Jump to content

04 5.3 Sierra. New misfire problem

Recommended Posts

Few months ago I bought a 04 Sierra. Ran great for exactly 103 miles then problems started . Seller told me it still had 02 sensor code  showing ( I'll post pics)  even though he'd replaced all 4 twice . This going to be long cause I want it all out there. He replaced the following to my knowledge. 02's, map, maf, ect, PCM, throttle body, plugs, wires, coils , injectors, pretty much everything but knock sensors. I replaced 1 o2 because I saw 1 (new) was bent. Problems started with loss of power and MPG. Started running very lean. Using multimeter I've checked the wiring on everything and it all seems fine. While troubleshooting the power loss and gas mileage past few weeks , 2days ago it's developed a misfire. When you start it about 15seconds in it starts missing and sounds like crap. Like running on 3 cylinders or something. It's not everytime and when it happens turn off and on and your good . 

Last night I went through the plugs and gapped them at . 40. They were all . 32 for some reason.  I check fuel pressure, it holds at 52. So I'm thinking bad injector?

I've tested 5 so far (ran outta time) 1,2,4,and 6 show 13.1-13.6

#8 shows 12.9

Still have to check 3,5,7.

But as you see in pics , problem is on bank 2.

I know this is long post, I hope someone reads it all lol. 

Some of the spark plugs keep loosening. Like very loose. I  Never had that problem , they are the right size too .

The 2 pics of 02 banks volts I'm thinking shows at bad B2 CAT . I I right on that? 



Link to comment
Share on other sites

What brand and part number plug did PO use? These engines kinda like AC Delco and NGK and the correct number already correctly gapped to .040. Platinum or Irdium. Higher mileages, I'd just go with Plats. Answer my first question.


Also some other parts that have been replaced may still be your problem. Some aftermarket brands of O2 sensors seem to have issues, same with knock sensors.


Spec on fuel pressure is 55-62 @ key on, engine off. As you start straying below that you may begin to have issues. Your 52 reading...  is that after it has sat for a minute or is it KOEO and immediate check?


Who actually replace all this stuff previously? A qualified shop or shade tree Joe? You don't just pop a new PCM in without doing another step.


Have you done a compression test on all cylinders?

Link to comment
Share on other sites

Plugs becoming loose, that S_ _ t ain't right. txab is right; just because the guy installed this 'n that sure doesn't mean he bought first quality parts. Most better plugs these days are pre-gapped too so that's kind weird... Installing a PCM is big stuff, I have never done it (knock on wood) but it is certainly something that has to be done "By The Book", I'm a big btb guy having worked on Uncle Sam's stuff for 38 years.

  • Like 1
Link to comment
Share on other sites

The PCM would need to be programmed for the vehicle. A crank relearn would need to be performed also. Grabbing a junkyard PCM doesn't always cut it. I've swapped PCMs due to tuning. No issues when the steps are followed. Yes the loose plugs are concerning. That's why I wanted to know brand/part number.


I'd wonder what all brands parts were used. So many of those parts may still be involved. Something else to think about..... did he really replace all that or is he blowing smoke?

Link to comment
Share on other sites

  • 2 weeks later...

I appreciate y'all helping me out with this I really do. I'ma gonna try to add some pics n video. I'll have to look at the PCM for brand, but definitely looks new . The plugs I bought was ngk and we're already gapped. The ones the guy put in was the same ngks which again makes me wonder why he changed the gap to 32. I did find some significant exaust leaks by hooking up a shop vac and using spray bottle. The exaust manifold is leaking pretty good on passenger side. And the gasket connecting the flange gasket pretty bad leak.





Link to comment
Share on other sites

My fuel pressure is still low, idk if those leaks would do that. I just tried to link the video but says it's too large. ( only 10 sec).I'm not the best at this, I can turn a wrench but the diagnostic side is definitely my weak side. That's why I say I appreciate yall

Link to comment
Share on other sites

Fix the exhaust leaks first. Your O2 sensors aren't getting an accurate reading of what is happening in the combustion chamber. The PCM is trying to adjust the engine based on the lies the bank 2 O2 sensors are spreading. 


The screenshot of bank 2 sensors is telling. Sensor 2 is following sensor 1.

Link to comment
Share on other sites

5 hours ago, denver110 said:

I know you all told me these trucks like certain brands, I'm buying the manifold gaskets today and gonna work on it this weekend, is there a particular brand this old hussy would prefer? She's turning out to be kinda moody so I wanna do it right



Want it right, get OEM.  The gaskets and flange seals aren't that much, and the OE ones are quality stuff.   


Especially if you need O2 sensors.  GM vehicles do NOT like any aftermarket O2 sensors at all.  We see this at least once or twice a month at work.      

Link to comment
Share on other sites

A lean condition with low fuel pressure and misfires indicates fuel pump, has that been replaced?


How was lean condition determined? (Don't I remember you having low o2 sensor voltage?)


How was deficient fuel pressure determined?


Trying to see if its one problem or multiple.



Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.