2011 GMC Yukon SLT Flex Fuel 5.3 hard starts when cold

[Self]

Having an issue with my 2011 Yukon SLT Flex Fuel with the 5.3. First start of the day it runs and tries to die with rough idle and will occasionally start but continue trying to start while engine is running. If it dies and I try to start it again, it will continually turn over for a good 15 seconds or so and then start up with a slightly better idle (still a bit rough) and then run fine. Driving short distances, turning engine off, and then trying to start again will result in the 15 second hard start. If it gets to operating temp (or slightly over half way) it will start up just fine. I initially thought it was the starter being stuck while engaging, but then it does the hard start crap and turns fine. Then thought it was a fuel issue as the rough idle and hard start could be low fuel pressure. But that also appears fine. The fuel pressure key on engine off is around 56psi and drops to 54psi after a few minutes. Turning engine on is around 59/60psi and stays fairly steady at idle after the idle steadies out. Once at operating temp and moving it stays around 43.5psi which apparently is normal for flex fuel models. Turning engine off and waiting the fuel pressure climbs and continues climbing up to 81psi and stays there. No codes aside from traction control/stabilitrak from the steering wheel sensor going bad. Stock engine aside from having AFM/DOD deleted years ago (after a head gasket blew) using a kit from Texas speed and performance. MAF I believe was tuned out as it reads low and no change when unplugging it. Intake manifold gaskets are good it seems. I've read conflicting posts about the high fuel pressure after engine is off with some saying it should hold at 60ish psi but then found another post with someone quoting a gm service manual stating 72-87psi is normal range after turning engine off. Any ideas?

 

[Fless]

I can think of a couple of things.

What fuel do you run? Any with ethanol content above 10%?

Looking at some live data right as you go to start it cold, you might see an issue with something on the scanner.

Engine coolant temp sensor mis-reporting.
MAF not reporting, or reporting improperly. (I would suggest putting a good scan tool on it to see if it reports at all, to confirm the assumption that it's been tuned out. But I don't know why tuning it out would be advisable. What do the fuel trims look like?)
Intake or ambient air temp sensor mis-reporting.
Purge valve on engine leaking fuel fumes while engine is off, or when it's not commanded on.
Crank-cam sensors out of sync. (Do a quick CASE relearn to rule it out)

I'm sure there can be other things that would impact the starting, so I expect others will chime in.

 

[mikez71]

Your fuel pressure is normal. Upto 81psi is heatsoak and the pressure relief valve setting.

I like @Fless' suggestions, check the coolant temp readings. It's easy and you seem to have the issue only when cold.

 

[Self]

I can think of a couple of things.

What fuel do you run? Any with ethanol content above 10%?

Looking at some live data right as you go to start it cold, you might see an issue with something on the scanner.

Engine coolant temp sensor mis-reporting.
MAF not reporting, or reporting improperly. (I would suggest putting a good scan tool on it to see if it reports at all, to confirm the assumption that it's been tuned out. But I don't know why tuning it out would be advisable. What do the fuel trims look like?)
Intake or ambient air temp sensor mis-reporting.
Purge valve on engine leaking fuel fumes while engine is off, or when it's not commanded on.
Crank-cam sensors out of sync. (Do a quick CASE relearn to rule it out)

I'm sure there can be other things that would impact the starting, so I expect others will chime in.

Not sure on ethanol content off the top of my head, but it's generally unleaded 87 and have been using the same gas since I purchased this in early 2021. Engine coolant temp seems to be working and reporting properly. Maf is reading 0.01 lb/s and when unplugging the engine still continues to idle as normal but scanner then reports 0.00 lb/s. Believe the guy who did the afm/dod delete used a speed density tune which would make sense on why the maf isn't reporting properly. I'll get fuel trim numbers when I go back out to it. Ambient air temp sensor seems normal in comparison to the current temp at that time of day. When engine is off, it does hold fuel pressure for quite some time verified by a mechanical gauge attached to fuel rail. I have a cheap Foxwell NT630 plus scanner, don't believe it does case relearns. Any recommendations on a scanner that isn't super expensive to do the relearn with?

 

[Fless]

Unleaded 87 will typically be E10 or E15, so the ethanol content probably doesn't affect the issue.

Your scanner likely reports lb/MIN, not sec. Switch the scanner to metric readings and get the MAF g/sec at hot idle; that should be close to the engine displacement (e.g., 5.3) and will be a more accurate reading. If it's been tuned out, I wonder why? Is it running something else that reports the air/fuel ratio?

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

You are already receiving clarifying questions and sage advice from the knowledgeable folks on this Forum.

 

[mikez71]

Seems to be a popular tweak for people to run speed/density tune..
Requires flashing the ECM with an operating system patch.. Something about mapping virtual volumetric efficiency..
Probably a more responsive tune from what I can gather..

Maybe your crank sensor is dying?

This comes to mind because you said:
"First start of the day it runs and tries to die with rough idle and will occasionally start but continue trying to start while engine is running."

( Assuming the ecm relies on crank signal to know if running or not? )

 

[Self]

Cold start fuel readings

Short term bank 1 is -3.9 to -2.3 back and forth
Long term bank 1 is -17.2 to -16.9 back and forth

Short term bank 2 is -7.8 to -3.9 to -2.3 to -1.6 back and forth
Long term bank 2 is -15.6 to -21.9 back and forth

Close to operating temp fuel readings

Short term bank 1 is 3.9
Long term bank 1 is -24.2
Short term bank 2 is 1.6 to 2.3 to 3.1 back and forth
Long term bank 2 is -24.2

Maf is now reading 6.02 g/s in metric. This foxwell scanner doesn't read lb/m it's in lb/s which is what was throwing me off thinking it was tuned out. Unplugging maf changes nothing with the idle though other than scanner reading 0

 

[Self]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

You are already receiving clarifying questions and sage advice from the knowledgeable folks on this Forum.

 

[Self]

Seems to be a popular tweak for people to run speed/density tune..
Requires flashing the ECM with an operating system patch.. Something about mapping virtual volumetric efficiency..
Probably a more responsive tune from what I can gather..

Maybe your crank sensor is dying?

This comes to mind because you said:
"First start of the day it runs and tries to die with rough idle and will occasionally start but continue trying to start while engine is running."

( Assuming the ecm relies on crank signal to know if running or not? )

If crank sensor was dying, wouldn't that want to throw the cel on and give a code? If it would toss a code at me I'd have better luck but this thing has me guessing all over the place.

 

[mikez71]

If crank sensor was dying, wouldn't that want to throw the cel on and give a code? If it would toss a code at me I'd have better luck but this thing has me guessing all over the place.

Good point!

But maybe.. I think it's one of those that may, or may not throw a code.. depending on the failure mode.
Hopefully someone who knows forsure can chime in..

Since you have a scanner (YAY!) maybe you can check injector pulsewidth so see if it looks steady? idk..
Since you say it runs fine when warmed up, maybe I'm barking up the wrong tree..

How long has this been happening?

 

[Fless]

Cold start fuel readings

Short term bank 1 is -3.9 to -2.3 back and forth
Long term bank 1 is -17.2 to -16.9 back and forth

Short term bank 2 is -7.8 to -3.9 to -2.3 to -1.6 back and forth
Long term bank 2 is -15.6 to -21.9 back and forth

Close to operating temp fuel readings

Short term bank 1 is 3.9
Long term bank 1 is -24.2
Short term bank 2 is 1.6 to 2.3 to 3.1 back and forth
Long term bank 2 is -24.2

Maf is now reading 6.02 g/s in metric. This foxwell scanner doesn't read lb/m it's in lb/s which is what was throwing me off thinking it was tuned out. Unplugging maf changes nothing with the idle though other than scanner reading 0

OK on the MAF. If you can reset the LTFTs with your scanner, do it, then monitor closely. Over time it's been running rich, which is why the LTFTs are so negative (they're taking away fuel). That could eventually take out the cats. Is there fuel in the oil? Vapors can cause a rich condition, as can a stuck purge valve on the engine.

Good fuel trims are within ± 10%; really good ones are within ± 5%. Add the ST and LT trims for each bank and you're way out of spec.

Example, at your operating temp:
Bank 1: 3.9 + -24.2 = -20.3
Bank 2: 1.6 + -24.2 = -22.6

I'd ask how old the O2 sensors are; if they're questionable, replace with OE sensors and monitor fuel trims.

 

[Self]

Good point!

But maybe.. I think it's one of those that may, or may not throw a code.. depending on the failure mode.
Hopefully someone who knows forsure can chime in..

Since you have a scanner (YAY!) maybe you can check injector pulsewidth so see if it looks steady? idk..
Since you say it runs fine when warmed up, maybe I'm barking up the wrong tree..

How long has this been happening?

I'm going to have to get a new scanner that does a little more as this one just does basic code reading, abs, srs, etc. Been meaning to get a new one just haven't. Lol. It started doing it a little while ago (Octoberish) and initially thought it was my starter as it would crank a bit longer than normal but now it's every time I start it cold and then it has the really rough idle which definitely isnt starter related. I don't drive it a whole lot as I drive my work truck more than my personal vehicles.

OK on the MAF. If you can reset the LTFTs with your scanner, do it, then monitor closely. Over time it's been running rich, which is why the LTFTs are so negative (they're taking away fuel). That could eventually take out the cats. Is there fuel in the oil? Vapors can cause a rich condition, as can a stuck purge valve on the engine.

Good fuel trims are within ± 10%; really good ones are within ± 5%. Add the ST and LT trims for each bank and you're way out of spec.

Example, at your operating temp:
Bank 1: 3.9 + -24.2 = -20.3
Bank 2: 1.6 + -24.2 = -22.6

I'd ask how old the O2 sensors are; if they're questionable, replace with OE sensors and monitor fuel trims.

Just did an oil change about an hour ago as it was just about due and did not notice any fuel odor from the oil. I'll do a little more reading on fuel trims as I honestly have no idea what good values even are. Don't think this scanner can clear the trims, but I'll be hunting for one that can do a bit more than this one. I would imagine the o2 sensors are the original sensors.

 

[Fless]

One other thing -- I think there may have been an update for the FF trucks due to it (the PCM) thinking that the ethanol content was high, causing it to add fuel. Find a scanner that can read the ethanol content -- as the PCM thinks it is -- to confirm that it's showing a correct value (10% or under, based on your fuel type in use statement).

Like the cam-crank correlation, it's something to check just to rule it out as a variable/cause.

 

[Coveman]

Since the fuel trims are trying to compensate for too much fuel you may have a leaky fuel spider that is dumping gas into the intake. Check that the fuel rail holds pressure steady after you cycle the key on and off.

 

[Self]

One other thing -- I think there may have been an update for the FF trucks due to it (the PCM) thinking that the ethanol content was high, causing it to add fuel. Find a scanner that can read the ethanol content -- as the PCM thinks it is -- to confirm that it's showing a correct value (10% or under, based on your fuel type in use statement).

Like the cam-crank correlation, it's something to check just to rule it out as a variable/cause.

Scanner shows 65.1% for alcohol fuel percentage.

 

[Self]

Since the fuel trims are trying to compensate for too much fuel you may have a leaky fuel spider that is dumping gas into the intake. Check that the fuel rail holds pressure steady after you cycle the key on and off.

Key on engine off it holds pressure around 56psi. Starting and turning off it will climb up to 81psi after a few minutes and stop.

 

[Fless]

Scanner shows 65.1% for alcohol fuel percentage.

BINGO!​

Get that reset right away, unless you're running E85! When you reset it, I believe that it also resets the LTFTs.

Not sure what part of the country you're in (?), but see the thread below for Tech 2 scanner owners; if there's one nearby, maybe get some help there.

Tech 2 Owners Map

There are quite a few members that have a Tech 2 and there are always people who wish they had one to use so I thought I would post this thread to try and help each other out. If you own a Tech 2 and would be willing to help out another forum member that might be local to you then post up your...

www.tahoeyukonforum.com

 

[Fless]

Since the fuel trims are trying to compensate for too much fuel you may have a leaky fuel spider that is dumping gas into the intake. Check that the fuel rail holds pressure steady after you cycle the key on and off.

No fuel injector spider on these GMT900s.

 

[mikez71]

If your scanner can't reset trims, I think unplugging the battery a few minutes should do it?