Stumble at first cold start - dealer can't find anything wrong

Mossyoakglock · July 7, 2015

For the last two days I have had the same problem at the first start in the morning and it's getting old.

Normally, when I start the truck in the morning after sitting overnight it will crank no problem and idle around 1,000 - 1,200 RPM and then slowly drop. The last two morning it will crank and start with no problem at all and jump right to 1,000 RPM but then instantly drop to around 400 RPM for about 2 seconds, idle rough, and then idle right back up to 1,000 and then everything is normal. It idles down smoothly like it is supposed to do. It's done this the past two mornings and will sometimes do it once every two weeks or so at random, sometimes 2-3 times a week. Again, at random. It hasn't done it in a while but just started it again this week.

Once the truck starts it runs fine. Good acceleration, etc. Although one day I was on a back road and decided to get on it a little bit and it just didn't go anywhere. Bogged down really bad and then about 2-3 seconds later it took off. After that it was fine.

Until I am able to check the fuel pressure this weekend, any thoughts on what it could be? Throttle body and MAF are clean and I just ran a bottle of Techron through it about 500 miles ago. Truck has about 91,000 miles.

My first thought is that it could be the fuel pump starting to go bad. Thoughts? I had left it with the dealer a few months back when it was doing it but they of course couldn't find anything wrong.

Mossyoakglock · July 13, 2015

So I checked fuel pressure over the weekend and this is what I found. Bear with me as I did a few tests and tried to do every variable I could think of. I made a 5ft extension hose so I could cycle the key and start it and view the pressure right from the driver’s side door. I confirmed that there were no leaks at any of the connections with the extended hose as well.

First Attempt: w/ gauge extension hose

Key on/Engine off (cycled 3 times)

10 psi
18 psi
60 psi

Removed extension and used original hose

Started truck – 60 psi then settled on 44-48 psi at idle, idled for about 3 minutes

Then did Key on/Engine off

56 psi then fell to 40 psi in about 30 seconds
30 psi and fell to zero quickly

Second Attempt (truck sat for about 2 hours)

w/ extension – wanted to make sure that it had the same reading at idle as it did with the stock hose and readings were consistent

Key on/Engine off (key cycled 2 times), also wanted to make sure readings were consistent with first test:
1. 10-18 psi
2. Roughly 60 psi
Started truck
1. 60 psi then settled on 46 psi at idle (idled for about 3 minutes) - same as with stock hose on test gauge so all readings should be accurate with and without the hose extension.

The truck started perfectly fine because I had already primed the fuel pump and fuel system. If I have time I'll test the pressure again and start the truck right away and see what the pressure does.

So, based on these tests does it sound like the fuel pump? I tested for any vacuum leaks and could not find any anywhere.

Doug_Scott · July 9, 2015

You trucks fuel pressure after sitting overnight is zero. It is zero after about an hour of not running. Try turning the key on for 20 seconds before trying to start the engine. That will ensure you have full fuel pressure at start up. You may be able to hear the fuel pump running. If you can, turn the key on and leave it on until the pump stops, then turn key off, and then back on again, until the pump stops, then start the engine. Not sure how the GM actually works the fuel pump, it may just run the pump on key on for x number of seconds, then turn off, or it may run the pump until a set pressure is reached, then switched off. I know my Yamaha FZ1 bike runs the fuel pump for 5 seconds, then shuts off. I have to turn the key on then off about 10 times after draining the fuel bowls on carbs for long term storage. It takes a while to fill the 4 fuel bowls with only 4psi pressure from the pump.

Mossyoakglock · July 9, 2015

I know I can hear the pump run for about 3 seconds if I just turn the key.

I was under the assumption that there was always pressure in the fuel system so that it is "ready to go" on start up. Could it be that the pump is not priming itself fast enough before I actually start the truck?

Edit: I'm going to start with the start fluid since that doesn't take long and is easy to do. Also, I attached a video of what the truck does. It's blurry for some reason but you can clearly see the needle drop. It doesn't look like much but you can definitely feel it sitting in the truck.

http://vid44.photobucket.com/albums/f38/Cheezeit9/IMG_1715.mp4

calgator73 · July 13, 2015

_{Might be worth performing an engine oil analysis to see if you have a leaky injector.}

certified22 · February 2, 2016

He doesn't have a loss of fuel pressure pretty sure he went over that already I could be wrong but I believe what I read he ran a fuel pressure test and it came back fine plus nobody wants to spend 1000 dollars on your JL38569CMFM IN LINE fuel pressure gauge and scan tool and the one they let you borrow at autozone won't do what you've described and for sure the chevy house won't let you borrow theirs we all get how the truck mixes air fuel ratio you did a good job of copy and pasting the instructions you got off some other web sight you did a half ass job of sounding like you might know your shit dealing with fuel related issues on Chevy trucks except this is not a fuel issue and you have done an absolute worthless job of adding and knowledge to the issue we have here if you don't have anything helpful to add or are just trying to sound intelligent by copy/pasting some diagnostic schematic off the internet then go buy a ford and swallow that heavy load cause this here is for Chevy truck owners looking for knowledgeable help from other knowledgeable Chevy truck owner who bought Chevy trucks to haul heavy loads not swallow them now it's the dam intake gasket the dam factory installed gasket is susceptible to ethanol gas even in the flex motor you need to replace the orange Canadian made factory gasket with the American made teal one and all you DIC codes will go away your rough starts will cease you'll be in great shape till your truck hits 130k-150k when you have to replace the o ring on the pick up tube for the oil pump but this is a intake manifold gasket issue for sure and to the other guy no shit up stream of the MAF sensor you ****ing Retard how many intake manifolds you ever saw down stream of the MAF sensor I'm not the smartest man on the planet but I dam sure don't go muckin up a perfectly good blog site where people are looking for intelligent answers to real issues by trying to sound ASE certified talking bullshit about an issue I obviously have no clue about now I'll probably be banned from the group but if this is the level of intelligence I have to deal with I'm cool ban away one last time this is a intake manifold gaskets issue anyone with some knowledge tips or tweaks that can share I'd love to hear from you cause I'm bout to tackle same project and I won't have any problems start to finish pretty simple procedure but I welcome any info from anyone who knows what they are talking about in case there are any pointers that will save me an hour of scratching my head thank you

Mossyoakglock · July 17, 2015

The issue is very likely tied to the low pressure on the initial fuel "priming". This really sounds like something is shutting the pump off early or too soon on first startup from a long time sitting. Do you know anyone that has a similar truck to yours that you can compare initial fuel pressure, and what the pressure is on first key on of the day? By similar I mean same generation of GM truck, preferably same size engine as well. I hate putting parts on to "try" it. As I previously stated, my bike has a very similar issue with the fuel pump not running long enough to put enough fuel in the carbs to start the bike after carbs were drained for storage. There is likely some fix for it somewhere, but, I know that just switching the kill switch will run the fuel pump for 5 seconds at a time. If it were me having the issue you are having, I would either ignore it, or prime they system twice before starting it. But that is just me. Not suggesting that as a solution.

So the hose extension that I was using was causing a large pressure drop over that distance. I used the original hose and got 60 psi w/ KeyOn/EngineOff.

I'm going to test it tomorrow morning from a straight start and see what it does.

When I tested the fuel pressure at idle when I got home (motor was hot) from work it was bouncing between 50-54 psi which the Haynes says 42-45 psi so it's a little high. It dropped about 3-4 psi in 1 min and when I shut the engine off it stuck at 50 psi and dropped to 20 psi in about 15 min.

I'm going to pull my fuel rail this weekend and prime the pump and see if I get any leaks. If no leaks withe the injectors then I would say it's the fuel pump?

It it just as simple as removing the 4 bolts that hold them down and pull up and remove them? I just need it high enough to see if they are leaking. I don't want to have to remove anything that isn't necessary and need all fuel lines connected so there is fuel in the lines. I'm going to get a new set of o-rings when I replace them just in case. I don't want to take a chance.

newdude · February 1, 2016

Starting Mode

When the ignition is first turned ON, the ECM energizes the fuel pump relay for 2 seconds. This allows the fuel pump to build pressure in the fuel system. The ECM calculates the air/fuel ratio based on inputs from the engine coolant temperature (ECT), mass air flow (MAF), manifold absolute pressure (MAP), and throttle position (TP) sensors. The system stays in starting mode until the engine speed reaches a predetermined RPM.

Fuel Pressure Regulator:
The fuel pressure regulator is contained in the fuel sender assembly.

.

After the fuel pump is turned OFF, verify the fuel pressure does not decrease more than 34 kPa (5 psi) in 1 minute.
If the pressure decreases more than the specified value, perform the following procedure:
Ignition OFF, relieve the fuel pressure. Refer to Fuel Pressure Relief.

Install the J 37287 Fuel Line Shut-off Adapter between the fuel feed pipe and the fuel rail.

Open the valve on the J 37287 Fuel Line Shut-off Adapter .

Ignition ON, command the FPCM Fuel Pump ON with a scan tool and bleed the air from the CH-48027 Digital Pressure Gauge .

Command the FPCM Fuel Pump ON and then OFF with a scan tool.

Close the valve on the J 37287 Fuel Line Shut-off Adapter .

Monitor the fuel pressure for 1 minute.
If the fuel pressure decreases more than 34 kPa (5 psi) within the specified time, locate and replace the leaking fuel injector(s).

If the fuel pressure does not decrease more than 34 kPa (5 psi) within the specified time, replace the fuel tank fuel pump module.

Relieve the fuel pressure to 69 kPa (10 psi). Verify that the fuel pressure does not decrease more than 14 kPa (2 psi) in 5 minutes.
If the fuel pressure decreases more than the specified value, replace the fuel tank fuel pump module.
Operate the vehicle within the conditions of the customer's concern while monitoring the fuel pressure with the CH-48027 Digital Pressure Gauge . The fuel pressure should not drop off during acceleration, cruise or hard cornering.
If the fuel pressure drops off, test, inspect, and repair the items listed below. If all items test normal, replace the fuel tank fuel pump module.
Restricted fuel feed pipe

Inspect the harness connectors and the ground circuits of the fuel pump for poor connections

Sounds like it could be an intermittent sticking injector, or potentially the pressure regulator (which is internal to the pump/sender). Pressure shouldn't drop off that fast in 1 minute.

white1500 · February 1, 2016

My 2000 5.3 also has stumbles a bit on completely cold startups but is 100% fine the rest of the time.

From what I have researched it is due to the intake manifold gaskets leaking air (upstream of the MAF).

Although I replaced the throttle position sensor and it did help slightly.

Doug_Scott · July 10, 2015

Propane is safe and easy to use, just make sure you only have it leaking out the hose. You do not need to have it coming out so fast you can hear it coming out.

You can use a lot of things that will either burn or not burn. You have to be cautious of what the effect on the o2 sensor is. Some fuels will damage the o2 sensor. Propane, co2, acetylene, are some safe ones to use. You have to watch brake cleaner, it contains some chemicals that when burned can release some really nasty gases, and you cannot control the flow of gases like you can with propane or acetylene.Do not ever use oxygen from your ox-acetylene torch kit.

I have never heard of using smoke to detect vacuum leaks. I can only imagine two different ways it could work, one is to effectively pressurize the intake with smokey air and watch to see where it leaks out, that would be near impossible to control since there is always at least one intake valve open on an engine that is not running. The other way would be to use smoke coming out of a hose and you watch for disturbances in the air around the running engine at the spots where a vacuum leak could be. That would require virtually static air around the running engine. That is near impossible as well.

Leave your fuel pressure gauge on overnight to see what morning pressure is before turning the key for the first time. I think you will find it at zero. You may want to not leave the gauge on though, it may be leaking itself and that would drain the pressure down on its own. You just need to know what is left after an overnight(or more than 6-8 hours of being parked) session. It is normal for it to leak down to zero overnight I think. Pretty sure there is a spec for how fast the leakdown is. Too fast a leakdown can impact idling quality.