blockage at either one will lower pressure.  I changed the primary filter, and started the engine.  Pressure still 3 psig.  Then I changed the secondary filter, and..........STILL 3 psig!  So the filters were probably in good shape, but it never hurts to have new ones in place.  Time to move on to step 4.

Step 4.  As Pierce has been explaining (with excellent links provided), the overflow valve is another factor in the fuel pressure equation.  They are very simple and fairly reliable, but they DO fail.  The spring can become weak, or the valve seat can erode, or trash can foul up the works.  Since my injection pump was original, and the overflow valve had obviously never been changed, this was the logical next step.  In the case of my engine, the valve is VERY difficult to access.  It is tucked under the AC compressor mounting plate, wedged between the injection pump and the cylinder head, and held in place (even when un-screwed) by a metal fuel line.  The hardest part of the operation was loosening the little bolt that holds the fuel line bracket.  I thought I would never get it to break free, but it finally did.  There is JUST enough space to unscrew the valve, then slide the fuel line and valve together out from between the pump and cylinder head.  It is NOT a fun job.

Once out, you can see (in the photos below) that my original valve does not have a retainer bolt for the valve spring, and therefore cannot be "modified".  No matter - I already had a new adjustable Tork Tek valve ready to install.  Even though the two valves are shaped slightly different, there was still enough room to install the new piece.  Getting it and the fuel line slid back in place was just as difficult as removing it, but it can be done...very slowly...very carefully.  Once it was installed and the fuel line was reconnected, it was time for the test.  I cranked the engine, walked to the rear hatch (AGAIN) and looked at the gauge:  19 psig at idle!!!!

I believe I have found the root cause of my low fuel pressure.  If the new overflow valve had not cured the problem, the next step would have to be replacing the fuel line from the tank to the primary filter.  You can probably guess that I am glad I did not have to go to step 5.  I still have to take the coach out for a test run, to see if the higher fuel pressure results in any noticeable improvement in engine performance.  Like I said in the beginning, I haven't had any problems driving it up till now, so I don't expect a dramatic change.  After the test drive, I will close out this thread with a final report.

TORK TEK CUMMINS ADJUSTABLE OVERFLOW VALVE OFV020 - Tork Teknology

Re: Fuel System Science Project

Reply #8 –
Thanks for the research! We have virtually identical motors so this will definitely carry over to our coach in the coming months!

Re: Fuel System Science Project

Reply #9 –
Any thoughts as to whether the 19psi is too low compared to TorkTek's literature saying that typical idle would be 23psi?

I've been busy with other things and haven't had a chance to hook up my gauges but will post my numbers here when I get a chance.

Also, did you have to remove the AC compressor bracket for overflow valve removal?

Re: Fuel System Science Project

Reply #10 –
Chuck,

Nice job with the photos and good logic figuring the pressure problem out. And glad you explained how tough it is to get a wrench back there to get the fitting off. Every time we would put a turbo on a normally aspirated engine, we would have to R&R the pump to add fuel, otherwise, the turbo just dropped the EGT with not much increase in HP. The pump is a real devil to get off. First, there is a single bolt at the far end to stabilize it and then you need a 1/4" ratchet with a tiny universal, about an 8" extension and a 13mm socket. I would wrap electrical tape around the universal so it wouldn't move too much and then try to get my hands down there to remove the 3 nuts holding the pump onto the engine block.

The Bosch injection pumps only get about 5 minutes on the calibration stand at the factory and to get the best idling and running Cummins (or Mercedes, etc), having a shop put it on the stand to calibrate it can make a difference. When installed on the Bosch test stand, it gets spun for an assigned amount of revolutions. It then disconnects and the amount of fuel in clear, graduated glass vials is measured for each cylinder. This is done at several different speeds and the end result is a smooth running engine from idle up to the governor limit. It is also necessary to check so each injector gets it's fuel at the precise number of pump degrees.

This video best shows the irregularities a stock pump can have. This is much like the machine we used. YouTube injection pump calibration at: Fuel pump test stand - YouTube  To do a pump right, it took an hour and a quarter or a little more unless new parts were needed.

When it was done, the cars were faster, smoother and didn't smoke like a lot of chipped PUs do now.

Pierce

Re: Fuel System Science Project

Reply #11 –
Quote from: rsihnhold –
Any thoughts as to whether the 19psi is too low compared to TorkTek's literature saying that typical idle would be 23psi?

Also, did you have to remove the AC compressor bracket for overflow valve removal?

I have seen a wide range of "recommended" numbers for fuel pressure on this engine (both the 5.9 and the 8.3).  For a stock, unmodified application anything around 15-25 psig (at idle) seems to be commonly accepted as OK.  Also, at the bottom of the the Tork Tek installation instructions it says (I added the underline):

"NOTE: Set the fuel pressure to OEM specs. 17 to 22 PSI at idle. Our valve typically idles at 25 to 26 PSI. Pressure at 2500 NO LOAD RPM is 29 to 32 PSI. Tork Teknology is not responsible for injection pump leakage or damage from running excessive fuel pressure or the use of a HP overflow valve."

Even though I'm not getting the 25 to 26 they say their valve "typically" produces, I AM right in the middle of what they say is OEM specs.  I'm happy with that.  Also, I have not had the chance to check the pressure at higher RPM - my DW was not available to manipulate the gas pedal yesterday when I was working on the installation.  I want to see what the pressure reads at higher RPM before I think about changing anything.  If I do decide I need to adjust the valve setting, at least it is now easy to do!

And to answer your second question, no, I did not remove the AC compressor.  After "eyeballing" the job I decided there was enough room to work around the AC mounting plate.  It would probably be easier with the AC bracket removed, but it's not absolutely necessary.

Re: Fuel System Science Project

Reply #12 –
Excellent piece of troubleshooting and repair. Nice work. I have the 5.9 engine on my U225 (with about 70,000 miles) and should look at this as a job for next spring.

Thanks for posting this (and for the great photos). And thanks to Pierce for significant commentary. :)

Craig

Re: Fuel System Science Project

Reply #13 –
Chuck, On most of our FT's you can increase idle speed using the set and increase switches on your cruise control.  On My 95 you used the increase switch in increments to set idle where you wanted it.  May differ on some models.
Gary B

Re: Fuel System Science Project

Reply #14 –
Gary,

First of all, let me say how happy I am that you finally found a buyer for your coach.  I know that is a load off your mind.  But I also hope, like many others, that you will stick around the Forum and continue to provide the benefit of years of experience with Foretravel products.  I was always especially interested in your comments, since we shared similar model coaches.  Don't go away - we need you here!

As to using the cruise control to set idle speed, I have read on the Forum about this "optional" function but cannot get it to work on my coach.  Either my unit does not support this mode, or something is not working right.  The CC seems to work fine going down the road, so don't know...

I'll just have to rely on my DW (as usual) to provide assistance.

Re: Fuel System Science Project

Reply #15 –
You can just reach around and pull on the throttle to speed up the engine. I can reach it from standing behind the coach reaching in the engine cover.

Re: Fuel System Science Project

Reply #16 –
krush, Thanks for that info!  I know old school carburetors, but I don't know squat about diesel fuel injection.  Today I am installing a new fuel shutoff solenoid.  While I'm "under the hood" I will get a good look at the injection pump linkage, and determine what or where I should push/pull to rev up the engine.

Re: Fuel System Science Project

Reply #17 –
It will be very obvious. The cable from the King Controls box goes to it. Pull/push it....just like a carb!

Were you having problems with the shut-off solenoid? I can't help but feel that sometimes people over-replace stuff. Remeber, there's tons of these engines out there on trucks going much further and working much harder than in our RV's.

Re: Fuel System Science Project

Reply #18 –
HEY ALL ......... If you reach around from the rear and move the fuel arm make very sure that you do not have a loose sleeve or anything else that can contact the belts or pulleys running,  IT CAN BE PAINFUL.

GaryB