6.0L LQ4 random misfire

[Mater]

ok first the vehicle 2003 silverado LS 2500HD 4x4 automatic former plow truck fleet vehicle 114,000 miles 1800 hours


ok the lq4 in the truck has developed a misfire at 2500rpm or above the only cylinders that are misfiring are 1,5,6,and 8 it is only throwing a p0300 random misfire code


i have changed plugs, wires, and coils all with AC delco parts from the dealer. i even switched out the fuel filter and it still persists at 2500 or above. it also idles rough but does nto show a misfire count


i am about ready to set this ####ing thing on fire

[Russell]

Dude that sucks, pour some Lucas fuel treatment in it. That's what I would do b/c I have no clue how to fix it.

[Mater]

might as well frist thought was crank sensor but only being 4 cylinders that is very unlikely if it was all 8 then i could see it.


will hook up my fuel pressure gauge and drive it and see what happens. fuel pump may be getting tired but still does not make sense only being 4 cylinders

[Andy4639]

Check the knock senors. If they are good check 0-2's they could be clogged up are not reading correctly. On your truck you should have 4 of them 2 before the cats and 2 after. As stated if it was the fuel pump all of them would be same as the crank senor. If all else fails find a tuner in your area that has HP tuner and have him look at it.

[Damon23]

I don't know the firing order on those motors but maybe a fuel injector or harness? My Excursion was having a issue like that a couple years ago and it turned out to be three bad injectors, probably from some bad fuel I got. Andy's advice is probably more in line with the problem though.

[Mater]

swapped coils on 1 and 3 stayed with one. swapped plug wires stayed with 1, swapped plug stayed with 1

knock senors are reading correctly. really do not feel like trying to get the o2s out of there dwelling as i do not have an instrument of fire. (former plow truck if they come out there are not going to be able to go back in)


i did check fuel pressure this morning as 1 and 8 are at the ends of the log and 5 6 are at the junction where it splits and i tried every thing else that was easy simple.

fuel pressure is acting weird i have a return style system as it;s an early 2003 and on start up it goes to 50 then falls to 45 at idle then under load high rpm when the misfires start up it drops to about 30. let off fuel pressure rises back up misfires go away until the pressure starts dropping again

i'll swapp the coil harness as i have a spare just to see but the fuel pressure is confusing me

[Mater]

here is the firing order sense some one asked

1-8-7-2-6-5-4-3

companion cylinders are misfiring not sure if this information will help or not but it is a thing.

[Andy4639]

Fuel pressure is the problem if it's like you say it is. These motors need close to 58 PSI when running.
The fuel pressure is low,these must have no less than 55 psi at all times 50 psi is not enough..perhaps the regulator is bad..or,most likely the fuel pump is getting weak.

[Mater]

Regulator is cheap and is easy to get to will change it out first. Also going to price a pump seems to be a very common failure

[Mater]

changed the fuel regulator (my original one was just that original and leaking out the vac line) fuel pressure has risen to 50psi but only 50psi with the vac line off it only make it to about 58psi then drops after it starts running to 53..and it still misfires on the same cylinders just takes longer to do so

except 6. it improved the most going forma 250 misfire count down to 50. 1 went form a 230 to 109. 5 and 8 are staying at a 260 count


this is all with in a 5 minute drive at 60mph in 3rd forcing the RPMs at 2700.

[Mater]

My pcv valve is rusty and the hose going to the intake is dry rotted.

The pcv was also sludged over on the bottom from what looked and smelled like mud. Under the rocker looked fine.

New pcv and hose did not fix the issue mpg went up though still have p0300 at 2500 and above

[Mater]

well found the fuel pressure spec 55-62psi so i am going to go ahead and swap out the pump. may even get a second fuel filter as the new one could of just got goobered

[JOES66FURY]

A little late to the party but I see this pretty often in the shop. I was gonna say fuel pump, just did one this weekend for the same reason. We hooked up a fuel press gauge to and it would pump to 60+ but when we turn the key it would drop off then build up again...then after driving it would drop off and run like poo tripping the misfire...I bet I've done 6 in the last 6 months for the same problem.

[Mater]

just bought a motorcycle jack to drop the tank. bonus i can also use it as a trans jack as it has tie downs on the lift portion

i love you harbor freight


but yeah i was just confused about it only being 4 cylinders but talked to the chevy dealer and one of the techs said it's normal as the other 4 are getting enough fuel but starving the rest

[Runner1972]

I remember seeing a 6.0 with busted bee hives that did the same thing.