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Topic: Fuel System Science Project (Read 5034 times) previous topic - next topic

Re: Fuel System Science Project

Reply #20 –
New shutoff solenoid is installed.

Note to those who are planning on upgrading this item: I encountered a problem during installation.  The new solenoid and the new bracket that comes with it fit perfectly - no problem there.  HOWEVER, due to the larger size of the new solenoid, my throttle cable bracket would not fit.  On my engine, it is held by two of the same bolts that attach the solenoid bracket.  Basically what I had to do was modify the throttle cable bracket so it could be moved "inboard" (toward the injection pump) about 1/4".  This required drilling 2 new mounting holes, and removing some material from the bracket.  Not a big deal - can be done easily with basic hand tools.  Your setup may be different, and you might not have the same problem.  Just thought I'd mention it.  Photo of new solenoid and modified bracket below.

Re: Fuel System Science Project

Reply #21 –
The injector pump 'overflow' fuel return flows directly back to fuel tank. Each injector also has a fuel return hose connection that seems to daisy chain from injector to injector.

Where does the last daisy chain of the injectors flow to?

One electric fuel pump I am looking at, the installation instruction says to tie injector fuel return with injector pump fuel return, and I don't know what they are referring to.

Re: Fuel System Science Project

Reply #22 –
Daisy chain goes into the return line.

Pierce

Re: Fuel System Science Project

Reply #23 –
Pierce,

I thought our fuel return rubber hose came directly from the injector overflow fitting. Where does the daisy chain connect to the return line, or does it connect to injector pump and then flow out the overflow valve?

Re: Fuel System Science Project

Reply #24 –
Good question Barry. There are two sources of excess fuel. The first comes from the high pressure injection pump. The fuel/lift pump produces more pressure than the overflow/bypass valve on the back side of the injector pump is set for. So, the excess fuel goes past the ball valve and out the fitting toward the tank.

The fuel injectors have a certain amount of internal leakage and the extra fuel is daisy chained either through metal lines or in fabric covered rubber lines from one injector to the next where it picks up a little more to the next and so forth. Once it leaves the last injector, it goes to a fitting where it meets the excess fuel from the injector pump and adds to the total amount of fuel returning to the  main fuel tank. The main fuel injection pump has to take into account the small amount of fuel loss at the injector when it is calibrated so the proper amount of fuel is injected into the combustion chamber or pre-combustion chamber.

The returning fuel from the injectors and injection pump is warm so you can feel it when you put your hand against the tank after it has been running for any length of time.

In the case of a Detroit 2 cycle, there are no injector pipes or fuel lines or even a single injection pump. All fuel travels inside the cylinder head(s) to each cylinder where there is a fuel injection pump and injector all in one compact package called a unit injector. All excess fuel is VERY hot and carries quite a bit of heat back to the main fuel tank. Putting your hand on a tank that is only partially full can almost burn your hand. That's why Detroits are so clean looking on the outside. Everything to do with the fuel system is internal in a very complex cylinder head.

Below is a diagram of a Bosch style inline injection pump with the lift pump mounted on the side of the main injection pump. An electric or engine driven lift pump remotely mounted may also be utilized. Engine manufactures may design the system in different ways but they all get the excess back to the tank. The returning fuel is under very low pressure. 

Since we don't have a Cummins, I can't describe exactly how or where the connections are made. I used the diagram below as it is the one I am most familiar with. This closely matches a Bosch pump mounted on a Mercedes 300SD engine. One of the differences is a primary filter is installed ahead of the lift pump (between the lift pump and the fuel tank) and there is only one secondary fuel filter.

Pierce

Re: Fuel System Science Project

Reply #25 –
Quote from: Barry & Cindy –
Where does the last daisy chain of the injectors flow to?

I can answer this one!

On my engine (may be different on others), the drain from each injector feeds into a manifold that terminates at the inlet fitting for the secondary fuel filter, where it joins the supply line from the lift pump.  The two lines are attached with a double connection banjo bolt.  The drain manifold is constructed of metal tubing "T's", one for each injector.  The "T's" are connected by thin plastic tubing that is a simple press fit onto the metal tubing.  It all looks pretty flimsy to me, and is obviously not meant to contain any appreciable pressure.  I think the excess fuel basically just "runs downhill" until it gets to the filter inlet, where it rejoins the flow into the filter.  There is a one-way check valve (banjo bolt #21 in the second drawing) at the double connection, I suppose because line #6 is under pressure from the lift pump.  In the first drawing below, fuel line #6 comes off the lift pump to the secondary filter.  In the other drawing, drain manifold #20 terminates in a banjo fitting, which is stacked on top of #6 at the fuel filter inlet. 

(Photos from Cummins QuickServe Online)

Re: Fuel System Science Project

Reply #26 –
So I got my GlowShift gauge temporarily hooked up with wires run through the living area rather than fished underneath for a few days.

I was getting 9 psi with the stock overflow valve at idle.  After changing out the valve, it is between 15 and 17 depending on exactly what speed I set the idle at.  17 psi occurs at an idle of 725 rpm, any lower idle speed results in a fluctuation of 15-16 psi. 

These numbers are still low from what I've read on other Cummins truck forums, especially considering that this is supposed to be a higher pressure overflow valve than stock, and I'm trying to figure out if there is something that should be looked at.  I'm also going to pick up a cheap mechanical gauge to compare to the GlowShift reading.  I'll probably fill up a 5 gallon tank of diesel and hook directly to the injection pump to see if there is anything in the fuel tank line causing the pressure drop.

I'm going to see if there are any leaks in the lines that go into the transfer pump.  I do have a spot down on the right side of the injection pump, when looking at it from the side, that is a bit wet.  It appears to be a vent but I'll be investigating it tomorrow. 

Re: Fuel System Science Project

Reply #27 –
Robert,

I haven't had a chance to get my coach out for a test drive since I changed the overflow valve (and both filters).  I don't know yet if there will be any improvement in driving performance, but I'm hoping for some noticeable change.  One thing that did definitely improve is my cold startup action.  For the past several months I have noticed a degradation in cold starts - my engine would start immediately as soon as I turned the key, but after about a minute it would falter and die.  Then it would require 2 or 3 re-starts before it would finally run smoothly.  After that, no problem.  After I changed the filters and overflow valve, this behavior has completely disappeared - every start, hot or cold, is immediate and flawless.  Idle seems smoother, also.

As to your pressure concerns, here is what I have observed.  Right after startup (as soon as I see good oil pressure, I get out and walk to the back of the coach) my fuel pressure reads around 22-23 psig.  As the engine warms up, it gradually drops to around 18-19 psig at 750rpm, and holds steady there.  So allowing for the difference in our two gauges, we are probably pretty close on the readings at idle.  I have not checked mine at higher RPMs yet.  I do not have any leaks in my system, at least none that I can see.  I will be interested in hearing the results of your further investigation.

Did you install the Tork Tek adjustable overflow valve?  If so, you could try raising the fuel pressure, but be careful.  I have read warnings about going too high, and causing damage to injection pump.  I plan to leave mine alone for now, and see how it drives.  I'm REALLY happy with the improved startup performance!

Re: Fuel System Science Project

Reply #28 –
Service bulletins (& forums) on the diesel in your machine will point you in the right direction.

In my case (5.9L HO Dodge) after learning that lift pump issues were common 100%, I replaced the fuel lift system with a FASS HD (electric) fuel lift delivery system which ended this problem.

Injection pumps have a light duty fuel lift feature which if compromised w/low pressure from the lift pump,
stresses the injection pumps' separate lift feature.  If this goes away the injection pump will no longer function.
 
A gauge (fit in the injection pump) will tell how much lift (pump) pressure is getting to the injection pump.

A gauge on the low pressure side of your high pressure injection pump is money in the bank. 
If you have a starting problem.. (PLEASE) check the fuel system. 

Keep the agenda thing in mind.  Cummins, Caterpillar and ALL the other manufactures will be happy to repair fueling problems. pc



 


 

Re: Fuel System Science Project

Reply #29 –
Thanks, pc.  I'll keep that info in mind.

I got back from a drive to a local campground to dump my tanks which has provided more useful information.  I'll probably end up crashing the FT since I've been staring at the 3in1 gauge with EGT, boost, and fuel pressure nonstop and noting how the values change based on terrain.  I do have some slight doubts about the accuracy of the gauges due to almost 50' of 16 gauge wire but I'm going to perform some comparisons in the engine area, without all the extra wire, to see if there is any change.

The trip was at 500' above sea level and mostly flat land though there are a few longer hills.  It was slightly windy.

As noted, I'm idling at about 725 rpm and have approx. ~16psi fuel pressure.  During my trip (and assuming that the gauges are reading correctly with no voltage drop due to the length) I saw a maximum of 23psi of fuel pressure and a low of 7psi.  The high pressure of 23psi, along with the highest readings in general, occurred when I had just let off the gas after aggressive acceleration and it did not last long before dropping.  The lowest pressure, along with all of the other lowish readings, occurred during hard acceleration and high boost. 

The average cruising psi at highway speed at about 1800rpm was in the 10-14 range which seems lower than ideal from my research.  This is with a new lift pump from Cummins, new fuel lines, new and clean fuel filters, etc.  I did note that after changing to the TorkTek overflow valve that the engine has more power and is more responsive.  I hit a maximum of 22psi of boost during the trip and slightly over 1100 degrees of EGT measured from a port drilled in the exhaust manifold. 

There was an inverse relationship between fuel pressure and boost/EGT, when one went up-the other went down. 

I'm going to do some testing to see if there is any voltage drop causing incorrect readings and trying to see if I can find any leaks at all.  If I can't find any problems and the readings are correct, I may end up playing around with the adjustable overflow valve to see if I can bring the values up closer to the recommended values.

Edit- the higher pressure of the overflow valve also seems to have smoothed out my engine a bit and the addition of the additional aluminized insulation over the top of the old and worn out insulation made a rather larger difference in sound.

Re: Fuel System Science Project

Reply #30 –
After testing with the gauges temporarily hooked up back at the engine, I feel comfortable saying that whatever differences there may be due to the length of the wiring are very small, if any, and that my numbers from my earlier post are correct.

I clamped off the return fuel line a bit and the fuel pressure went up to the 30s prior to letting off the clamp so I feel comfortable that there aren't any issues with the transfer pump. 

I'll probably start playing around with the adjustment on the fuel overflow valve to see if I can get it up to 25psi in the next day or two. 

Re: Fuel System Science Project

Reply #31 –
Robert,

Be sure to post your results from increasing the fuel pressure.  The way I understand it, when the engine is under heavy load, the closer you can get to holding 30 psig at the injection pump inlet, the better off you are.  Supposedly, anything over that is either not beneficial (with a stock engine), or can be harmful. 

Re: Fuel System Science Project

Reply #32 –
Quote from: Barry & Cindy –
One electric fuel pump I am looking at, the installation instruction says to tie injector fuel return with injector pump fuel return, and I don't know what they are referring to.

Is this the FASS HD 150 unit, Barry?  From everything I hear that thing is LOUD. 

I definitely think that you should be the guinea pig for the electric fuel pumps and tell us how it all works out ;). 

Re: Fuel System Science Project

Reply #33 –
Quote from: rsihnhold –

As noted, I'm idling at about 725 rpm and have approx. ~16psi fuel pressure.  During my trip (and assuming that the gauges are reading correctly with no voltage drop due to the length) I saw a maximum of 23psi of fuel pressure and a low of 7psi.  The high pressure of 23psi, along with the highest readings in general, occurred when I had just let off the gas after aggressive acceleration and it did not last long before dropping.  The lowest pressure, along with all of the other lowish readings, occurred during hard acceleration and high boost. 

If your fuel pressure drops to 7 psi under high load, either the lift pump can't supply the volume or you have a restriction somewhere between the tank and the injection pump. Your fuel pressure from normal cruise to full throttle should see little fuel pressure change on the low pressure side of the injection pump.

We used an alternate fuel source for the injection pump and then had the lift pump fill a graduated container and measured it against factory specs. They may provide the liters/minute/rpm information online.

Pierce

Re: Fuel System Science Project

Reply #34 –
Quote from: pocketchange –
In my case (5.9L HO Dodge) after learning that lift pump issues were common 100%, I replaced the fuel lift system with a FASS HD (electric) fuel lift delivery system which ended this problem.

Do you have anything to say about the FASS, positive or negative?  Was it worth it?

Re: Fuel System Science Project

Reply #35 –
Quote from: rsihnhold –
Do you have anything to say about the FASS, positive or negative?  Was it worth it?

Pittsburg power mentioned a .4mpg improvement and a handful of power increase when I called them recently.

Good videos on their site.  Depends on how much air and/or water is in your fuel I guess

Re: Fuel System Science Project

Reply #36 –
So after my last trip to dump my tanks and with a bit more turning of the adjustable overflow valve, I'm currently at ~23-24psi at idle in neutral along with a drop in pressure at higher RPMs.  cajk reported that he has gotten similar readings.

I'm still getting a major drop at WOT under load but now I'm bottoming out at about 9psi rather than the 7psi I was getting before.  Most highway driving results in low teens fuel pressure. 

I'm not sure how much further this valve will let me turn it but I'm going to find out.  The Cummins manual says that the P7100 injection pump can take up to 50 psi. 

On a side note, the turbo intake v-clamp was not lined up quite right which was causing a bit less boost than I am getting now.  The correct and full connection actually looks slightly off, like it is crooked, and I found that the sealing o-ring had slipped out of its groove and was preventing a good seal.  I'm now up to the occasional flash of 24 psi and a continuous 22-23psi.

On a side, side note.  The new engine compartment insulation has made an enormous difference in keeping the heat and noise in the engine compartment and out of the bed/bedroom.  The rear air basically never kicked on during my trip and the bed didn't seem to be warm at all. 

Re: Fuel System Science Project

Reply #37 –
Trying to finish our engine compartment insulation this week. How about a photo of what you have done?

If you are dropping fuel pressure on the freeway and further on grades, I suspect you either have a restriction somewhere in the fuel supply or the lift pump is deficient.  Depends on what fuel pressure Cummins says you need at full load/max RPM.

Turbo boost would seem to be pretty good.

Pierce

Re: Fuel System Science Project

Reply #38 –
Quote from: Pierce & Gaylie Stewart –
Trying to finish our engine compartment insulation this week. How about a photo of what you have done?

If you are dropping fuel pressure on the freeway and further on grades, I suspect you either have a restriction somewhere in the fuel supply or the lift pump is deficient.  Depends on what fuel pressure Cummins says you need at full load/max RPM.

Pierce

I didn't do anything fancy at all with the engine compartment, just covered the old stuff with new insulation and aluminum tape like a few other people here have done like Barry and Cindy. 

It seems to be fairly well acknowledged around the Dodge Cummins 5.9 guys that the stock mechanical fuel pump can't keep the fuel pressure up under load.  Only real path to take is the electric pumps which I'm thinking more and more about though it doesn't sound like it is quite as critical for us with the P7100 pumps as it was with the slightly later VP44 pumps.

Re: Fuel System Science Project

Reply #39 –
Looks good and will keep the foam from falling down on the hot engine.

Sounds like you have done your homework pretty well.

Pierce