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.

Doug_Scott · July 7, 2015

The dealership you took it to actually sounds like a good honest dealership. Your description says you maybe have one shot per day at having this happen, and when it does happen, you have about enough enough time to say "see, that's what I mean" and it is over.

Sometimes you have to wait for an issue to get worse before you are able to properly diagnose it. Best you can do now is to make a guess, throw a part at it, and wait to see if that was it. My guess would be to run some injector cleaner through it.

Mossyoakglock · July 9, 2015

I run cleaner through it every few thousand miles, usually about every oil change or so. I've heard that it could be an intake manifold gasket but if that was the case wouldn't it run rough for a few minutes at cold start until all the parts start expanding due to heat?

I have checked all hoses but I bought new rubber hose for my catch can just in case one of them has a small crack that I couldn't find. The only thing that leads me to believe that it could be something with the fuel pump is that it only does it after it's sat overnight. Maybe the pressure isn't dropping completely and still holds pressure but it drops enough to cause the stumble. Something odd did happen a few weeks ago. I was going down a backroad and decided to get on it a little. Pushed the throttle down about 1/2 and it just didn't go anywhere. Then about 5 seconds later out of nowhere it took off like a bat outta hell. Somdtimes the fuel gauge will creep up once started. It might be at half a tank and then about 2 minutes later it might move up to a little under halfway between 1/2 and 3/4.

Other than the stumble at the first cold start of the day, the truck runs fine with no codes or anything. After it does the 1-2 sec stumble, the truck idles normal. I am going to test the fuel pressure when I get home tonight and leave the gauge hooked up for a few hours to see if the pressure drops. Is there anything wrong with doing that?

I am also going to spray some starter fluid around the intake manifold and see if I get a change in RPMs too.

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

Doug_Scott · July 9, 2015

If it is fuel pump then it would do it more often. If by starter fluid you mean ether, be careful with it, ether is very flamable, it can combust just by compression in a good condition gas engine. You don't really need something that burns to test for vacuum leaks, you are looking for a change in rpm. I have never tried, but, maybe someone else has tried using compresses canned air that you use for cleaning keyboards. It is just carbon dioxide in a liquid format in the can.

Mossyoakglock · July 10, 2015

Well it's done it every morning this week which it's never done before. It will maybe do it 2-3 times a month, if that.

I've heard you can use brake parts cleaner as well to test for leaks too. I also checked fuel pressure last night and Key On, Engine OFF it was around 60 PSI. Then I started it and it was around 48-50 PSI once it dropped and stabilized. When I shut the engine off it stayed at the same PSI and I left it connected for about 10-15 min and the pressure dropped to around 40.

Mossyoakglock · July 10, 2015

Also, if I were to use smoke to test it, what hose would I use to inject the smoke?

*Edit - I found a video where a guy said you can use one of those blue propane torches as well. Put a hose over the end, crack the valve and probe around with the hose. As the gas comes out it gets sucked into the leak. Thoughts on that?

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.

Mossyoakglock · July 10, 2015

Thanks, Doug.

I'm going to pick up a can of the keyboard cleaner too since it's basically just air and then try the propane trick too. Going to test fuel pressure in the morning after it sits overnight so we'll see.

Thanks again!

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.

calgator73 · July 13, 2015

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

Doug_Scott · 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:
10-18 psi

Roughly 60 psi

Started truck
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.

There is a timer somewhere that keeps the pump running to prime the fuel system. When you start the truck without priming(in other words, jump in, and start the truck like you normally do)..On your tests the fuel system only gets to 18psi on the first timed prime. It should also be doing that if you don't prime. Start from zero fuel pressure, pressure gauge connected and watch fuel pressure during this process. You can't take your eyes off the gauge, or even blink. See if the fuel pressure is low when it stumbles, then clears up as soon as the fuel system is over 40psi. Depending on fuel pump design, the pump itself may be letting the system pressure bleed down over time. It is odd though that the rough idle only happens on the first start of the day. I would expect it to happen every time the initial static pressure is 0psi.

I am not convinced it is the fuel pump, to me it really sounds like the timer control is not letting the pump run long enough. The timer may be part of the pump though, and would require the replacement of the pump assembly. When did you last change the fuel filter(assuming yours is not part of the pump)?

No need to analyze the oil to see if there is any fuel in it. A leaky injector would have dumped enough fuel to be able to smell the gas if you are near the time for an oil change.

Mossyoakglock · July 14, 2015

I just changed the oil about a week ago, maybe two and no odd smells. I thought if it was a leaky injector that it would be hard to start because it is essentially flooded.

As for for the fuel filter, it's all internal and on the pump on the '08s. I also have a gauge that likes to creep up once started. not much but noticeable so I would most likely be fixing that issue at the same time. There is a fuel pump control module but not sure how I would test that.

Doug_Scott · July 15, 2015

I just changed the oil about a week ago, maybe two and no odd smells. I thought if it was a leaky injector that it would be hard to start because it is essentially flooded.

As for for the fuel filter, it's all internal and on the pump on the '08s. I also have a gauge that likes to creep up once started. not much but noticeable so I would most likely be fixing that issue at the same time. There is a fuel pump control module but not sure how I would test that.

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.