2007 5.3 misfires, rough idle, no other codes

torquemeister · December 15, 2019

I have a 2007 5.3L that started getting P0308 misfire codes on cylinder #8 a couple of months ago. There were no issues with how it drove or idled at that time. Last week it switched to the generic P0300 code and started idling really rough; it gets a little better if you put it in park or neutral with higher RPMs. I changed plugs and wires with AC Delco for both. I found the PCV tube broken and replaced that. I checked fuel pressure at the rail, it's 58-60 at idle and running, and 60 with key on engine off.

I bought a scanner tool that gives misfire counts and live o2 voltages. cylinders 7 & 8 both have hundreds of misfires over the course of a 10 minute idle in the driveway. All 4 o2 sensors read around .85V with very little fluctuation. The fuel status is listed as Open Loop FAULT.

The only code I've seen recently was the P0455 which I assume came from opening the gas cap when depressurizing the fuel system to connect the pressure gauge.

What should my next step be?

Thanks,

-Chris

davester · December 15, 2019

Also check fuel pressure when the engine is revved up. FP may drop a little while the engine is speeding up, but quickly return to full pressure once engine speed stabilizes at the new higher RPM.

riverbanks · December 15, 2019

With the scanner,do you see temp. rising to,195,200°?and what monitors are incomplete?

torquemeister · December 15, 2019

I never saw the fuel pressure drop below ~53, even while hitting the gas.

I didn't notice the coolant temp on the scanner; the dash gauge was reading 210 (normal) once it was warmed up.

I'm not sure what you mean about incomplete monitors?

Thanks,

-Chris

riverbanks · December 16, 2019

Trying to see why you are in open loop,scroll down on list,read codes,1,freeze frame,2,obd readiness,3,that's the monitors,and not in that order, just a reference

swathdiver · December 16, 2019

You're probably going to have to pull the valve covers and inspect the valve train on 7 and 8. Your upstream O2 sensors should be moving up and down and the downstream sensors fairly steady once warmed up. Problems like these usually take out the cats so once you get the motor fixed, watch the cats.

If you wind up replacing the lifters and trays and a pushrod or two, replace the VLOM as well with the updated one, along with the newest valve cover for the driver's side.

MaverickZ71 · December 19, 2019

On 12/16/2019 at 8:19 AM, swathdiver said:

You're probably going to have to pull the valve covers and inspect the valve train on 7 and 8. Your upstream O2 sensors should be moving up and down and the downstream sensors fairly steady once warmed up. Problems like these usually take out the cats so once you get the motor fixed, watch the cats.

If you wind up replacing the lifters and trays and a pushrod or two, replace the VLOM as well with the updated one, along with the newest valve cover for the driver's side.

What years of the GMT-900 need the updated VLOM?

swathdiver · December 19, 2019

17 minutes ago, MaverickZ71 said:

What years of the GMT-900 need the updated VLOM?

The updated VLOM came out sometime in 2011. Anytime the system is worked on, it is SOP to replace it with new along with lifter trays.

MaverickZ71 · December 19, 2019

4 minutes ago, swathdiver said:

The updated VLOM came out sometime in 2011. Anytime the system is worked on, it is SOP to replace it with new along with lifter trays.

First time I've heard that. VERY GOOD TO KNOW! Thanks!

torquemeister · December 24, 2019

Merry Christmas! I finally had some time to get back on this yesterday.

I was really hoping to avoid pulling the valve covers. I'm not familiar with the VLOM so I'll have to read up on that. I replaced fuel injectors 7 & 8 and it idled rough but without codes. That was before I put the air hose and resonator back on. Once I put it all back together, it started throwing the P0300 code again for misfires and 7 & 8. That makes me think something like not getting enough air. I don't see any holes or cracks in the plastic from the air filter to the throttle body. Could it be a MAF issue? While running without the air hose/resonator, it went into closed loop and had no misfires. Once reconnected, it went back to open loop - fault and started misfiring. The upstream o2 sensors would go down to about .05 when I pressed the accelerator and then back up to .8 when idling. No codes for MAF, even when it wasn't hooked up to the air hose and was reading 0.00. I thought that would throw a code for sure.

Thanks,

-Chris

sefiroxx · December 28, 2019

Get some MAF cleaner.

Normal o2 voltage is .5. If you are bouncing around .8, you are running rich. MAF is probably reporting too much air.

torquemeister · January 5, 2020

When watching the scanner live data, I'm seeing it jump to 45-50 degrees of timing advance on #1 cylinder when RPM are in the 2500 range. It settles back down to 10-15 range at idle. Is this normal? I would never have guessed timing would be that advanced on a stock engine, so it might be a clue. The misfires go away completely if I just unplug the MAF sensor, but it still runs really rough and throws the stabilitrak disabled codes, which cause violent 1st to 2nd gear shifts.

Thanks for everybody's help on this,

-Chris

riverbanks · January 5, 2020

Do you have 12v and ground at MAF?pink,power,black w/white ground

torquemeister · January 5, 2020

with engine running I had 14.4V on the power and ground. Hz readings were 2.7kHz at idle and it went up to around 4.5kHz when it was around 2500 RPM. It seemed to be responding correctly. The scanner's live data on the MAF was about 6g/s at idle and all the way up to about 33g/s at 2500 RPM.

Thanks,

-Chris