Warm Start Problems

[mcsc1]

Good afternoon all. I’m hoping someone here can guide me in the right direction. It’s a long one, but I’m gonna give all info and things I’ve tried.

My issue is I’m having difficulty starting when warm. When it’s cold, it fires right up. When I put my foot down to clear flood mode it will sometimes fire, usually it “may” help it start.

I’ve replaced fuel pump, all 8 injectors, coolant temp sensor, evap purge solenoid, 8 spark plugs.

Checking pressure on a snap-on scanner, we saw it fall from 42-43 at idle to (key on engine off) down to lowest of 16. Key on to prime, it got to 24 roughly.

I’ve pulled all 8 injectors still attached to the rail and put paper towel underneath to see if any leaked, and none are.

If it’s not injectors, but clear flood mode helps start it…what in the world is going on?

 

[Fless]

Tell us more about what the vehicle is: make, model, engine, etc.

 

[OR VietVet]

It's a "secret".

 

[mcsc1]

My apologies. It’s a 09 Yukon SLT, 2wd. 198k miles. 5.3, vin code 3, flex fuel.

 

[Foggy]

Look at your IAT when cold, driving and hot restart...
The sensor or circuit if screwed up could cause an issue
You'll need a bidirectional scanner to get live data

 

[Doubeleive]

Good afternoon all. I’m hoping someone here can guide me in the right direction. It’s a long one, but I’m gonna give all info and things I’ve tried.

My issue is I’m having difficulty starting when warm. When it’s cold, it fires right up. When I put my foot down to clear flood mode it will sometimes fire, usually it “may” help it start.

I’ve replaced fuel pump, all 8 injectors, coolant temp sensor, evap purge solenoid, 8 spark plugs.

Checking pressure on a snap-on scanner, we saw it fall from 42-43 at idle to (key on engine off) down to lowest of 16. Key on to prime, it got to 24 roughly.

I’ve pulled all 8 injectors still attached to the rail and put paper towel underneath to see if any leaked, and none are.

If it’s not injectors, but clear flood mode helps start it…what in the world is going on?

that pressure it too low, koeo should be 50-65, running it should be minimum 43 (i think) mine runs about 44
i would try what foggy suggested otherwise I might be batting a eye at the fuel pump or fuel pump control module, module may be getting hot and failing, you can pull the harness off and check it for corrosion or water intrusion as that has been known to happen before. the module is mounted under the spare tire area under the frame.

 

[mcsc1]

Look at your IAT when cold, driving and hot restart... The sensor or circuit if screwed up could cause an issue You'll need a bidirectional scanner to get live data.

I’ve seen IAT on a scanner at idle and key off engine warm. It read 107 when outside temp was 81. Where does it get IAT at? The MAF sensor?


D, I’ll check that out. I was curious about the module, but wasn’t certain. The pump did have a broken bowl which I rigged with my old bowl. That may or may not be my issue but it’s under warranty.

 

[Doubeleive]

I’ve seen IAT on a scanner at idle and key off engine warm. It read 107 when outside temp was 81. Where does it get IAT at? The MAF sensor?


D, I’ll check that out. I was curious about the module, but wasn’t certain. The pump did have a broken bowl which I rigged with my old bowl. That may or may not be my issue but it’s under warranty.

either maf or map pulls double duty, I think it's the maf, after 160k miles they are always suspect in my book, they can just go goofy after that

 

[mcsc1]

either maf or map pulls double duty, I think it's the maf, after 160k miles they are always suspect in my book, they can just go goofy after that

So my wife has a 2011 Denali, I swapped her MAF with mine and it made no difference in starting. Hers runs perfectly normal so I assume it’s a working MAF.

 

[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 sage advice from the knowledgeable folks on this Forum.

 

[mcsc1]

Updating this.

Replaced fuel pump again since it has a warranty. Didn’t change anything. It was, however, a different looking pump than originally put in.

Swapped MAF from my wife’s Yukon and nothing changed.

When cranking, the dash shows volts going below 12. After firing it sits around 14 or slightly less.

I’m beginning to think it may be a spark issue as I’ve tried everything else and found no leaks. But the 1 thing that I go back to is the “clear flood mode” working sometimes.

 

[Doubeleive]

Updating this.

Replaced fuel pump again since it has a warranty. Didn’t change anything. It was, however, a different looking pump than originally put in.

Swapped MAF from my wife’s Yukon and nothing changed.

When cranking, the dash shows volts going below 12. After firing it sits around 14 or slightly less.

I’m beginning to think it may be a spark issue as I’ve tried everything else and found no leaks. But the 1 thing that I go back to is the “clear flood mode” working sometimes.

my next chioice in the parts cannon would be the fuel pump control module.
you can get them cheap enough (even used) but it will need to be programmed to the vin
it sucks that is only a "warm start" issue as that makes a official diagnosis more difficult, from what I have read it could "maybe" also be the cam position sensor based on it being affected by heat? not sure that could just be another rabbit hole
it could even be a electrical connection somewhere

 

[mcsc1]

my next chioice in the parts cannon would be the fuel pump control module.
you can get them cheap enough (even used) but it will need to be programmed to the vin
it sucks that is only a "warm start" issue as that makes a official diagnosis more difficult, from what I have read it could "maybe" also be the cam position sensor based on it being affected by heat? not sure that could just be another rabbit hole
it could even be a electrical connection somewhere

Thanks for all the suggestions. I too have thought control module but having it programmed to the vin doesn’t sound fun or cheap.

One spark plug smelled like raw fuel where the others all smelled like burnt fuel. So that could be my issue.

We’ve tried checking grounds in the engine bay and cleaned everything up a little. But nothing changed after doing that.

I had thought cam position sensor, but I hate just throwing parts. I’m unable to leave it for more than a day due to needing it for work.

 

[mcsc1]

Also, before getting the AFM turned off I was getting a p3400 code for afm malfunction. May be related, may not be.

 

[Fless]

A cylinder balance test, and an injector balance test, might show an imbalance in the spark or fuel delivery. Even if it's not tested when the issue occurs, one of those might show a weak point.

Also, look at individual misfires with live data to see if one cylinder or bank is acting up.

What do your short- and long-term fuel trims look like?

 

[Doubeleive]

a tech2 would probably be helpful, they pay for themselves in the long run. there is a owners map thread maybe someone near you has one

 

[mcsc1]

A cylinder balance test, and an injector balance test, might show an imbalance in the spark or fuel delivery. Even if it's not tested when the issue occurs, one of those might show a weak point.

Also, look at individual misfires with live data to see if one cylinder or bank is acting up.

What do your short- and long-term fuel trims look like?

Can that be done using a scan tool? If not I may have to rely on a shop to do that as I don’t have the tools for it aside from the scan tool.

 

[Fless]

It will need to be done with an advanced scan tool like a Tech 2 or another with bi-directional capability.

Some scan tools let you look at live data (like misfires) and most will show fuel trims.

 

[Sam Harris]

I may have missed it, but what brand fuel pump did you put in (and replace a second time)? We’ve seen a good amount of non OEM pumps failing out of the box.

 

[j91z28d1]

definitely needs the correct cranking fuel pressure.

put a gauge on it and cycle the. key a few times till pressure comes up and then crank it


we had a truck at work that would bleed down the fuel pressure when off, it would build when primed but take a sec, before that happened if you stated cranking instantly without pausing for a sec to build pressure, it would crank for a long time and then act like it was flooded put once it did start.

the fix was turn the key on, wait a second and then crank. fired every time. from tuning cars, cranking fuel is a bit of a hassle to get right. I guess being a bit low and then flooding it messed it all up. now this was a older truck with a return style system and the ecm doesn't know the fuel pressure like these. so not sure what it would do. but worth a try