02 sierra 2500hd cylinder 7 misfire

EvergreenNJ · September 26, 2019

Since I bought this truck used ive had a misfire in cylinder 7 (p0307). Since then ive had a tune up considering the truck had 160k when i bought it. So all new spark plugs. Changed the ignition coil. Swapped around the plugs and wires but it still comes back to cylinder 7. There isnt a pattern to the misfire. Could happen once a week or every other day. Only misfires upon starting (heavily) and then after about 5 minutes stops or usually if i turn it off and back on it stops. Never happens in the middle of driving. Not even close to being a mechanic but im thinking it could be the injector or maybe a clogged fuel rail? Could really use some insight if anyones had this problem.

Thanks,

Rich

CamGTP · September 26, 2019

More the injector first, see if that helps.

Then I'd be looking more into that cylinder, as in removing the valve cover and checking the valve spring. Checking cold compression on that cylinder and comparing it to the spec and the rest of the cylinders on that bank.

I'd even perform a crank/cam or another name a case relearn to sync everything back up with a scan tool.

davester · September 26, 2019

With a diagnostic computer and a fuel pressure gauge, you can perform a leakdown test on the injector, to see how that injector is working compared with the rest (this isn't a definitive test, just will tell you if it's basically working ok).

Short of someone packing dirt into the fuel rail manually, there is no way it gets plugged without every cylinder having fuel problems, as every injector would be significantly plugged with dirt. That said, your truck has a fuel filter along the inside of the driver's side frame rail, that you may want to replace if it hasn't been done recently.

You could maybe pull the spark plug, and compare how it looks vs other plugs, and maybe do a cylinder leakdown and compression test (separate tests) to see if there are problems inside the cylinder.

muddkatt · September 26, 2019

Cylinder 7 is the lowest point on the rail and the first one that can stop up if trash gets into the lines. Pull the injector and examine the filter screen. Back on those years they have an in-line fuel filter. Replace it with either a AC-Delco or Wix. I had a Fram let crap get though it and it clogged the #7 injector.

EvergreenNJ · September 27, 2019

Thank you guys for the quick responses, very informative.

I did forget to mention that I did a leak down test on the fuel pressure system. It held pressure but had a very slow leak which someone had suggested may be a leaking injector rather than a failing fuel pump/ check valve. I also took the cap off the fuel regulator which was sucking air, and i didnt smell or see any leaking gas. Also watching the gauge which was around 45-50 psi, it jumped up about 10psi upon taking it off the regulator. Figure id add this info as it might be helpful.

So moneys a bit tight and id like to look into doing the compression test over having a mechanic do it. I will try replacing the injector first. If you guys have any good links to either reading material or videos on some of these procedures please post them up.

@davester i think the fuel filter is long overdue and i will be looking into replacing it this weekend. Im sure its housing is probably covered in rust as was everything else that ive replaced on this truck

@CamGTP ive been researching this issue for a while and ive heard some others talking about a relearn and after doing their problem was resolved.

@muddkatt just ordered the acdelco from autozone

Any more info is appreciated, thanks.

Rich

riverbanks · September 27, 2019

45,too low,do the filter change,start there,lube those fittings for a day or so,first

EvergreenNJ · September 27, 2019

@riverbanks it should be in the 50-55 range correct? This im guessing would indicate a clog in the filter or a weak pump?

The fuel filter is rusted terribly. Should be a fun job replacing it. Im going to spray it tonight with some PB blaster and let it sit for about 24 hours before i attempt it.

So I ordered the more expensive ac delco injector along with an 8 piece ac delco O ring set(figure ill change the o rings on the side i already have accessible). What type of mineral oil or lithium grease should i use on the o rings when putting the injectors back in? Any brand reccomendations? A little unclear on what im supposed to be using.

riverbanks · September 28, 2019

50 is the bottom,baby it,it works,but if you have another weak link,like that injector,it's the domino effect, probably,if you had 58 to 62 psi, you would still have a misfire count on #7,but less,and engine oil is what I use for orings,good luck

EvergreenNJ · September 30, 2019

Update:

Pulled the fuel rail and all the injectors. Cleaned them the best i could with carb cleaner. Changed every O-ring. Replaced #7 with a brand new one. Reinstalled and turned the truck on and, misfire. Fuel filter is next to get changed. Still letting it soak because it is badly rusted.

Gagliano7 · October 3, 2019

Does the truck run like it has a misfire or you just get the code? I know i read somewhere that a bad crank or cam sensor(can't remember which one) can cause false misfire readings.

Richard Carrino · March 1, 2020

Few months later and I’m having some more issues with the truck. Mid drive today I got a check engine light and it reads p0300 random misfire. So I’m a bit lost on where to start diagnosing. I checked the coils, they all had spark. Sprayed brake cleaner around the valve covers and didn’t see a change in the idle. Truck seems fine when it’s driving but once you hit 50 mph it won’t go any higher, you can feel it start to shake and lose power and at idle you can feel it shaking. Tomorrow I’m going to run a compression test on all the cylinders and see what the results come back. Any advice on what could be causing the issue and where to start testing? Season just started for my new business, can’t be without a truck.