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P0307 Cyl #7 Misfire, P0505 High Idle, P0404 EGR Valve

Troubleshooting & Repair

P0307 Cyl #7 Misfire, P0505 High Idle, P0404 EGR Valve

Postby patman [OP] » May 24 2013, 8:10pm

So I started to post this as a blog but I was asked to move this to a forum topic so it'd be more visible and for better searchability. I'll be outlining the steps I took to diagnose and fix a few issues with my Sierra:

Background info and list of issues - 2000 GMC Sierra 1500 SLE, Ext. Cab, 5.3L. 2WD

This is going to cover a misfire on cylinder #7, a high idle I caused fixing the misfire and a little bit on an EGR Valve trouble code.

The truck had a noticeable misfire when I picked it up. Scanner showed P0307 Cyl #7 Misfire and P0404 EGR Valve codes set in the computer. There was a slight miss at idle and the motor was a little down on power when driving and was rough and obviously misfiring from the vibration coming through the drivetrain. The Check Engine light was not functioning when I first picked up the truck but after repair the light would frequently flash indicating that the computer was detecting a misfire that was bad enough that it was worried about damaging the cats.

Step 1 - Ignition

Ignition is a good place to start when tracking down misfires.

  • Inspect #7 spark plug, wire and connections to coil - Old plug looked OK but was wet, wire & coil appear to be OK.
  • Replaced #7 spark plug with new plug.
  • Swapped #7 & #5 spark plug wires.

If misfire goes away then spark plug was bad, if misfire moves to cylinder #5 then plug wire is bad, if misfire stays on #7 then suspect #7 coil pack or fuel injector, a vacuum leak at intake manifold gasket around cylinder #7 or (god forbid) a mechanical issue with that cylinder.

  • Cleared codes and test drove truck - misfire persisted, P0307 code set again.
  • Tested coil #7 resistance of primary & secondary coils. The manual wasn't clear about the correct readings of V8 coils but they matched the readings I got off other known working coils.
  • Swapped #7 & #5 coils

If misfire moves to cylinder #5 then coil is bad, if misfire stays on #7 then move on to fuel, vacuum leaks or mechanical issues.

To be completely thorough, I should have checked for spark on #7 too but on a test drive I discovered that the misfire went away at large throttle openings which led me to believe that it was getting spark and that a vacuum leak was more likely as a leak becomes a relatively smaller source of unmetered air as you open the throttle and it's easier for air to get in through the proper metered path. I also didn't have the proper tool to check for spark as the manual advised against the old trick of just taking a spark plug out and grounding it to the block while it's running as it could damage the coil and/or PCM - plus I always get the crap shocked out of me when I do that no matter hown many gloves and rubber handled pliers I try to insulate myself with. Also, the PCM should usually give you a code if the wiring to the coil is damaged or it's shorted.

  • Cleared codes and test drove truck - misfire persisted, P0307 code set again.

At this point I could pretty much rule out ignition problems as the source of the misfire. An internet search indicated that cracked intake manifold gaskets were a very common source of misfires on 5.3L motors, especially on cylinder #7. Gunk collecting at the end of the fuel rails and clogged injectors were also noted to be common sources of trouble.

Step 2 - Fuel & Air
  • Inspect wiring to fuel injector - appeared OK
  • Check for fuel injector operation by placing a looooong screw driver against injector and listening for clacking sound of solenoid operating - injector sounded good

The fact that the #7 sparkplug was wet indicated that fuel was making it to the cylinder so the injector must be working to some degree so a vacuum leak seemed like the more likely culprit

  • Sprayed area around #7 intake port with carb cleaner - inconclusive, no change in RPMs but leak could be on backside of manifold where it can't be easil reached

At this point, some disassembly was necessary to determine anything further so the fuel rails and intake manifold were removed as per instructions in Haynes repair manual. Top of engine had collected a bunch of loose gunk and leaves so I vacuumed as much out with a shop vac before removing anything to avoid dropping crap into the intake ports.

  • Inspected fuel injectors and rails - gunk on end of some injectors but that was almost certainly picked up after the fuel rail was removed by dragging in the accumulated gunk on the exposed surface of the head and valve cover. Rails appeared clean.
  • Cleaned injectors externally and flushed rails
  • Replaced injector O-rings
  • Removed and inspected lower intake manifold gaskets - gaskets didn't look too bad at first but after they'd dried out for a few minutes there were very obvious cracks and sizable gaps in the rubber gasket seals.
That appeared to be the problem right there so off to the parts store for a new set of lower intake manifold gaskets.

  • cleaned cylinder head mating surfaces, engine valley cover and valve covers
  • cleaned intake manifold mating surfaced and inside of intake runners
  • Checked throttle body for carbon buildup - appeared clean
  • Replaced intake manifold gaskets
  • Reassembled engine
  • Cleared codes and test drove truck

Misfire totally gone, engine idles & runs much smoother, has much more power. Can't believe how nicely it runs for 210K miles

Oops. P0505 - High Idle
After repair I noticed the idle seemed a little high in some situations, it took a long time to drop down to low idle when coming to a stop, at low speeds you had to fight the engine a little on the brakes and coasting down hills it really picked up speed quickly. The PCM set a P0505 code after a few trips. My first guess was a vacuum leak at the EGR tube where it goes into the intake manifold behind the throttle body - it was difficult to get it lined up right and I suspected it had not seated properly

  • Visual inspection revealed that EGR tube flange was not sitting flush against intake manifold
  • With engine running, air could be heard being sucked in around the throttle body in the area of the EGR tube
  • Sprayed carb cleaner around EGR tube inlet and RPMs rose with each squirt
  • removed EGR tube bolt from intake manifold
  • removed EGR valve to cylinder head bracket bolts to allow freer movement of tube
  • removed EGR tube from intake - found O-ring had been pinched and didn't seal properly
  • reinserted O-ring into intake manifold - orifice is stepped - make sure O-ring is seated fully against step and it's not folded or bunched anywhere
  • lubricated O-ring and end of EGR tube
  • reinstalled EGR tube taking more care to line it up straight and tightened it down (why is the flange made of such thin, bendable metal but then only bolt on one side to the intake? It's bound to get bent, not seat flush and for the tube to go in at an angle!)
  • reinstalled EGR bracket bolts
  • started engine - idle back to normal
  • sprayed EGR tube area with carb cleaner - no change in idle speed

So, that took care of that.

P0404 - EGR Valve

Truck also had a P0404 code set which indicates that the EGR valve is not opening to where the PCM thinks it should be. While I had the top of the motor apart I pulled the EGR valve off hoping that cleaning it would fix it as they often get gunked up with exhaust gasses byproducts. The pintle was frozen closed with corrosion and carbon but I was able to free it up pretty easily. I cleaned it as best I could and put it all back together but the P0404 code has returned. It probably just needs to be replaced - I suspect either the servo burned up trying to open the stuck valve, or it died first and then the valve froze shut from disuse - but as it doesn't cause any real driveability or reliability issues while stuck shut and i don't have to pass emissions, its been pushed down on my list of things to worry about. I'll get back to it eventually after I take care of more urgent stuff.

  • Bad intake manifold gasket
  • Didn't line the EGR tube up very well
  • EGR valve is probably toast
2000 GMC Sierra 1500 SLE
Ext. Cab | 5.3L | 2WD
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Year: 2000
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