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2003 GMC Sierra 4.8 running rough with multiple fault codes


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MarineGrunt
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Mar 1, 2020, 1:06 PM

Post #1 of 28 (886 views)
2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

Hey everyone...hope all is going well. First off I want to say I always appreciate the help. You've taught me so much over the years. Anyways, the other day the engine starting running kind of rough and threw some codes. It's not super rough but you can feel and hear the mild sputtering at idle and can tell it doesn't have full power at higher rpm's. The check engine light was also flashing while driving. Here are the codes.

P0300 Random Misfire Detected (generic)
P0171 System Too Lean (Bank 1) (generic)
P0174 System Too Lean (Bank 2) (generic)
P0449 Evaporative Emission System Vent Control Circuit Intermittent
P0455 Evaporative Emission System Leak Detected (gross leak/no flow)
PO101 Mass or Volume Air Glow Circuit Range/Performance
P0300 Random Misfire Detected (generic) (has it twice for some reason but maybe just my scanner?)
P0455 Evaporative Emission System Leak Detected (gross leak/no flow) (has this code twice too for some reason)

About two months ago I rebuilt the transmission after losing 3rd and 4th gear. While I had the trans out I went ahead and replaced fuel lines since I had to fix a small leak in the return a few months back. I also replaced plugs and wires. I did not replace with iridium plugs and used platinum if that makes a difference. Ran fine up until a few days ago. I tested fuel pressure and was about 48 psi. The fuel pump has had a whining sound for about a year so I already had a new pump on hand. I replaced it and pressure is now at 52 psi. It didn't make it run any better. I also pulled the vacuum line off of the fuel pressure regulator and pressure jumped to about 58 psi. The MAF sensor was replaced about a month ago. I checked for spark using one of those cheap testers you hook inline with the wire and it flashes with the same intensity for all of them.

After the fuel pump install I cleared the codes. I didn't pull the truck out of the shop but it did idle for a few minutes. The following codes popped right back up. Not sure why the both popped up twice on the scanner.

P0300 Random Misfire Detected (generic)
P0449 Evaporative Emission System Vent Control Circuit Intermittent
P0300 Random Misfire Detected (generic)
P0449 Evaporative Emission System Vent Control Circuit Intermittent

I didn't take it out for a drive so maybe the other codes would've came back up. I will take it for a spin if you think I should. The emission code was thrown in the past well before the truck started running rough.

Can I please get some help troubleshooting? I have a cheap scanner but it does have some very basic live data if any of it will help. It's an Innova 3160b. I think the exhaust seems to smell gassier than usual if that makes sense. Could it be as simple as a bad plug? Maybe a plugged cat? I'm just not sure how to attack things here. If you need anymore info just let me know.

Thanks in advance for the help.


Tom Greenleaf
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Mar 1, 2020, 1:45 PM

Post #2 of 28 (882 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

OMG, it's really YOU! Great to see you here again never mind fixing anything :-)


Isn't your operating fuel pressure a tad low? Does it say "Vortec" on this engine spelled that way?
Not sure on the evap codes so much yet but I recall where you live and temp swings have been insane I do watch that nationwide. If you fill a tank full and force it when really cold fuel it will barf it backwards but that problem ends usually and clears out.


Converter? Not so fast it should tell you OR test with a vacuum gauge at real intake (actual vacuum) at idle (will be bouncy now you said it isn't right) but see if that reading and again up at 2,000 or tad more with no load is the same. It should be.


Oh my again this thing is now 17 years old, the rusted fuel line plus just don't trust rubber crap for vacuum anything by now. Can be fine or not have to really check - grommets, gaskets near anything if off the show is off too.


That's a place to start anyway for now,


Tom - same twit you recall all these years here!



Hammer Time
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Mar 1, 2020, 2:27 PM

Post #3 of 28 (875 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

Hey Grunt, How ya been?

That Mass Air Flow code could be the key to everything. Check the intake snorkel that it isn't sucking air anywhere. A bad sensor or related wiring could cause most of those issues.



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MarineGrunt
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Mar 1, 2020, 5:14 PM

Post #4 of 28 (868 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

Things are going pretty good here. Finally have some warmer temps so that's a plus.

I actually threw on another MAF sensor I had. It's aftermarket but new in box. I just checked the intake snorkel and I don't really see anything out of the ordinary. Clamps are tight and no holes. The MAF sensor wiring harness looks good visually. I just tried testing for power on the harness. When key on engine off I was getting some voltage but I think I accidentally blew a fuse. I went to test the other wires and then noticed I didn't have any volts in the initial wire I tested plus the truck won't start now. Do you know what fuse the MAF sensor would be? I pulled quite a few but haven't found one blown.

I think fuel pressure is pretty close to where it needs to be Tom. I've read that with key on and engine off you want somewhere in between 48-54 psi. I'm not sure how I would go about hooking a vacuum gauge up to the intake on this thing.


Hammer Time
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Mar 1, 2020, 5:46 PM

Post #5 of 28 (864 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

The MAF is powered by the PCM1 fuse in the underhood fuse box.

It also powers all the trans shift solenoids and the Purge solenoid.
The fuel pressure spec is actually 55-62



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(This post was edited by Hammer Time on Mar 1, 2020, 5:52 PM)


MarineGrunt
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Mar 1, 2020, 6:07 PM

Post #6 of 28 (852 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

I actually didn't blow a fuse I just didn't turn the key back on when I started testing the harness again...lol. Although, it's not wanting to start and wasn't doing that until after I messed with the wiring harness. It has 5 wires and in the order listed below. I have only tested them with the key on engine off and I get the following volts.

Black
Beige 5v
black
pink 11.6v
yellow 5v

The fuel pressure gauge is reading 57 after the engine cranked. It looks like you found my problem with the wiring though just because fiddling with it definitely changed something. Should I go ahead and get a new harness and go from there?


Hammer Time
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Mar 1, 2020, 6:13 PM

Post #7 of 28 (850 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In





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Tom Greenleaf
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Mar 1, 2020, 6:25 PM

Post #8 of 28 (845 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

OK, Fuel pressure is too low but isn't the only game going on. Blowing a fuse? IDK, check with an ohmmeter not looks. It's not running at all now so that's first.
MG - that age old test is OLD but valid now. "T" in a vacuum hose to a gauge so no leaks just watching it. If you have a hand held pistol vacuum pump with gauge that set comes with all that don't need to pump it just use the gauge if short of all kinds of tools for things.
Pre-dates converters but valid still for any reason exhaust is restricted. Tells of single cylinder faults, vacuum leaks but not where and so on.
Add cancelling cylinders manually look for even drops fuel or spark issue or one bad cylinder?


MG: Have the same vehicles I did when you came here a 97 Chev P/U the grounds were falling off so was adding them all over the thing with better braided wire or real copper make some up.
What goes wrong is erratic when grounds lost? Yes, electricity is stupid just wants to find ground will do so any place it can find or if it can't like now you aren't even running is in the cards.


Gas tank has to be grounded, fuel neck so no sparks with cap off filling it just because. Those may go to a P/U's body or frame on this or both.


Exhaust system should be grounded if not add one or more. When know good spray or brush grease all you can find.


I know we both deal with the rust-belt games that's the trick that fuel line and all could have been saved or bad luck.


I have to use the truck cars don't even get wet so doesn't count.


Keep looking physically for flaws IDK if codes are screwed up if things are now so rusty, wires may be chaffed and so on. If not the upfront problem isn't a waste to do that stuff just time,


Tom



MarineGrunt
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Mar 1, 2020, 6:58 PM

Post #9 of 28 (835 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

I didn't blow a fuse Tom. I just thought I did because I made the dumb mistake of not turning the key back to run.

I installed a new fuel pump the other day. I've tested fuel pressure in the past for other issues and have always been around 50 psi. It's a little higher now with the new fuel pump. The lower reading might even be my gauge so might be worth picking up a new one.

The truck isn't that rusted. Sure, the frame has some surface rust but nothing extreme and especially for 17 years in the rust belt. I'll definitely look things over though.

HT mentioned earlier it could be a MAF sensor or wiring could cause some of the codes. The MAF sensor itself was only a few months old and I also had another new one laying around that I threw on just for the heck of it and it didn't help. I then decided to see what voltage the MAF sensor wiring harness was getting. Now the truck won't start only after fiddling with the harness. Time to start tracing down the wires.


MarineGrunt
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Mar 1, 2020, 7:02 PM

Post #10 of 28 (832 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

I'll have to pay close attention to the wire running to the back right of the engine. I had a tough time getting the starter off when pulling the transmission. I also replaced the oil pan gasket. Maybe I pinched a wire and it finally rubbed enough on something to ground out.


Hammer Time
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Mar 1, 2020, 7:05 PM

Post #11 of 28 (828 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

Don't discount Tom's first suggestion of a possible clogged Cat. That will also give you most of those codes, including the MAF.



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MarineGrunt
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Mar 1, 2020, 7:48 PM

Post #12 of 28 (821 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

I thought about pulling the O2 sensor earlier. Is that a good way to diagnose the cat? I have a Mighty Vac or probably some other gauges I could rig up but nothing to try and T into any hose.

I was kind of hoping the wiring harness was messed up somehow due to the no start right after messing with it. I tested each connector for continuity and it seems okay. I just don't get why it would all of the sudden not want to start when it started right up fine before. I get it's not running right but just seems like a big coincidence that it started happening right after that.

Not sure if this means anything but there's a lot of condensation coming out of the exhaust. There was a decent size puddle from only starting it long enough to check for spark for each coil.


Hammer Time
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Mar 1, 2020, 7:54 PM

Post #13 of 28 (819 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

You could always pull the plugs and look for water in the cylinders.
If you can't get it started, you can revert back to the Crank, no start testing.

You can test for bad cat in the 0/2 hole if you have a pressure gauge that reads down to 1 PSI and a fitting to screw into it. The vacuum gauge would be easier.



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MarineGrunt
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Mar 1, 2020, 8:38 PM

Post #14 of 28 (809 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

I'll go pick up a tee tomorrow and use a vacuum line since that's the easier route.

Check this out. I just took the brand new MAF sensor out and put the one back in I got two months ago. The truck started right up. It's still not running right but the new sensor obviously has an issue. The two month old sensor is the same brand. Am I better off using oem for a MAF sensor? It's not worth saving $50 if it's going to cause headaches every few months if it happens to be the issue.

I saw a video where you can test a MAF sensor using a multi meter. Can you get accurate results doing so? I tried cleaning it this morning but that didn't change anything.


Tom Greenleaf
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Mar 1, 2020, 11:22 PM

Post #15 of 28 (793 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

Missed a lot bear with me.
* Dump fuel pressure tester or 2nd one it can't be right.
* The remove O2 sensors: If a while they wont come out. Have Oxy/Acet always AYOR and plan on a kit to fix threads those pipes are from Mars type metal I never had great luck with those heard of welding in new threads - too, thread chasers just expect some hassle or nasty cost for just that crap!
* Oil pan + pinched wire. You do have to know check or redo that.


MG > You always did have some unreal tough ones this is no exception but am with you see how far we can go.
Rusted crap the nightmare of our areas and fast again this is OLD (like me now) does this to holes in less than 10 years never mind this old if real ROCK SALT used it will eat thru anything - the orange rock looking stuff if you see it fresh = OMG why do they use that for anything it's easier to learn to drive than fix ROADS too over that crap!


T


Hammer Time
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Mar 2, 2020, 3:49 AM

Post #16 of 28 (786 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

An aftermarket sensor shouldn't be that different. There is something else going on there.

I won't be available for the rest of today. Spending the day at the hospital. I had a heart valve replaced in December and even doctors get comebacks.........LOL
Talk to you later.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



MarineGrunt
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Mar 2, 2020, 8:52 AM

Post #17 of 28 (772 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

Wow...hopefully you're on the mend. Good luck at your appointments today. We'll talk to you later on today.

I don't know if this is even worth mentioning but I did get gas at a station I've never used before. The problem started after burning through about a 1/4 of it. It's never that easy for me though...lol

The wiring from MAF sensor runs to the O/2 sensors on the passenger side so not in the area where I thought I could've pinched something off. I'll still take a look at the them.

The spark plugs are pretty sooty. Every single one.

I'll try and test for vacuum, check for injector pulse, clean throttle body plate, and maybe clean injectors too. I guess it won't hurt to check compression too.


Tom Greenleaf
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Mar 2, 2020, 9:08 AM

Post #18 of 28 (764 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

Just me today I think MG.
GAS/Fuel or whatever junk we get?
Try to find a glass container get that clean and dry. Pour contents of fuel filter newer or not into that so you can see what the hell comes out.


Problem is out there as Ethanol absorbs moisture and or morphs with (I'm serious) over 200 formulations depending on where you live! How could I speak for every place all over it's nasty watched here with sign off tests at pumps or available inside places.


Just check it the alcohols would burn off if only that the water when/if excessive corrode lines, tank and on down EVERYTHING.


If you don't like the looks of what you get I do have real luck with SeaFoam brand just easy how much and quit if it works. I don't own one but those "Motor-Vac" things I'm told are forcing that stuff thru injectors as the trick!


If metallic yuk got thru and hardened up after filter it's going to be real hard to know but you said up top lots of evap codes is now more interesting with the possible fuel problem,


Tom


MarineGrunt
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Mar 3, 2020, 2:56 PM

Post #19 of 28 (727 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

I think I figured it out. I pulled codes again earlier and I had a P0118 Engine Coolant Temperature Circuit High Input which I've never had before. I pulled out the coolant temperature sensor connector and the connector was broken. The locking tab was missing and the plug part also had a piece missing. It actually had a zip tie through the 2 wires and was somewhat tightened around the sensor. It could'n't have been doing too much. Anyways, I'm sure I bumped it when pulling the plug or wire right there and that's what caused it to throw the code. I plugged it back in and the code went away but was still running rough. Since the connector won't be in until tomorrow I decided to rig something up from an old plug I had laying around just to make sure it had a good connection. I mainly wanted to be able to continue troubleshooting today. Anyways, once I hooked up the temporary homemade plug it ran fine and no more P0118 code.

I assume a bad coolant temp sensor connector could've been the culprit of all my issues? I'm sure it explains the fouled plugs and the gassy exhaust smell. What about all of the other codes? I'm just doubtful because I'm never so lucky to where it's just a $15 two wire connector. Normally I have to pull the transmission, rear end, or rebuild something...lol


Hammer Time
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Mar 3, 2020, 3:01 PM

Post #20 of 28 (724 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

Well, yes and no. It could have caused the "no start", the lean codes and some of the symptoms but there is no way it caused a MAF code.



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MarineGrunt
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Mar 3, 2020, 4:20 PM

Post #21 of 28 (719 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

How'd your appointments go yesterday? Hope you're healing up well.

I did do the vacuum test earlier and I got about 18. When I did the throttle tap it dropped to near 0 and quickly jumped up to 25 and then settled back at 18.

I looked at the O/2 sensor live data on my cheap scanner and they were mainly around .8 or so but would jump down to .1 or so and back up. I took a video but, from my understanding, if one is bad they don't usually jump around and just stay in place? I really need to just get a good scanner and learn how to use it and read live data. I've been talking about it for years. I never need to because you always figure my issue out...lol

I'm not getting any codes at the moment but I didn't go on a very long drive. The P0300 and evap codes were popping up right away after being cleared until I took care of the coolant temp connector. The original MAF code and lean condition codes never did come back up after initially being cleared even when the engine was still running rough. One of the first things I tried was cleaning the MAF sensor so maybe that was the initial cause of the P0101 and was only partially to blame for my rough engine? I'll drive it over the next few days, see what codes I get, and go from there. Unless there are some other things worth checking.


Hammer Time
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Mar 3, 2020, 4:29 PM

Post #22 of 28 (717 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

They didn't make any repairs on me yet. Yesterday was a day full of inner probing and measuring. I guess they will let me know what the next step will be.

Yeah, get everything cleared and reset and start from scratch so you get a clear picture of what is going on. You really should always read codes, then clear them to see what resets. A false code triggered by something you did can send you on a wild goose chase.

Your vacuum test pretty much eliminates the cat, assuming you held those RPMs up for a little while. That's when the problem shows up.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



MarineGrunt
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Mar 4, 2020, 2:41 PM

Post #23 of 28 (679 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

It's amazing what all the medical field can do nowadays. They're especially good at taking our money...lol.

I drove it for about 75 miles today and I'm just getting the..

P0449 Evaporative Emission System Vent Control Circuit Intermittent

P0300 Random Misfire Detected

I've had those two codes in the past even before my engine started running a little rough. The misfire hasn't been there for quite awhile though. The evap code has been hanging around for longer than I care to admit. I really need to dig into them and get it figured out.


Hammer Time
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Mar 4, 2020, 2:47 PM

Post #24 of 28 (677 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

The EVAP code has nothing to do with the engine running so put that on the back burner for the purpose of this conversation.

The P0300 is still a problem and you need to find out what the cause is. I would take a hard look at the fuel pressure and the way it is acting. We already know it is too low or you have an inaccurate gauge. See if it is holding rest pressure. Leaking fuel injectors and regulators are a big problem with Vortecs. The low pressure could be triggering the lean codes and misfires too.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



MarineGrunt
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Mar 4, 2020, 3:15 PM

Post #25 of 28 (673 views)
Re: 2003 GMC Sierra 4.8 running rough with multiple fault codes Sign In

Sounds like a plan. I'll do as you say and check rest pressure and also work on getting another gauge. Just to recap I installed all new fuel lines and a filter a few months ago and installed a new fuel pump so everything is new back there except for the evap box and the lines from it to the pump.

I've never had to touch the top end of this engine. It has a little over 150K now so I guess anything is possible. I'll let you know what I find. Thanks






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