Warm Start stumble/multiple attempts... where to start looking?

2014 Yukon SLT 4x4 183,*** miles... 5.3 swapped to 6.0 with BBP tune, 50lb. injectors, Low Buck truck cam (truck norris clone). (I've already messaged the tuner, but looking for more thoughts/opinions here)

I've been running my current tune for a couple months now, and there has been an ongoing issue with hard starts AFTER the engine is warmed up, or has been running for a while, parked for a few mins, then restarted. Sometimes it won't start at all... just stumbles at low rpms, then dies.

Cold starts are rock-solid in any temperature.

I've been trying to ensure that there has been plenty of adaptation time after swapping in new O2 sensors & resetting ECU, but I feel like it should start fine no matter how cold or warm the engine is.

Definitely sounds like something we need to address with the tuner.

On a side note, your coolant temperature gauge operating correctly? The ECM uses that as well as ambient temperature in its calculations for fuel mixture..... Thought I'd throw that out there in case your temperature sending unit is acting up. Even your mass air flow sensor checks incoming air temperature.

Hook up a fuel pressure gauge. It was pretty common on the older vehicles for this to happen and usually was fuel leakage into cylinders after shut off (injectors) or FPR returning fuel into tank and causing low pressure on next start-up.

Not sure if the second case if common at all now with return-less fuel system? IDK.

In the first case, the engine basically floods next time you start it warm. To check for that, press the gas pedal all the way down and that will momentarily cut off fuel (clear flood mode.) Then see if it fires up.

I have noticed fuel pressure rising after shutting down.. Gets upto around 100psi if engine is warmed up and you shut it down.

Also notice this table, I have NO IDEA if it's related to your issue or not.. And I don't see the stage 2 table anywhere either..

definitely check the fuel pressure with a mechanical gauge if you can, to make sure it's correct.

and then I agree it's on your tuner to adjust. that hot heat soak restart can be difficult to adjust. lots of tables vaguely described tables in play. also your 50lb injectors, what brand are they and do you have good injector data for them? or I guess does your tuner's have good data.

some of these ecm's even do a bunch of math to come up with an injector nozzle temp vs flow rate. usually oem gm injectors that you can find a os from and copy over all the data is the best. or that high end expensive aftermarket ones come with good data sheets.

rdezs said:

Definitely sounds like something we need to address with the tuner.

On a side note, your coolant temperature gauge operating correctly? The ECM uses that as well as ambient temperature in its calculations for fuel mixture..... Thought I'd throw that out there in case your temperature sending unit is acting up. Even your mass air flow sensor checks incoming air temperature.

Click to expand...

Coolant temp sensor is good, brand new as of the summertime, and working fine, as far as I can tell.

strutaeng said:

Hook up a fuel pressure gauge. It was pretty common on the older vehicles for this to happen and usually was fuel leakage into cylinders after shut off (injectors) or FPR returning fuel into tank and causing low pressure on next start-up.

Not sure if the second case if common at all now with return-less fuel system? IDK.

In the first case, the engine basically floods next time you start it warm. To check for that, press the gas pedal all the way down and that will momentarily cut off fuel (clear flood mode.) Then see if it fires up.

Click to expand...

When you suggest pressing the gas-pedal to the floor, is that right before I turn the key to start?

j91z28d1 said:

definitely check the fuel pressure with a mechanical gauge if you can, to make sure it's correct.

and then I agree it's on your tuner to adjust. that hot heat soak restart can be difficult to adjust. lots of tables vaguely described tables in play. also your 50lb injectors, what brand are they and do you have good injector data for them? or I guess does your tuner's have good data.

some of these ecm's even do a bunch of math to come up with an injector nozzle temp vs flow rate. usually oem gm injectors that you can find a os from and copy over all the data is the best. or that high end expensive aftermarket ones come with good data sheets.

Click to expand...

The tuner provided me with a part# to search for, and I linked them the product page so they knew exactly which brand.

Charlie207 said:

When you suggest pressing the gas-pedal to the floor, is that right before I turn the key to start?

Click to expand...

While cranking. Gas pedal all the way in. It actually "shouldn't" fire, until you release the gas pedal. Sounds odd, but that's how it it's supposed to work.

Did I read that right, fuel pressure rises to 100 PSI? You're lucky if the only thing leaking is the injectors into the cylinders! If that fuel pressure reading is correct, verified with the mechanical gauge off the fuel rail, I'm thinking it's time to drop the tank and change out the fuel pump. Before one of your quick disconnect fittings pops off right on the exhaust pipes