I've been having this issue for about 7 months (started in December 2019) and have poured tons of time and now money into figuring it out with no solution. When my truck gets up to temp, and especially after highway trips, I have a problem starting it up after shutdown. Sometimes it won't start at all and I have to crank for over a minute (in intervals) or pop the hood and let the engine bay cool down for it to start. Usually it takes about 20-30 minutes to pop up after the truck is warm and its more prevalent now in the summer months. I've replaced FCA, transfer/gear pump on the back of the CP3, injectors, fuel pressure sensor, rail pressure relief valve, and maybe some other little parts I'm missing. Filters currently have about 7500 miles on them and I'm running an aux. cat 2 micron/baldwin separator in addition to the stock filter. I've run return tests to the death but it seems like everyone from my manual to people on the internet have their own opinion on how to run the test/what the real parameters are so I don't really consider it anymore. In tank pump is still providing fuel to the CP3 and I've verified that at all temps. Honestly I've done everything short of replacing the CP3 and the rail. I'm about to give up on this truck because I can't keep having this problem pop up when i'm out on a job site or need to get somewhere. Does anyone have any clues as to what the heck is happening?

Edit: this only happens when the truck has gotten up to temp or it's really hot outside (heat index of 100+). In the morning, it starts fine, no issue.

 

Mine will do this on occasion as well, I have found on mine if it doesn't start right away to turn the key off then back on and it will start. Like you I have spent a ton of money and time on this and so far nothing has helped .. It is a 05 with a manual tranny and all stock

 

You didn't mention what year RAM or how many miles, but probably doesn't help much. You might want to look at this thread where the OP replaced the CP3 and all seems well again.
Back to morning brew . . .

 

Its an 07 with 428k. I'm starting to think that may be it. I've tested the pump flow, but only when cold, never when the truck was warm, so I might do that. Same goes for the lift pump.

 

Is it a fuel issue or a starter issue? Starter not turning the engine over at the needed rpm speed to fire off..............I have seen older semi trucks where the starter and such would get heat soaked after a shut down in high heat and then turn over slower then needed and not start til they cool enough to spin fast enough to fire..Easier to let em idle. Also is the engine temperature at a good level and its simply not running hot and when turned off it gets too hot and not allowing engine to be turned over at proper start speed. Kinda a slight seized condition


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Your issue of trouble starting when hot, rather than cold, is contrary to most hard/no start common rails.
Most issues are from low rail fuel pressure when cold.

Everyone with a common rail should monitoring low side and high side fuel pressures. This is critical for these trucks. For troubleshooting, for performance, for tuning, etc, etc.
These gauges are worth every penny.

Confirm that the lift pump fuel pressure is adequate when the truck wont start.
Confirm that the fuel rail pressure is adequate when cranking the engine over and it wont start.

You never mentioned DTC's but I hope you have checked for these. Plus battery voltage when the truck wont start.

 

I don't think its a starter issue, truck spins over at 150rpm no problem.

Both batteries are ~12.6V and no DTC's.

I have a rail pressure gauge, but it only reads pressure once the engine is running. I might see if I can hotwire it to read while it cranks though. Truck also has a fuel filter minder which should flash the WIF light if there is low suction pressure coming into the CP3, but I also have a mechanical fuel pressure gauge on order that I'll splice into the CP3 suction line after the factory filter.

Just got home from a day of site visits and long driving and tested my CP3 flow to the rail and I'm at 50mL/10sec which is well above the minimum of 90mL/30sec that my manual states is required.

The only other thing I can think of is my FCA which I've replaced twice now. the one that came with the truck is a knockoff Chinese unit (probably) with no markings on it. The other two are Bosch units (one from ebay and one from Genos). When I connect HPtuners, it shows a desired FCA duty cycle of 35-40% at idle, but the knockoff unit will read a ~1600% DC and the two bosch ones will read ~3-6% DC. Haven't been able to find anything online about FCA duty cycles but that seems suspicious to me.

 

Here's a picture of what's happening with the FCA. This is about an hour and a half after the truck shut off.

 

Ok, so I think its solved. Ended up giving Todd at T&C Diesel a call and after a long conversation about the symptoms and what I've done so far, he suggested replacing the CP3. Took a gamble and so far it seems to have solved the issue. I have to take the truck out of town tomorrow so that will be the real test, but so far, none of the issues that I was having earlier

 

Not saying this is or was your problem, I suffered the same "no start when hot" with my 92 D250. Mechanic said that my VE infector pump was shot due to burning to much bio=fuel. He filled a two gallon container of water and told me to pour it on the pump. This worked! I finally located a rebuilt pump and installed it.. No more no starting problems, starts with just a bump on the ignition swich.

 

Not sure about the bio fuel. I talked to the guy i bought the truck from (friend of mine) and he said he originally bought it as a flood truck that the owner didn't wan to salvage. He rebuilt the engine, but kept the original cp3 so I'm suspecting that some water may have entered the cp3 at some point and it was a gradual decline