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


Mossyoakglock

Question

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.

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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!

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Might want to consider this from past forum.

 

 

Subject: Cold Extended Crank - Rough Running - And/Or SES Light With DTCs P0172 And/Or P0175 - Check Fuel Alcohol Content

 

Models: 2010-2011 Cadillac Escalade 2009-2011 Chevrolet Avalanche, Express, Silverado, Suburban, Tahoe 2009-2011 GMC Savana, Sierra, Yukon with E85 Compatible V8 Engine (RPO Code L20, L77, L94, L9H, LC9, LH9, LMF, or LMG)

This PI was superseded to update model years. Please discard PIP4792C.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern

On rare occasions, an extended crank on cold start, brief rough running on cold start, black exhaust color, and/or SES light may be experienced.

If a SES light is experienced, DTC P0172 and/or P0175 may be stored. If engine performance symptoms are experienced, they will generally diminish within 1 minute of startup.

This may be a result of the ECM learning the fuel ethanol content incorrectly even though the customer does not use E85 fuel.

 

My 2010 had this very issue and a recalibration of the ECM by dealer corrected the problem. Even though you have a 4.8, I would still consider this.

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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.

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I do have a vehicle I can test. Dad has an 07 GMT-900 Avalanche with the 5.3. He doesn't drive it much so I'll test it tomorrow and see what it shows.

 

*Edit - Primed the pump this morning with two cycles of the key (let it sit for about 10 seconds each time) and the truck started no problem. Smooth as it should be. I can hear it prime for the 2-3 seconds that it's supposed to as well.

 

My guess is that pressure is bleeding off somewhere at the pump since everything on the fuel system is now in the tank as part of the fuel pump.

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Tested again Thursday morning and this morning on first cold start with the stock hose to get a better reading.

 

Yesterday (6 am): KeyOn/EngineOff - 60 psi, no drop in 1 minute

Today(Friday, 6 am): Started truck (did not let pump prime) - Straight to 60 psi with no fluctuation and settled on 46 psi, no PSI drop in 1 minute.

- Since the gauge was hooked up I let it sit for 2.5 minutes and 5 minutes, only 1 psi drop in 5 minutes

 

Yesterday afternoon I checked fuel pressure when I got home from work and it was bouncing between 50-54 psi. Turned truck off and sat at 50 psi. There was a 3-4 PSI drop in 1 minute and a 30 psi drop in 15 minutes.

 

So I don't know what to do next. I'm at a loss. Pull the fuel rail and prime the pump and see if any of them are leaking? Not sure if I mentioned it earlier but I tested for leaks with propane and got no change in idle.

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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.

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Did you resolve your,issue cause I think the senior enthusiast above no desrespect is only a senior enthusiast cause of his age not his knowledge I could be wrong about the intake manifold gaskets but I bet I'm not it's definitely not a fuel problem after reading your test results and reading more that day your truck fell on its face then took off it was just having a day like anything else there could be a million reasons why that happened and it could never happen again curious to know weather you resolved this issue

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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)

  1. 10 psi
  2. 18 psi
  3. 60 psi

Removed extension and used original hose

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

Then did Key on/Engine off

  1. 56 psi then fell to 40 psi in about 30 seconds
  2. 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

  1. 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
  2. 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.

 

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.

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Check you r intake gasket that's the number 1 cause of rough cold starts check engine lights and replacement off everything under the sun avoiding replacing that gasket but in the end you'll end up,replacing it has any one noticed check engine lights running 87 or 89 octane cause I only run 91and ^ or my check engine light comes on

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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.

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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.

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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.

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Certified22 - You certainly don't hold back. Vent my friend, vent....but I agree, that this seems like useless info from 15HDriver - but bleeding the air out of the system might help some people - who knows?

 

mossyoakglock - did you resolve the issue? I ask since I am searching for a solution to what sounds like a similar issue. Here is what I have done and what I am experiencing.

 

I have a 2010 GMC crewcab with the 4.8 engine.

 

I started experiencing rough starts with misfires when starting my truck cold (sitting for about 8 hours) it would then settle down after a minute or so and run fine (open loop vs closed loop?) I checked the forums and everything seemed to point toward an intake manifold gasket so I tested it when cold with propane and received a RPM spike when pushing the propane down on the passenger side so I replaced the gasket and have been having fuel pressure issues ever since - with codes for "too rich" P0175 & P0172. My brother in law is a diesel mechanic and he decided I screwed up the intake manifold gasket - so he did the job again. I am still receiving the same codes but I no longer have rough starts or misfires so I am 95% sure the intake gasket needed replacing but now I have a fuel pressure problem.

 

I recall that the first time I did the gasket I bled the fuel rail and there was a lot of gas the came off - the second time we did the job there was not anywhere near the same amount of gas that came out to the rail - air pocket???? Hard to say - it still runs fine once warm but now it just starts hard when cold - ZERO fuel pressure when starting cold but then after a minute or so the pressure creeps up to normal (43ish). When the engine is warm and I check pressure it is in normal specs - 60 when primed (key on and engine off) and 43 when idling. At this point I need to floor the accelerator to get it to start - causes truck to shut off fuel since flooring it puts it into "flooded condition" and that gets the engine running. RPM's spike, drop off and then idle back up to normal. Although yesterday it stalled once - lack of fuel???

 

What have I done besides the gasket?

 

- new MAF

- cleaned throttle body

- replaced O2 sensors

- tried running for a week with stock air filter and intake - no change so I put the cold air kit back on (non-oiled K&F filter)

- replaced the fuel pump based on advice from mechanic at work who said zero fuel pressure = bad pump. No change at all - still zero fuel pressure when cold (Anyone need a fuel pump?? - I have a perfectly good one sitting in my garage since I am not doing that job again......)

 

Now I have found forums and articles that point toward the FPCM (Fuel Pressure Control Module) which is mounted above the spare tire. Some people have experienced water getting into the unit since it is mounted in an exposed area. I read about one guy who opened his up and let it dry and then sealed it back up and it worked fine after that. Other people suggest unplugging it while others replace it, which requires a visit to the dealer since it does not come programmed.

 

I am at the point where I am leaning toward taking my FPCM out and opening it up to see if it is wet. I hate to throw any more money at this since all the advice thus far has not yielded any results other than ruling out the fix I just did...

 

The other option would be to rule out the fuel pressure sensor on the fuel line under the drivers door - $20 part on amazon - but again seems like a waste of money after buying O2 sensors and a fuel pump that I apparently did not need.

 

I must say though that in reading all of these forums you learn a lot from all the great advice but no one seems to go back on and report back as to what finally fixed the problem.

 

mosseyoakglock - WHAT FIXED THE PROBLEM? What was the final solution? I am on the verge of going to the dealer but hate the idea of doing that since I have done so much myself already - I must be close to the solution...I know, some of you are saying I would have saved money by going to the dealer first and having them tackle the issue but I am stubborn and like getting my hands dirty.

 

Any help??

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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?

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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.

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