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Discussion Starter · #1 ·
Recently I've noticed an issue with my 07 sierra. It has a 5.3L with 67K on it. After sitting for a while sometimes it hard starts. It doesnt take a lot of cranking, but it starts and the RPMS go high then drop very low then steady out all in about 3 seconds. If it doesnt sit for a few hours and I start it it's fine. Once it's running I don't have any issues. It idles smooth, accelerates and holds speed smooth and no issues with fuel mileage. Today I replaced the plugs and wires and cleaned the MAF and Throttle body to see if that would help at all. I won't know for sure until the truck has sat for a while, but once I got done with all of that I hooked up a fuel pressure gauge. With the key on, but engine off the pressure went up to around 57 psi and over about twenty minutes it dropped to around 45 psi. With the engine running the pressure holds steady at about 62 psi. My question now is do I have a problem and if so what would it be? From what I thought (which is probably wrong) if the fuel pump check valve is bad the pressure should drop pretty quickly, and if the injectors are leaking I would be experiencing other symptoms like rough idle and issues accelerating and at speed. Also if it is an injector issue would I need to replace all injectors or just the faulty one(s). Thanks in advance for any help.
 

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Mine does something very similar. It cranks, turns over, drops RPM's for a second then revs and idles fine. No other issues at all and only happens when it's been sitting over night or longer. I've checked my electrical system. Battery is new and alternator tests fine. I have new spark plugs and wires. Cleaned all the items you mentioned such as throttle body. I hope someone has some insight. If it doesn't get worse I guess it's no big deal but I have a feeling it's gonna progress.
 

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Weak battery maybe cell going bad?? Just a guess. May be hard to start and rev up for the alternator then level out.

Fuel pressure may be a separate issue. Possibly this great non service able filter. Idk if they are adjusting the pressure on this system or not but it's possible.
 

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Discussion Starter · #4 ·
apcnc said:
Weak battery maybe cell going bad?? Just a guess. May be hard to start and rev up for the alternator then level out.

Fuel pressure may be a separate issue. Possibly this great non service able filter. Idk if they are adjusting the pressure on this system or not but it's possible.
I would think a power issue too but the alternator is good and I just replaced the battery three weeks ago

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Ya, I changed my battery that was fairly new and it still does it so I don't think it could be that. Could be a connection. I checked all my cables and connections and from what I can see there is no corrosion or loose connections.
 

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Discussion Starter · #8 ·
So possible fix but too soon to know for sure... from what I was seeing I was thinking I possibly had leaking injectors. I figured before pulling them and sending them to get cleaned or replacing them I'd try something myself so I went out and got two cans of sea foam. Dumped one can into my gas tank that was a little less than half full and i did another half can in the vacuum line just for some extra cleaning. Drove around to less than 1/4 tank last night and parked it. Just got in this morning and it started with no issues. Once I get back I'll let it sit a few hours again and see if it starts smooth again, but right now I'm hoping the sea foam cleaned whatever deposits were keeping the injectors from closing fully.

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