Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
This pickup has no engine codes. I cleaned the throttle body and the mass airflow sensor. I also checked the fuel pressure. The fuel pressure seems to be in spec and holds fuel pressure for a while but will drop to 0 psi after a few hours. Would the fuel pressure regulator in the fuel pump or check valve cause this problem?
Fuel pressure dropping to zero over several hours isn't really indicative of a fuel system problem, IMO. If you'd said 30 minutes, I'd say you're onto something.
What are we qualifying as a stumble? Start and stall? Long cranking time?
Also, once running and warmed up does it get better? How are your fuel trims? May be the ol' intake gasket that only shows up when cold. Of course it could be other things too, ha. Let's get some data and clarification and go from there.
The pickup doesn't start and stall. It just stumbles for a few seconds part of the time during startup. Last weekend I drove it a 100 miles and shut it off where it set for a couple hours. During startup it just stumbled for a couple seconds and then run fine. I don't use gasohol either.
Does anyone have the fuel specifications for this vehicle on the fuel pressure and how long the fuel pressure should hold in the fuel rail? This is a flex fuel vehicle.
Update on this pickup. Changed the spark plugs and they are running partially black. Anything I can check for this rich problem with no codes. This pickup does a lot of short trips.
There are a bunch of drivability tsbs but nothing I saw without an accompanying dtc. Everything from clogged injectors to new valve cover.
What makes you think this is a rich condition? If the fuel trims are normal, then I'd definitely smoke that intake while it's cold. The idea is that the intake gaskets leak while it's cold on start up then warms up and seals, so fuel trims may not show the issue.
ECT sensor can cause enrichment issues and a leaking injector or few can cause the starting issue you have. If you suspect the injectors, you could do a injector balance test, but I'd also expect to see negative fuel trims with this happening.
Coolant leaking into the combustion chamber can do it too, but I don't think I'd expect the plugs to be dark if that were the case.
If you have a scope, use an amp clamp to check the starter armature wiring. A bad contact in the brushes would cause what you're describing....
If you wanna be a parts changer, throw a starter at it and Id almost put money on it that the problem goes away...
But if you wanna verify it first, remove the starter, connect it to a battery with an amp clamp on the power side. If you zoom in far enough, you'll see humps in the signal where the brushes contact the different plates on the commutator.
If those humps are all even, I'm wrong.
If one or two are smaller, there wont be enough flow to get the engine really turning until the kinetic energy built up from the contacts.
After you verify it is the problem, you could be really cheap and tear down and clean up the contacts and brushes to get another year or so out of it. or just buy a new one with a lifetime warranty. Those tend to last better in "Lots of starts vs miles" type trucks, I find...
If you don't have a scope, you could check it with an ohmmeter while slowly turning the armature to check the resistance at all the plates. But at that point, you might as well just tear it down and clean everything with bronze wool and electrical parts cleaner. It's about as much work as check with an ohmmeter and doesn't hurt anything to do it anyway. Inside the motor, everywhere that makes physical contact should be *Shiny*. Seriously shiny, like brand new metal shiny. If it isnt, it isnt making a solid connection.
Tutti57 wrote: If the fuel trims are normal, then I'd definitely smoke that intake while it's cold. The idea is that the intake gaskets leak while it's cold on start up then warms up and seals, so fuel trims may not show the issue.
ECT sensor can cause enrichment issues and a leaking injector or few can cause the starting issue you have. If you suspect the injectors, you could do a injector balance test, but I'd also expect to see negative fuel trims with this happening.
Coolant leaking into the combustion chamber can do it too, but I don't think I'd expect the plugs to be dark if that were the case.
All of those are things that cause issues AFTER startup.... What he is talking about is a stumble DURING startup... while it's still cranking.
Does your truck have the displacement on demand lifter setup? A sticking lifter can have similarities to stumbling until it frees itself. An easy way to find out is use a pulse sensor in the intake like the pcv port at throttle plate, disable injectors by pulling the two fuses and crank it.
Also what is your oil pressure readings at 1000 rpm, 2000 rpm and 4000 rpm when engine is at operating temperature? Minimum psi at 1000 is 6 psi, minimum psi at 4000 rpm is 24 psi.
The reason I think it is a rich condition is the spark plugs were fairly black when I pulled them. The stumble in this motor is during the initial cranking and startup of the motor causing a longer than normal start.
One question I have is whether there is any software updates available for this pickup. I went to a GM dealer and asked them about this. The service manager told me the software updates are only on the newer vehicles. He said there won't be any software updates on a 2008 Chevrolet Silverado 1500. Anyone have any knowledge on this topic?
Rapidrobert1 wrote: One question I have is whether there is any software updates available for this pickup. I went to a GM dealer and asked them about this. The service manager told me the software updates are only on the newer vehicles. He said there won't be any software updates on a 2008 Chevrolet Silverado 1500. Anyone have any knowledge on this topic?
It's possible for you to check this yourself using GM Tis2Web:
To determine if there's an update available, you'll need the VIN and the Cal ID's for the PCM. You can find these in Mode $09 in the Global side of OBDII. Navigate around in Tis2Web and you'll get a page like this:
Under 'System', the PCM has an update for P0128. Note that this only tells us what calibrations are available. It's up to you to compare the Cal ID's to the Part Number on the left to determine what calibration you're at.
I used this website Tyler. The website worked perfectly and there are a few software updates on my pickup. I talked to the dealership again and the service department said they only go back so many years on those software updates and my pickup is old enough where they couldn't do a software update on it anymore. None of the software updates on this pickup pertained to the problem I have. Thank you so much!
Have you noticed a slight loss of coolant over time in the coolant reservoir?
To rule out coolant leaking into cylinder during engine off, you should consider doing a coolant system pressure test.
A stumble at cold start up might also be due to leaking injector. Inject balance test will show results
Another quick test is to start up the engine (cold), and immediately turn off the ignition when it starts to stumble. Pull each spark plug and look for evidence of wetness on plug. A bore scope would be handy also in this situation
The following user(s) said Thank You: Rapidrobert1
I would also still check for a leaking injector(s), coolant in the cylinder, ECT sensor. I know it was mentioned that these impact issue after start up but I've seen them cause this. Just a few weeks ago a head gasket cause this for me and we've all seen a fuel flooded cylinder not start right away.
At this point, I'd at least rule them out.
What exactly are your long and short term fuel trims at idle, and 2500 rpms? I know you said they are fine, but it might help us to know. If it's running rich, this data will show it.
Rapidrobert1 wrote: I used this website Tyler. The website worked perfectly and there are a few software updates on my pickup. I talked to the dealership again and the service department said they only go back so many years on those software updates and my pickup is old enough where they couldn't do a software update on it anymore. None of the software updates on this pickup pertained to the problem I have. Thank you so much!
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It's up to you to compare the Cal ID's to the Part Number on the left to determine what calibration you're at. 
