Crystalflow

I am new to this forum and would appreciate any help that you could provide.

I have a 2006 Express 2500 with a 4.8l engine.
I have misfires on cylinders 1,5,6 and 8. This only happens if I rev the engine above 2500 rpm, or I'm at cruising speed above 80kph.

my fuel trims are correct at idle and no misfires at idle. The fuel trims go up under the misfire conditions.

What I've done so far:

new plugs and wires
sprayed intake manifold to check for vacuum leaks. No change in fuel trims or misfires.
swapped coils from cyl 7 (no misfires) to cyl 5 (most misfires) this didn't change anything. I'm still missing on 5.
ran a compression test on cylinder 5. Over 150 psi. Seems good to me.
I have not swapped injectors as it's a royal pain!

Any suggestions are appreciated.

mountainmanjoe

The diagnostic flowchart for misfires is kind of long. I'd suggest picking up something like a Hanes manual
https://haynes.com/en-us/gmc/savana-2500/1996-2010

Here's a few easy things you can check.
- First, is it actually misfiring? Can you hear knocking? If not, it could just be a bad knock sensor, or it's picking up vibrations from somewhere else.
- Spray water on the secondary ignition components using a spray bottle. Look and listen for arcing or misfiring.
- Check the exhaust system for restrictions
- How many miles/kms on the engine? The timing chain wears out eventually.
- Read https://www.obd-codes.com/p0300

The PCM monitors information from all of these sensors to control spark timing:

• The throttle position (TP) sensor
• The engine coolant temperature (ECT) sensor
• The mass air flow (MAF) sensor
• The intake air temperature (IAT) sensor
• The vehicle speed sensor (VSS)
• The transmission gear position or range information sensors
• The engine knock sensors (KS)
• Crankshaft Position (CKP) Sensor
• Camshaft Position (CMP) Sensor

Crystalflow

I don't hear or feel any misfires but my fuel economy is worse and the CEL flashes at highway speeds.

if it was fuel pressure, maf, vacuum, then I would see misfires on all cylinders. I sprayed carb cleaner around the intake manifold and it made no difference.im thinking injectors at this point or maybe a crankshaft sensor relearn?

mountainmanjoe

Then maybe you don't have any.

Of course. The engine is not running efficiently.


I didn't say anything about fuel pressure. Bad ignition timing caused by faulty sensor reading do not necessarily cause misfire in all cylinders all the time. Also, misfire codes are not exact. There can actually be more or less than the PCM reports because knock sensors are not perfect.

I did see that in your original post. However, as I said, spraying water around the ignition components can reveal electrical arcing. (do it in the dark).

tech2

if the check engine light is flashing...you have misfires. Chevy has misfire counters...you can go into scan data and view the accumulation of the misfire counters.

at high rpms high backpressure from broken or clogged cats can cause a misfire.

Hammer fist the cats to see if anything rattles...if no....unscrew the 2 pre cat 02 sensors out of the exhaust pipe so the exhaust can bypass the cats.... take it for a run...there will be a solid check engine light but its should not flash if the misfire is gone.

mountainmanjoe

This is taken straight from GM documentation:

tech2

p0300 and a flashing mil have different severity. motorist.org/articles/check-engine-light

Crystalflow

I will try the exhaust trick today.
I do have a scan tool that sees the live data misfires and counters. The misfires are on both banks in random locations.

StanVan

"Random" might just be an intermittent ground somewhere.

Crystalflow

I was thinking about that but the misfires are always on the same 4 cylinders. #5 is always the worst one.

There also must be a reason why it's fine at idle, but appears when I rev it, or I'm at cruising speed.

it wont misfire when I floor it on the highway.