Misfire at certain speed

[Silasyac]

Hey guys I’m getting a po308 code cyl misfire 8. Changed distributed cap and rotor. Still persists. The misfire only comes on at around 140 kmh at around 3k rpm in overdrive gear. Putting a feeler out to see if any of you guys had this problem or any ideas on what this could be. Any advice appreciated

 

[Road Trip]

Good morning Silasyac,

Welcome to the GMT400 forum. There are a lot of folks in here who share a
deep appreciation for how this generation of the General's trucks look, run, and
work. Big bang for the buck, if you know what I mean.

But before anyone here can hazard an informed guess as to what's ailing your
vehicle, we first need to know the year, model, which engine/transmission/rear
end gearing/tire size, whether the vehicle is stone stock, or has it been modified
over the decades?

For example, troubleshooting the ignition system on the '88-'95 TBI versions of
our vehicles is different than the '96+ Vortec versions. Actually, it can be quite
a bit different once you start chasing cam (distributor) <> crank phasing, resulting
DTCs, etc.

In other words, help us help you. Pretend that we are sitting in closets, and we don't
see anything that you don't share on the screen. Photos really are worth a thousand
words, especially when looking at the underside of a distributor cap and looking for
a failed electrical pathway or carbon tracking.

Please reply with the requested data, and I'm sure that someone who's solved a similar
issue in their own engine bay will join in and help you keep your GMT400 on the road.

Best of luck with the troubleshooting hunt.

Cheers --

 

[Road Trip]

Hey guys I’m getting a po308 code cyl misfire 8. Changed distributed cap and rotor. Still persists. The misfire only comes on at around 140 kmh at around 3k rpm in overdrive gear. Putting a feeler out to see if any of you guys had this problem or any ideas on what this could be. Any advice appreciated

You have mentioned changing the cap & rotor with no change. How old are the wires and the plugs? Is it possible to
share a clear photo of the #8 plug showing the condition of the electrodes and the color of the porcelain? And if it
obviously looks to be off, possibly pull a second plug from another cylinder that never shows a misfire and take a
photo of both so that we can compare & contrast them?

The troubleshooting thought process is that we are trying to figure out which scenario are we looking at?

* Global issue affecting all cylinders, but #8 is the first cylinder to cross the error detection boundary.
* 7 cylinders running fine, problem isolated to #8 cylinder as the outlier.
* Problem being reported by the computer as if it's the #8 cylinder, but the computer is being misled by
other issues and the problem is physically elsewhere. (Lower probability, but included for completeness.)

Again, the more detail you share the more that the talent in here can refine their guidance. And as much
recent history of what's been done to the truck will be helpful. We need spark, fuel, compression *and* a
good feedback loop around all this stuff in order for these constantly computer-tuned engine bays to work as
advertised.

 

[Schurkey]

Hey guys I’m getting a po308 code cyl misfire 8.

What is the misfire COUNT for that cylinder?

How about all the other cylinders?

Changed distributed cap and rotor. Still persists. The misfire only comes on at around 140 kmh

87 mph.

at around 3k rpm in overdrive gear.

Engine under substantial load. Ignition voltage fairly high, and also fuel demand.

What is the fuel pressure under load?
How old are all the rest of the ignition parts?
How old are the O2 sensors?
When was the MAF last cleaned/checked?

Putting a feeler out to see if any of you guys had this problem or any ideas on what this could be. Any advice appreciated

When my truck had a high-speed misfire, it gradually got worse--appearing at progressively lower speeds. Eventually it stalled at idle and wouldn't re-start. But that was a TBI, and the TBI distributor had a faulty magnet on the pickup coil. Doesn't apply to a Vortec engine.