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LS-one · 02-16-2025, 09:40 AM

I have a 2000 Monaco Diplomat ISC 315. Shortly after my purchase in 2017, I discovered the lift pump leaking and had it replaced. The coach had 42,000 miles on it. I now have 108,000 and have had no problems. I change my fuel filters very regularly.

A good friend of mine has a 2001 Dynasty 68,000 miles ISC 350. We were on our way to Quartzite mid January from Central California and he lost his injection pump. Yes his lift pump was bad. Luckily he was only 50 miles from home and 10 miles from the shop that did my lift pump. It’s been a five week process to get it repaired. No fault of the mechanic, waiting on the remanufactured pump from the company my friend ordered it from.

Is there anything I can do or have done to gauge the fitness of my injection pump?

Planning a pretty lengthy trip later this year?

He also suffered from the cracked exhaust manifold awhile back. That repair he did himself.

justcheckn · 02-16-2025, 10:27 AM

This is a CAPPS injection system.It’s inherent faults are the pump and transient suppressor. Replace the transient suppressor and install a FASS ind165g lift pump system.
MANY posts on the forum about this
Bobby in Colorado

jacwjames · 02-16-2025, 05:08 PM

Lift pump is a common cause of injection pump failure.

Cummins had a TSB on the lift pump having shops check for leaks. The TSB has the the replacement gasket listed, two kinds, there is a source on Ebay for these. I use to carry spares.

The problem is if the lift pump leaks during the ~30 seconds it runs, it will suck air when the engine is running. Air in the injection pump is what causes it to fail.

In 2020 I found my lift pump leaking, I tightened up the 3 bolts on top of the lift pump and that stopped the leak. Took my time and researched the FASS lift pump install and decided that was the way to go. So I bought one and installed, also put a pressure sensor on top of the secondary filter and the put a gauge on the dash.

I mounted the FASS pump at the Fuel Tank passenger side, this made the electrical easy since the wiring harness was long enough to reach the drivers side electrical bay. The plumbing was also pretty easy.

Bypassed the old lift pump and installed the fooler relay so the ECM think the lift pump is still there.

Turn the key, hear the FASS pump running, pressure gauge goes to 16 psi and pretty much stays there unless pulling a steep grade, it may drop to 13psi but recovers to 16 psi,

LS-one · 02-17-2025, 09:48 AM

Thanks for the replies. Maybe I didn’t ask the right question. Since my own lift pump failure in 2019, I’ve done extensive reading on lift pumps, gasket replacements, Fass pumps, air leaks, injection pump cavitation.

I actually stopped by the Fass booth at Quartzite twice this year and both times the booth was unmanned.

What I am wanting to know is are there any diagnostic tests that could done to determine if my injection pump is in good shape? I am trying to be preemptive rather than coasting to the side of the freeway with a failed injection pump.

Alpine36 · 02-17-2025, 11:22 AM

I don’t know of any preemptive diagnostic tests on a CAPS pump, aside from scanning for codes. Every CAPS failure I’ve read about started with occasional hard starting, maybe rough running or low on power for a bit and then becoming a pedestrian in short order. My only recommendations (aside from a FASS/AirDog install being first and foremost) would be to change fuel filters at every oil change and use a lubricity additive such as Power Service. Keep an eye out for any small fuel leaks and address them immediately. One small fuel leak or partially restricted fuel filter will eventually ruin your whole RV experience and deplete your maintenance fund. Also a good idea to change out that old Transient Suppresser as preventative maintenance. It’s a classic example of pay for preventative maintenance now, or gamble and pay a whole lot more if or when the CAPS pump heads south. I think you know where the smart money is…

Scepter40 · 02-17-2025, 01:52 PM

The only other way to determine if the CAPS pump is doing it's job is a fuel pressure gauge (that I can think of). Before and after the pump if that's possible. Folks with more experience would know if that's possible. I have a vague recollection of someone having a fuel pressure gauge here.

I don't know what the expected pressures would be, but that's an old trick for the 12 Valve Cummins motors. On those we just plumbed them into the side of the injection pump.

jacwjames · 02-18-2025, 08:47 AM

There is a procedure in the maintenance manual for my 8.3ISC. I downloaded the whole manual ~10 years ago from the Cummins Quickserve site. They don't give you that option but you can search and find the info.

The minimum pressure for the lift up is 5psi. So you could install a gauge on top of the secondary filter to monitor. Someone would either have to turn the key while you monitor and/or you turn the key and quickly get back to the rear of the coach and watch it as the lift pump runs.

The manual also lists a minimum flow rate, but can't remember what it was, it is pretty small amount

LS-one · 02-24-2025, 08:52 AM

Thanks for all the replies.

Skip426 · 02-24-2025, 09:15 AM

The electric fuel pump on the Cummins ISC with CAPS injection pump , operates only as a primer pump approx.30 seconds from ignition key on , then a mechanical portion of the CAPS injector pump takes over when the engine starts .

The gasket that was the problem on the electric pump then allowed air into the CAPS system and contributed to the injector pump failure .