Intermittent stall after crank when hot

[johnmyster]

I know what you're thinking. It's loosing prime. Rebuild filter housing and/or check fuel hoses, put a FASS on it. Stay with me.

1) Dad owned truck for 3 years. He removed the factory fuel filter assembly and put a BMP "lift pump" filter housing on it. Never experienced this he says. He towed a ~10k 5th wheel camper. (He also doesn't live near much elevation change.)
2) We got truck a year ago. No issues observed.
3) June, we bought a ~9k 5th wheel camper. I replaced every coolant line, fuel line, and hydraulic line under the hood to solve some weeping fuel lines in the valley. July 4th, took first trip with camper. Headed into the hills on a hot day with AC on max. No issues.
4) Got home, changed fuel filter as PM.
5) Very next trip. Arrived at campground. Backed into spot. Unhooked camper. 15 minutes later, restart, truck cranks, runs for a few seconds, and stalls before I can put it into gear. I figured residual air in the filter. Lift pump was running. I purged from test port and got it to start.
6) Came home, did the same thing after we unloaded. (Probably 90 minutes of heat soak time.)
7) Subsequent camping trips, same thing on hot restarts. Starts, runs for a few seconds, and stalls before I can put it into gear. Eventually it will restart. Sometimes right away. Sometimes after a few tries.

Casual observation is that I didn't get the new fuel filter on right, and it's sucking in air. Read on.

Additional info:

• Never does it unless camper is involved. This summer it was like clockwork.
• I did errands and did short trips when it was still summer (hot out), and could not duplicate. No stalling and steady rail pressure at idle. Only after towing.
• It's not my daily driver. I can hot park the truck for a week and it'll crank right up. (Leads me to believe this is not a prime issue.)
• Before/during/after the hot stall, I notice engine tone will oscillate, observable on rail pressure readout on scangauge if I look fast enough. Once driven a bit, oscillation goes away.
• Late fall trips (not as hot) it hasn't stalled on hot restarts. Instead, I might observe the rail pressure oscillation but engine will manage to stay alive and get past it. Stab the throttle or drive it a bit, oscillation goes away.
• We can hit the hills and get it hot. If I let it idle while unhitching and let it sit overnight (complete cool off) it'll crank and idle smooth.

Truck has what I assume is stock injectors, stock pump, with a bunch of miles and a 5 position switch. Other than towing I drive it easy but I've never seen a P0087. CP3 seems to keep up with WOT pulls.

My theory is that the fuel pressure regulator is in the earliest stages of going bad. It takes lots of heat in the CP3 to cause it to act up. When hot, the plunger doesn't move easily and causes the typical oscillation of a bad CP3 FPR. Somehow it's fine when running, but shut it off and restart, it oscillates and hangs so bad it dumps rail pressure and stalls. New FPR should resolve?

Am I on the right track?

 

[2004LB7]

Bad regulators typically cause hunting or looping when bad. Fuel pressure will fluctuate during idle but not really anywhere else.

I'm going to guess it has something to do with the fuel getting hotter and possibly thinning out. The LB7 was known for having excessive fuel return with worn injectors when hot. Not sure how the LBZ was affected with the same issue

How much pressure is your lift pump making at idle?

 

[johnmyster]

Bad regulators typically cause hunting or looping when bad. Fuel pressure will fluctuate during idle but not really anywhere else.

I'm going to guess it has something to do with the fuel getting hotter and possibly thinning out. The LB7 was known for having excessive fuel return with worn injectors when hot. Not sure how the LBZ was affected with the same issue

How much pressure is your lift pump making at idle?

I haven't had a gauge on it. I believe the BMP uses a style of pump that can't generate much pressure. It's enough to squirt from the test port but the system doesn't have a regulator in it like the FASS/AD does. I can see if I have a low pressure gauge around and get a hose barb on it. I know what you're thinking - an aggressive lift pump can cause hunting, but I'd expect that hot or cold.

The hunt/oscillate/stall pattern when it happens is what clued me towards the FPR. It only happening when hot and only during/at startup is the weird thing.

I'll pull into the campground with a smooth idle and steady 4300 psi of rail pressure. (IIRC, I think that's warm idle rail pressure. I know cold is higher.) At that point I will have driven easy for some of miles and idled during check-in, leveling the camper, etc. I'd think heat soak in the heads after shutoff would be minimal. 10 minutes later or two hours later, it'll fall on its face on restart. But not at the hardware store, or at the barber shop, or at the gas station. The only thing I can see that is different is that my return fuel temps will be significantly higher on tow days than on lighter duty days. But still, it will idle fine at the stoplight, truck stop, etc.

FWIW, there's a FPR sitting on my workbench, waiting to go in.