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

[Charlie207]

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.

 

[rdezs]

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.

 

[strutaeng]

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.

 

[mikez71]

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..

 

[j91z28d1]

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.

 

[Charlie207]

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.

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

 

[Charlie207]

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.

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

 

[Charlie207]

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.

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

Delphi GM 50lb/hr LS Truck Flex Fuel Injectors 12613412-8

Find Delphi GM 50lb/hr LS Truck Flex Fuel Injectors 12613412-8 at LSXceleration with Free shipping on qualifying orders!

www.lsxceleration.com

 

[strutaeng]

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

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.

 

[rdezs]

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

 

[mikez71]

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

Yes, after shutoff the heatsoak causes it to go up. Think it's normal.

 

[rdezs]

Wife got home with the 2014 Escalade, she has a 20 minute commute at highway speeds. Did the fuel pump about 9 months ago, an ACDelco unit. It was 58 PSI idling. Turned it off and crept up to 63 PSI, never went higher.

I'm not 100% sure, but I don't think your fuel system is rated for 100 PSI..... No idea what the safe limit is though.

 

[j91z28d1]

I've seen mine high, but I don't remember exactly what.

 

[j91z28d1]

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

Delphi GM 50lb/hr LS Truck Flex Fuel Injectors 12613412-8

Find Delphi GM 50lb/hr LS Truck Flex Fuel Injectors 12613412-8 at LSXceleration with Free shipping on qualifying orders!

www.lsxceleration.com

that's good. I think you just need a touch up to that part of the tune.

atleast I hope so.

 

[mikez71]

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

I may have exaggerated. I only find a couple logs where it climbs to 82psi..
That was only a a 60 degree day, but the pressure does seem to plateau right there.
Perhaps the fuel is leaking past the pump, or some pressure relief valve, back to the tank?

Took about 2 to 2.5 minutes from shutdown.. ECT was 194 degF on shutdown for both.
(sorry the 2 graphs are zoomed different amounts, top one took 2.5 minutes, bottom was 2 minutes)

 

[rdezs]

That still seems rather high. I'd check with a mechanical gauge on the fuel rail to verify.... Even 82 PSI raises safety questions. GM used to have a return line on the ls engines which used a pressure regulator. I'm thinking they did away with that around 2005. After that I believe the pressure was regulated by the fuel pump module in the tank. Normally when the pressure rapidly drops after shutdown, that's the fuel pump in the tank leaking excess fuel backwards.... I'm betting yours has failed in just the opposite way. I can't say I've seen that before.

 

[rdezs]

But by all means check with a mechanical fuel pressure gauge. If the mechanical gauge shows the readings are incorrect and you have normal fuel pressure, then your electronic fuel pressure sensor is likely the problem. For example, it could be telling the ECM you have more than enough fuel pressure, in the ECM will not allow the fuel pump to come on... Which would also give the symptoms you have. And I don't think there's a trouble code for excess fuel pressure.

 

[mikez71]

I don't have any symptoms (that I know of).
I was just trying to throw info out there that might be related to the OP's problem.
But I will look into it now, thanks!

 

[Doubeleive]

I don't have any symptoms (that I know of).
I was just trying to throw info out there that might be related to the OP's problem.
But I will look into it now, thanks!

you are not wrong heat soak does happen, seen mine do the exact same thing with a mechanical gauge attached. but I don't think heat soak is the issue
I would be looking mainly at the throttle body, but also maf, map, temp, voltage. if all that passes then maybe start looking at injectors/coils
mine used to have a little "sputter" that happened randomly for no apparent reason, it was the throttle body. in fact I thru a LS7 throttle body on it and it hasn't done it since and that was back in 2018

 

[j91z28d1]

high fuel pressure after shut down is completely normal for these dead head style systems. mine goes into auto stop anytime I'm under 30mph and not accelerating hard enough to need the engine and fuel pressure spikes very high every time. soon as it spins the motor over(opening the injectors) it goes right back to 45ish.

check valve in the fuel pump, high under hood Temps and no return line. it is what it is.