24v Hard start hot
 

The truck just had a new lift pump (airtex style) and injection pump installed when brought to me, I found the airtex pump to be bleeding fuel back to the tank and causing a hard start hot every time!, I replaced the airtex pump with a delphi style pump (oem) and solved the issue, or so i thought, two weeks later they bring the truck back to me and it does the same thing but only when it wants to, like driving to store and cutting off, going in getting a drink, come back out, hard start, but if you get it hot, cut the key off and crank right back up no problems, the fuel pressure is draining back to the tank obviously! does anyone know if the return check valve can cause fuel drain back to tank? any help would be greatly appreciated. :argh:

Sounds like an injection pump or as you said loss of prime, these trucks rarely suffer from loss of prime issues. Even less likely with intank pumps. Whats the fuel pressure at now, how many mL in 10 seconds, and with initial lift pump start up is there any air in the fuel ? A failed overflow would cause a low fuel pressure issue over a loss of prime

You talking about something other than your 96, obviously?

No it will not drain back because of the return on injection pump if that's what you're wondering. Do you have a fuel pressure gauge? Possible your getting air in the lines somewhere, maybe?

With the truck idling no load the pressure is about 12 psi, and wot no load about 7, i havent checked the flow yet, and this isnt an in tank pump, this one still has the block mounted pump, i just changed it from the cheap airtex style to the delphi oem style. How do i check to see if it is the overflow causing drainback other than replacing it?

You could remove it, and visually inspect it. Unless you can find a way to regulate 18psi towards it, it is a visual inspection or replacement part. If memory serves me correctly, it is less then $30.

I seriously doubt its the overflow. The more I read about your situation the more I feel confident you have a vp issue. To verify you could do a bucket test using clear tubing, and command the pump on with a scan tool or bump the starter, and view the first 30 seconds of fuel flow. We use a bit more advanced method but this would be a simple way to check without a expensive gauge set.

Agreed. Classic symptom of a cracked diaphragm is hard start when hot. You can try, and probably waste the money on a overflow valve, but I bet your issue is much larger.

It's would be easier to buy an overflow than confirm with the test I explained, that's for sure.
Our gauges have multiple inlets and a way to regulate the the output to the bucket. So with a gauge showing filter head pressure,(eliminating the line between the filter and vp, using the gauge manifold as fuel line) one at the vp, we verify we have equal pressure on both, lock the bucket line down and watch the pressure drop. With the bucket line being higher than the vp, and the filter side locked down, any overflow leak will be indicated in the level in the bucket line lowering. You effectively created a water (fuel in this case) level.
One more edit and I'm done I swear lol.
The erratic no start and as Mr Williams said the no start when hot is The true giveaway. The chances of intermittently having a loss of prime issue is extremely unlikely.

I thought the same myself when they initially brought the truck to me, but they had just had the vp and the lift pump replaced and this hard start hot issue didnt start until after the new pumps were installed, bad pump from reman possibly? :argh: :humm:

I've got a shop pump for that, eliminate the entire system and determine if you can recreate the issue again.

doesn't mean that the lift pump they put on was working very well could of killed the injection pump already. get rid of the stock style lift pump and put on a fass

Not my truck and its bone stock, i doubt they want to spend the money for a fass after spending 2k on injection pump and lift pump and paying me to fix a problem that created. I finally figured out that the truck cranks fine if you wait after turning on the key and waiting for the lift pump to run, but it takes 10-15 sec after key on when warm for lift pump to run, could i possibly have an ecm issue causing the lift pump to not run like its supposed to?

Are you bumping the key to start or just keying on? Was thread sealant used on the fuel line fittings at the suction side of lift pump

when i bump the key the lift pump runs like its supposed to if i just key on it takes 10-15 seconds to run, all this while engine warm, it has no issue starting cold. the suction side fittings are all banjo bolts and seals but new seals were used.

We had a customer install a lift pump a few months back and he didn't have seals so he found it resourceful to use thread sealant...
The next step would be to pressurize the tank with just a touch of shop air. Any suction side leaks will show up that way.
I would also use a scan tool and verify if its calling for lift pump at key on, and the pump is on when its called for. I hate to say what is normal operation, we have had issues where the pcm "pulses" the pump for 10 seconds and cuts off, yet after the truck starts, the pump operates normally. Graphing of the lift pump circuit showed the pulses and the computer showed it as desired.

Thanks a lot mysterync, are you associated with L&N Performance? because the truck in my signature i actually bought from a guy in boone and it had a L&N Performance sticker on the tailgate, off topic but just wandering. I will try to get ahold of a scan tool or drive it to work (Cummins Atlantic, Inc Charlotte, NC) and see if i can monitor the pump commanded versus the actual pump and let you know what i find.

Yep that's us. It looks like a truck we had some dealings with. Pm me the previous owners name and I'll let you know.

I dont remember the guys name, but the truck has 370 marine injectors that were installed when i bought it, it didnt have the stacks or the tow mirrors on it when i bought it though. could you do any thing with the VIN?

I think it was a guy named Jessie I know.

well i found out that the pump runs everytime the ecm commands it to, but i still dont understand why my pressure is dropping so fast when the engine is cut off. should these stock systems hold residual pressure for a certain amount of time?

There is a small hole in the pressure regulating valve that will allow fuel to escape all the time.

Yep, it won't hold pressure at all with pump off.

so do you think maybe the ecm is messed up and needs recalibrating to cause the lift pump to run every time the key is turned on instead of a 5-10 sec delay when the engine is warm? Im grasping at straws now, this thing has driven me crazy! :dang: :argh:

I seriously doubt that will help. In some cases systems were devised to prevent aftermarket pumps from running right off. The vp doesn't actually pressure right on start up. Not to mention it will start with 0 lbs on trucks with failed pusher pumps. If you can't isolate some type of drain back or loss of prime the next item would be the vp44.