5.3L hard start when warm

[W8TVI]

I have a weird issue with my the 5.3 in my 2000 GMC Yukon XL (135k miles).
When the engine is warm, or it's a hot day, I'll have to hold the starter cranking for a lot longer than you would expect to be normal, and it will run rough for something like 3 or 4 seconds, and then is it fine.

When it isn't hot out, and the engine is cold, it almost always starts right up almost instantly (like a new truck).

The engine has plenty of power, and doesn't stumble or loose power if I put my foot in it. I can take it to WOT, and it doesn't complain or hesitate.
It had a couple of old codes in the computer (when I got it last fall) about one upstream and one downstream O2 sensor (I don't remember what two it was) being out of range on the voltage (I think). I cleared the codes and I have had no new codes since getting it.

I haven't done a tuneup, yet, just an oil change.

I haven't taken the truck to have anyone look at it, but I was wondering if there was a list of easy stuff I could check to figure out what might be causing this problem.
I've don't google searches, but I can only find people talking about hard cold starts, not hard hot starts.

Thanks!

 

[M1Gunner]

Your MIL lamp may not be illuminated but I would still start by checking for codes. If no codes are present then you have to check for the three things an engine needs to run fuel, spark, oxygen. In this case I'd start with checking fuel. As your fuel sending unit may be heating up and not producing enough fuel pressure and/or volume.

 

[W8TVI]

Following up after a year, The Yukon still does this. I've checked with two different types of code readers and there are no codes stored whatsoever.

When the engine is warm, I'll have to hold the starter cranking for a lot longer than you would expect to be normal, and it will run rough for something like 3 or 4 seconds, and then is it fine.

When the engine is cold, it always starts right up like a new truck.

The engine has plenty of power, and doesn't stumble or loose power if I put my foot in it. I can take it to WOT, and it doesn't complain or hesitate.
Since I cleared the codes when I got it, I have had no new codes show up.

I have yet to do a tune-up, but I'm planning on pulling the wires and checking them, and re-gaping the plugs.

For new information, whenever it is hard to start, there is a strong smell a fuel for a few seconds, and then it goes away.
A couple of days ago, I watched my wife drive start the truck, and I noticed that from the tail pipe, there was a quick puff of some sort of white vapor that dissipated very rapidly. It didn't linger in the air like the smoke you get from it burning oil.

 

[CountryBoy19]

Have you check fuel pressure while that is happening? Autozone should have a fuel pressure gauge set they will rent you for free. That's where I would start. As the previous, and only poster, suggested, it sounds like the fuel sending unit (fuel pump) isn't functioning the best when it's warmed up.

Cold fuel pump = just enough fuel to get it started
Hot fuel pump plus low system voltage while engine is off & starter is cranking = not quite enough pressure to get good injection
Hot fuel pump w/ engine running (higher system voltage) = enough pressure to run things

 

[retiredsparky]

There are other things that heat affects. A heat soaked battery can produce low voltage. Loose/corroded battery cables can act different when a hot engine is cranked. Sensors that get heated by an engine that is not running/no coolant flow/no air flow can move just out of operating range.

With all that said, I agree with Mark's suggestion to check fuel pressure. Your fuel pressure regulator may allow pressure to leak down when hot, the fuel filter may be dirty, the fuel pump may be weak. Any combination like this could create a vapor lock in the fuel lines supplying the injectors.

When your engine is cold, the ECM or PCM or ECU depending on the name that GM uses for the engine control module enriches the fuel mixture by leaving the injectors open a little longer for each injection/combustion event. This is called open loop operation. The ECM ignores signals from O2 and other sensors during this time until the engine reaches a predetermined temperature.

Once the engine heats up, it is more carefully controlled by the ECM for emissions/power/efficiency by paying attention to all of the sensors. If a temp or other sensor is providing incorrect data, the ECM could provide too much fuel for the engine to start easily. The reason I bring this up is that you said you saw a white vapor. This could be unburned fuel or you could have the Castech heads that leak a small amount of coolant which could wet the spark plugs when the engine is hot and the coolant pressure is high (20psi).

My point? It may take some time to troubleshoot this. You should consider that if this vehicle has a lot of miles, it isn't a bad idea to replace some parts like a fuel regulator and or fuel pump.

Larry

 

[Wmac]

I have a weird issue with my the 5.3 in my 2000 GMC Yukon XL (135k miles).
When the engine is warm, or it's a hot day, I'll have to hold the starter cranking for a lot longer than you would expect to be normal, and it will run rough for something like 3 or 4 seconds, and then is it fine.

When it isn't hot out, and the engine is cold, it almost always starts right up almost instantly (like a new truck).

The engine has plenty of power, and doesn't stumble or loose power if I put my foot in it. I can take it to WOT, and it doesn't complain or hesitate.
It had a couple of old codes in the computer (when I got it last fall) about one upstream and one downstream O2 sensor (I don't remember what two it was) being out of range on the voltage (I think). I cleared the codes and I have had no new codes since getting it.

I haven't done a tuneup, yet, just an oil change.

I haven't taken the truck to have anyone look at it, but I was wondering if there was a list of easy stuff I could check to figure out what might be causing this problem.
I've don't google searches, but I can only find people talking about hard cold starts, not hard hot starts.

Thanks!

I also had the same problem and just fixed it by putting in a new fuel pressure regulator. Which also somehow fixed my AC blower motor problem as well.

 

[DirtDigler]

Having same situation here. Have replaced Delco fuel pump, pressure regulator, injectors , plugs, and wires. Seems the fuel pressure drops off too much. I dont remember the actual readings though. I think its accurate at start up, and when priming pump, but it drops off. It's not necessarily every time the engine is hot, it's kinda hit or miss. Mostly starts fine though. No codes either btw.

 

[Bobsquatch]

I'm having the same exact issue of warm hard starts and raw gas smell from the exhaust. I recently replaced the fuel filter then subsequently the fuel pump when it failed less than 5000 miles ago, with factory AC Delco parts. The pressure regulator is not leaking gas so I suspect the fuel filter which I should have changed again after installing the fuel pump. Or the pump itself as the fuel level sending unit is not accurate. I am going to check my fuel pressure then change the fuel filter and the regulator in sequence looking for any changes in pressure at each step. This 5.3 has around 150,000 miles and runs beautifully. The only issues it has are the knock sensors which I replaced last year with factory AC Delco and one failed immediately or was bad on installation and the other just recently. Kind of a pain in the butt job I have been putting off. And a pretty bad oil leak from the rear main seal or that PITA cover. Never the easy one... What should I be seeing for fuel pressure? 50-60 psi?

 

[Fless]

I'm having the same exact issue of warm hard starts and raw gas smell from the exhaust. I recently replaced the fuel filter then subsequently the fuel pump when it failed less than 5000 miles ago, with factory AC Delco parts. The pressure regulator is not leaking gas so I suspect the fuel filter which I should have changed again after installing the fuel pump. Or the pump itself as the fuel level sending unit is not accurate. I am going to check my fuel pressure then change the fuel filter and the regulator in sequence looking for any changes in pressure at each step. This 5.3 has around 150,000 miles and runs beautifully. The only issues it has are the knock sensors which I replaced last year with factory AC Delco and one failed immediately or was bad on installation and the other just recently. Kind of a pain in the butt job I have been putting off. And a pretty bad oil leak from the rear main seal or that PITA cover. Never the easy one... What should I be seeing for fuel pressure? 50-60 psi?

What year truck?

KOEO (Key On, Engine Off) specs for NBS V8 engines:
Non-Flex: 55 to 62 psi
Flex: 48 to 54 psi

Running pressure seems to be around 43 psi.

Next time you get the hard start, floor the accelerator to put it in clear flood mode. You could have an injector leaking down, dumping fuel into a cylinder.

 

[Bobsquatch]

It's a 2002 Yukon. I will try the flood clear floor it method. I was unaware of this feature. It's funny, same technique as a flooded carburetor but with computer assisted defueling I presume. Thank you for the tip.