Cummins 8.3 Caps Fuel System

[Lane Stefano]

Ihave a mx 240 with about 4500 hrs on it and its starting to throw codes that signal a CAPS fuel system failure in the future (at least that's what folks are telling me). Anyone replaced one of these lately? Cost? I need some direction on where you guys are getting them redone or go to a p pump conversion. Give me some direction. The tractor still runs and seems to start good. The codes were:

How many hours are you getting on these rebuilt CAPS form The Diesel Store. I hear nightmares about getting less than 1000 hrs on them before replacing again. Is there a filter in the tank of these tractors that should be changed? Cant help but wonder if there are things we could do to the tractor itself to get these things to last longer. Better charge pumps etc..

We have put 3-4k hours on a MX 270 and never did anything besides change the filters for preventative maintenance. They don't like being starved for fuel. Now I'm sure it will fail now that I said this ....

We had a MX285 throwing those same codes...ended up needing fuel filters changed. The other thing to check is the electric fuel pump. Ours had 4200 hrs on the original CAPS pump and still going. We just traded it off and I would not have been afraid of it. Key to those pumps is keeping clean fuel in it, change the filters(inline and spin on), before you start it turn the key on and let it pump for 30 secs then start, and make sure the electric pump is good.

Thanks all. We needed a bigger tractor and the low houred 72-89 series are just too expensive. So, with some reluctance, I bought this. I really like it so far. Reliability seems to be a issue. I tend to think in a lot of ways, all the newer stuff isnt as reliable as the old. Can we buy the software to fix these things yet? I wish they would put the MX cab on a 8940. It would make the perfect tractor.

First things first, change the fuel filters(especially the one under the LH front corner of the cab that is the same inline filters the classic Magnums). See if this gets rid of your 329/456 fault codes....they are usually caused by fuel restriction or air in the fuel system.

You might want to look at where the fuel comes out of the LH fuel tank down at the bottom of the tank....make sure that suction hose has no bends, kinks, etc. in this area. Next, follow that suction line up to the top of the fuel tank. Have seen this line get pinched between fuel tank and side of the transmission case....especially if the tractor has ever been split for transmisssion service/repair.

Also, look at the electric fuel pump itself....see if there are any signs of fuel leakage there. You will see that that fuel pump is held together by 3 small screws. They tend to loosen up over time, causing air entering the fuel system. There is a gasket where those 3 screws hold the 2 halves together that can get damaged, allowing air to enter.

After some intense research I have decided to modify the existing fuel system design to eliminate the root cause for the CAPS fuel injection pump failure which is low fuel pressure to the fuel injection pump. I spoke to a RV repair facility that has done several of these, the longest running unit has been out for over 7 years and has accumulated just shy of 75,000 miles with no problems. This unit had two previous CAPS pump failures, none since. After doing my research speaking to one ISL owner that modified his I think I am going to tackle this for insurance. Here is my reason; the factory design of the Fuel injection pump pulling/sucking fuel from the tank 40' in front of it through a pump that shuts off after 30 seconds and through two filters is crazy, it strains the pump and it ultimately starves itself causing it to self destruct.

The conversion will give the system a pump close to the fuel tank that constantly runs supplying filtered fuel to the CAPS injection pump so it doesn't starve and will always have fuel coming at it when the ignition is on. My plan is to relocate my primary filter/separator in the fuel tank bay with the pump mounted just after it, pushing fuel down to the engine's secondary filter then onto the CAPS injection pump. All returned fuel will remain as designed by Cummins and be routed from the Cylinder head and CAPS pump back to the tank via the Cummins lift pump that will be there only to serve as a manifold where the returned fuel all meets up to go back to the fuel tank. The factory lift pump will be capped where the original fuel supply lines connect (so the returned fuel does not leak out and its all sealed up) the electrical connection get's unplugged, the connector gets a relay plugged into it so the ECM see's a draw, assumes its the lift pump and doesn't set a fault code or turn the engine light on, I am remote mounting the relay in my rear electrical bay just as if the factory put it there. I will incorporate a electronic fuel gauge into the design for my coach and put it in the dash as if it came that way from Holiday Rambler so I can monitor fuel pressure, when it drops I will know its time to spin new filters on, no more guessing or waiting for it to starve the pump and begin to buck. All parts are on order except any hoses I will have to make up for the installation so far the tally is at $750.00 this will be the most cost effective insurance policy yet for this coach.

CAPS design; ISL,ISM,ISX the lift pump shuts off after 30 seconds (just to prime the system) and relies on the injection pump suction to feed itself with this design you cannot install an aftermarket warning system to alert you of a problem since you only have fuel pressure for the first 30 seconds. There is no ECM strategy to turn the pump on when fuel volume is low, fuel flow is not monitored, in fact the only item monitored is the lift pump if there is an electrical draw, if its working or not it has no idea, it just will cause the $5500.00 injection pump (parts only not labor) to self destruct.

ISB and ISC; the ECM will regulate the lift pump and send fuel to the Injection pump as needed. The ECM on these will often not supply enough fuel to the injection pump causing them to fail or the lift pump just fails and you might not realize it since the injection pump will also pull fuel to itself, just not enough. BD diesel makes a kit with a dash mounted alarm incase fuel pressure drops below 5 psi, so you have a chance in saving yourself the cost of an injection pump replacement.

You have done a lot of research. Here are some "thinking out loud" things. Please do NOT take them as "Brett knows this is right/will work." I have NOT researched the issue, but agree with you, fuel system on DP do leave something to be desired.

Brett, thank you for the response and I understand, I am glad you responded! When I spoke to Cummins and asked them if I could send positive pressure into the existing lift pump they told me absolutely not. Something is weak inside, they wouldn't elude to what but the weak link is on the inlet side (I pried), I believe the check ball is plastic and will break apart and go through to the next filter. I would love to see a cut away of that pump.

The CAPS pump will accept up to 25 PSI (per Cummins), what it doesn't need it will return to the tank with no problems. The pump I ordered is the same one the RV dealer I spoke to swears by (I didn't want to be the test pigeon), its adjustable and the factory setting is 17PSI. I spoke to an injection pump rebuilder that is local, he bench tests the CAPS pumps (on the larger engines) at 25 PSI for an integrity test. Since the factory lift pump supplies 7 psi and the CAPS needs 10 minimum for proper lube and cooling...... which baffles me since there is no PSI coming to it now, its just volume that no one including Cummins has a spec on record.

I thought about building a manifold to connect the return lines (one from the head, one from the CAPS pump and the 1/2" hose that actually feeds the tank) but the more I thought about it...I already have it. So two 5/8 JIC female plugs for the lines and I am done. Since I changed that pump in 2014 those were the two that were actually simple to access, when I pull my floor they are right there.

I think you are good to go. I think the factory setting is right about where I would set it. The excess gets re routed to the tank. Having enough fuel and above the minimum pressure will add to reliability.

Brett, that's my fear. After speaking to the dealer that has done a few coaches and a bunch of Dodge Rams I felt better. They were kind enough to share their experiences with me, good and bad. They have only completed two ISL's several ISM's and a bunch of ISC and ISB coaches. None of theirs relocated the pump close to the tank on a DP. Apparently I'm fortunate to have space to relocate the separator and the pump down stream and have open access to the fuel tank. Fass recommend the placement of the pump as close to the tank as possible which on a Monaco works out since the next compartment forward is my power distribution. locating a ignition circuit should be simple, plus I have a bunch of open fuse spaces for adding circuits so I can make it look like it belongs.

Bill I'm shocked how many Rams have had this done and how many companies make kits to do it on those. Most include performance upgrades which I have no interest in. I just want it to stay together and go down the road.

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