Cold start hesitation

mikez71 said:

Could it be weak ignition? Original plugs?

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Sparksalot said:

I had a pinhole fuel leak between the tank and engine that caused similar behavior in thethirdtwin. It would lose pressure in the line after sitting several hours or overnight. Turned out the fuel line had abraided against the frame since new. Only took 8 years to manifest.

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j91z28d1 said:

oh well, with a try. in theory the leak also sucks in air and can make your o2 sensors read lean. so your long term trims adjust to richer than it actually is. I don't know that it would bother start up. but I did the external clamp on mine, it has worked well so far.

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dwinters14 said:

How did you get it behind the head? It looks like there's zero room. I'm gonna look at the stud on #7 and see if it's broken off in the head or the stud is sticking out so I can wiggle it out with vice grips with the manifold off. Definitely adding it to my to-do list, I just don't want to have to buy/learn to weld to get the studs out Not there yet lol

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donjetman said:

Its doable. I put them on my engine in 2018. Its still going strong. Here's a post w/pics I did:

https://www.tahoeyukonforum.com/threads/exhaust-manifold-bolts-no-bueno.108605/page-3#post-1296439

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dwinters14 said:

I tested the coolant temp sensor tonight and want to confirm everything before I potentially continue down that rabbit hole. I checked the sensor cold with the key in the on position and it read 30*C. Now keep in mind the temperature outside is 20*C, I'm not sure if that could potentially cause my cold start issue or not, but It's something to note. I also checked my resistance and it came in at about 2100 ohm's which is right in spec for 30*C.

Also I have to say for "check valves" being such common points of failure there's barely any concrete information on testing them which is frustrating. I'm going to cycle the key 3/4 times tomorrow before starting the truck to see if that changes anything. I'm gonna be p/o'd if it's bad considering I just bought this pump and have been chasing my tail for 6 months.

Morning Update:

Left the fuel pressure gauge on and primed the pump. Jumped to about 50psi, primed it 3 more times jumped up to about 58psi and I tried to fire her. Same long crank and struggle to turn over. Now I think it's safe to assume the fuel pump assembly is doing what it's supposed to and the problem is somewhere with the injectors.

I used a stethoscope on them when the truck was hot yesterday and they were all firing fine. I wonder if one is leaking or is clogged.

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dwinters14 said:

I believe I've figured out my issue but I wanted to get a second opinion and also create a thread for future reference for others with the same issue.

Vehicle:
2007 Suburban LT, 5.3L, 2WD Flex Fuel

Problem:
Delayed start when cold. Truck always starts, and idles afterwards, and will fire first crank the second time.

*** Most probable cause E85 ***

Troubleshooting:

Firstly, NO CODES. I exclusively fuel my truck with E85 for the cost savings and also the drivability compared to regular octane fuel. In my live data the truck read 69% alcohol content. In the past two years I've replaced the following items associated with starting. Battery, alternator, purge valve on engine, vent valve by fuel tank and fuel pump. I haven't changed anything else. The first thing I did to narrow down the issue is check cold cranking voltage on the battery which has been a steady 12.2v to 12.8v. Starter fires every time. I recently replaced both the purge valve on the engine and verified it's working properly as well as the vent valve by the fuel tank and also verified it's working properly.

The next test was to check out the fuel pump/check valve assembly. I threw a gauge on the fuel rail and here were my numbers:

Dry: 0 PSI
Key in, on position: 50 PSI
Started and idling: 58 PSI steady
Off: 51 PSI
10 Minutes off: 49 PSI
1 Hour off: 25 PSI

From here the only other thing I can extrapolate besides fuel is something deeper, timing, cam position sensor, ECM issue with the alcohol content or a multitude of other gremlins. Once on, the truck holds and idles very well and has power throughout the entire powerband through every gear. The engine fires every time when warm as well.

I did do some research on E85 and it seems problematic when igniting on a cold engine. This issue has come and gone since I've owned it, but I never knew where to point the finger as all the other parts listed above were bad. Now that I've narrowed them out I'm looking at what's left. The easiest way to confirm or deny my concerns is to run the tank dry and put regular octane fuel in it and see, or combine it with the ethanol already in the tank to see if it helps ignite better.

Before I get to that stage, does anyone have any other potential ideas? I like to explore all avenues as I have frequently found really odd things can disguise themselves in their symptoms.

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