Cummins ISC lift pump run cycle

[LAW6619]

Hello everyone,
Does anyone know how long the lift pump should run once the key is turned on? I have a 2001 Monaco Diplomat with Cummins 8.3 liter ISC engine. Typically, I turn the ignition key and let the lift pump run until it stops before turning the key all the way to start the motor. I'm not certain, but it seems the length of time it takes for the lift pump to stop running is increasing it time. The lift pump runs for 60 seconds and shuts off, then I crank the engine and it typically starts right up. As I mentioned, it seems the time it runs has lengthened and I want make sure the pump is not starting to fail. I'll monitor this closer now but I'm just curious if anyone else sees the same 60 second run time and if that is correct. Does anyone also know if the ECM senses pressure to the fuel system thus controlling the time the lift pump run time until it builds to a certain pressure or does the lift pump run solely based on a fixed, timed 12 volts applied to the pump when the key is turned?
Thank you for your input.

[jacwjames]

Not sure that the ECM senses pressure.



I've heard different times on the lift pump, mine use to run 30-45 seconds. I say use to because my pump started to leak so I abandoned it and installed a FASS pump. It runs continuously.



The lift pump is prone to failure and leaking. It will leak when it cycles on offf BUT then will suck air when the engine is running. This is BAD for the CAPS pump and potentially cause it's failure. A CAPS pump replacement is ~$5K. The FASS pump coast me ~$1K to install vs a new lift pump of ~$400. So I bite the bullet when my lift pump started to leak in 2001


I was aware of the problem. Cummins actually put out a TSB on the issue. Initially Cummins would replace the lift pump when they found it leaking but then put out the TSB (see attached) and let owners take care of it. The TSB does list gaskets based on what lift pump you have, there is a seller on EBAY.



I knew of the TSB and potential for leak so I kept an eye on it. In 2020 I serviced my rig, no leak present, then took it for a drive. Parked it and decided to clean/lube my jack cylinder stems and when I crawled under the coach I heard a drip hit the muffler and sizzle. Yup, transfer pump leaked. I tightened the 3 bolts on top and it stopped the leak. But then decided to just go ahead and install the FASS and bypass the lift pump.



So check your lift pump for leaking, you can access it from the bedroom hatch.

[Alpine36]

There is no fuel pressure sensor controlling the lift pump duration, at least that I’m aware of anyhow. The lift pumps cycles for a pre programmed amount of time by the ECM, usually somewhere around 45 seconds. I do know the ECM monitors the fuel pump electrical internal resistance. That is to say if the fuel pump is faulty (electronically inoperative) or disconnected it will turn on the check engine light. If you upgrade your fuel pump with a Fass or Airdog system (and you really should consider that) there is a work around to bypass the OEM lift pump circuit by adding a “tricker” relay in place of the OEM lift pump. This keeps the check engine light off, while the Fass pump supplies a constant 15-17 psi pressure in the fuel supply line to the CAPS pump at all times. Much better than operating in a negative pressure, thus saving the CAPS pump from cavitation damage due to any small leak or partially restricted fuel filter. There’s a ton of posts here regarding the benefits and installation of these systems.
Money well spent IMHO.

[dons2346]

As said, the pump runs for about 45 seconds. If it is not leaking, I would not worry about the extra 15 seconds.
You said it "seems" like the time is increasing. With all the clicks and other noises you might be overthinking the situation.

[LAW6619]

Thank you for the input. At this time my lift pump is not leaking and I'm not having any starting or run problems. I just wanted to make sure it wasn't possible for the run time on the lift pump to be lengthened due to low fuel pressure. Sounds like I'm alright for now but I'll definitely keep a close eye on the left pump for leaks. While reading some of the other posts about the FASS pump upgrades I did see that it's possible to install a fuel gauge on the OEM left pump manifold which is interesting to me. Does anyone know what the pressure should be at this point with a factory lift pump? This would allow you to easily monitor the pressure and possibly see any abnormalities before damaging the CAPS pump.

[Ozzsp250]

Law6619.


I do not know the pressure of normal lift pump, however you would need a transmitter that reads positive and negative pressure. When the lift pump shuts off and the mechanical pump is operating [engine running] it draws fuel from the tank at around negative (-) 10 psi.

[jacwjames]

While lift pump is running you will see pressure. But while engine is running it is actually pulling fuel through the filter so you will see a (-) pressure. That is why it's important not to have a leaking lift pump, it will allow air to be sucked in as the engine is running. Air is BAD for the injection pump.



That's one of the benefits of a FASS, it runs all the time. It is preset to pump about 16psi to the filters/injection pump. So when I turn the key I see 16 psi on the gauge and after I start the engine I see the same thing. Normal running it will show 16 psi but under heavy load it may drop to ~12-13psi.



The fass pump also does continual polishing of the fuel. It will cycle the fuel ~7-8 times between fillups. First time I change the filters on the FASS I found this in the primary FASS filter. See picture.

[Skip426]

On the ISC with CAPS ; the electric fuel pump is a primer pump , and has a timed cycle ( approx. 30 secs ) controlled by the ECM from ignition key on.
Unlike the ISB of the same vintage the pump doesn't operate once the engine is running .

[ArtJoyce]

Best way to determine if the pump is failing is with a fuel pressure test. I believe the time it rusis a time function.

[LAW6619]

From another post I read, I believe it was imnprsd that suggested use of a positive/negative, oil filled gauge which could be piped into the OEM lift pump manifold using one of the 10mm ports. I thought this was a great idea and could be used to monitor the start pressure or negative running pressure to help detect a leak (lower negative pressure while running). Knowing where the pressure should be, you could compare the pressure before each trip to confirm it's correct. I do like the idea of scrubbing the fuel but hopefully your fuel filters will do their job before anything gets to the injector pump. Anybody see any problems with this concept?

[jacwjames]

I suggest you create an account on the Cummins Quickserve site, it is free.



https://quickserve.cummins.com/qs3/p...ice/index.html


Once you create an account register your engine using the SN.


From there you will have access to the maintenance manuals and parts. Look under the Service tab to find the manuals. It may take you a while to figure out how to use the site but well worth the effort.



I did a quick search and it shows the minimum lift pump pressure is 5 psi. Also has a minimum flow rate but it isn't much.

[Ozzsp250]

LAW6619.

I elected to add 2 gauges, one pump outlet, the second at the secondary filter outlet, that read (-) to (+) values. These can show pressure drop across the secondary filter. I also have a fuel pressure gauge in cockpit [transmitter located in the secondary filter outlet] but it only reads (+) value. In the event of a FASS pump failure the pressure drops to zero.

[Alpine36]

Originally Posted by LAW6619

From another post I read, I believe it was imnprsd that suggested use of a positive/negative, oil filled gauge which could be piped into the OEM lift pump manifold using one of the 10mm ports. I thought this was a great idea and could be used to monitor the start pressure or negative running pressure to help detect a leak (lower negative pressure while running). Knowing where the pressure should be, you could compare the pressure before each trip to confirm it's correct. I do like the idea of scrubbing the fuel but hopefully your fuel filters will do their job before anything gets to the injector pump. Anybody see any problems with this concept?

Nothing wrong with that concept at all.

The major issues with the CAPS system running in a negative pressure fuel supply line is fuel cavitation. One small leak anywhere and air is introduced to the CAPS pump, starving it of crucial lubrication. One partially restricted fuel filter and it is also starved for fuel. Think of it this way, your sucking air thru a straw 30’ long. On small crack or restriction and it’s a big problem. It’s something the Cummins and Coach designers overlooked. This really wasn’t an issue on straight truck chassis because the fuel tanks were typically much closer to the CAPS pump.
So the $5000 question is it worth the investment for a $1000 fuel system upgrade? I think so.

[LAW6619]

All good points to consider. Thank you once again for all the valued support.