OK, I changed the crankshaft position sensor and the fault code related to that hasn't returned. It's been about 2 weeks. And the problem of stalling occasionally also seems fixed as it hasn't stalled since replacing that. Since then the hard starting when warm continues, however it's not once been so bad whereas power is reduced after starting. So now although it starts hard, it runs fine once started. Fault code 1023 did return however.
Now I have a full tank of fuel and start further testing. I hooked up the durametric, erased the fault code and then set up to watch the fuel pump signals. Upon start, both activate. After about 5 seconds, fuel pump 2 signal drops to zero as expected. Truck continues to run and no additional faults. Pull fuse 14 (I believe this to be fuel pump 2) and it continues to run no issues. Since that pump isn't active anyway, I would expect that. I replace the fuse then pull fuse 13. Takes about 10 seconds and the truck dies. Watching the 2 fuel pump signals, it seems as it dies fuel pump 2 is activated but the truck still dies. So I leave fuse 13 out, restart. Both pumps get the signal to activate but since fuse 13 is still out it's obviously not running. The truck starts and as seems typical, the signal for pump 2 drops leaving pump 1 signal active. Again the truck dies after about 10 seconds and again pump 2 signal goes active but too late. When the motor stalls both signals are dropped.
Looking at faults, now I have 1632 "Fuel pump "B" control circuit/open - No signal/communication". Also have a 0230, "Fuel pump primary circuit - No signal/communication". So I clear these.
Now both fuse's in, restart. No faults. Pull fuse 14, no faults. Turn off the motor, restart and fault 0230 returns. Since fuse 13 is still in place the truck contuse to run. I put in fuse 14, then pull 13. After 10 seconds the truck dies and registers fault 2632. Code 1023 hasn't returned yet.
So it seems both pumps are active on starting, then pump 2 shuts off after about 5 seconds. This appears to be normal behavior. I would have thought though that pulling fuse 13 and killing pump one that at some point pump 2 would kick in and keep the truck running. It almost seems like that's how it should and want's to work but it kicks in too late - or perhaps that 2nd pump is the problem.
Any insight on this one from anyone?
I suppose the next logical test would be to check the fuel pressure. I don't have the kit yet and don't know yet where to plug in either. I believe I've seen this somewhere either on this forum or on rennlist. I know deilenberger ran into a similar situation and it ended up being the check valve that's part of the in tank fuel filter. Sure seems like a potential candidate, but - would or could that contribute to a fault 1023? I'm assuming possibly yes but am not sure. From what he was saying the check valve should maintain 4 bar for 10 minutes or so. How quickly it should or could fall after that is unknown. I know that if I restart within minutes after shutting down it will typically be a hard start. Same if it's been 30 minutes to an hour.
Jury is still out...
Does anyone know what the failure mode is on the HPFP? What fails only when warm?