David Purdy

I'm getting a consisten misfire above 2000 RPM on four cylinders. Looks like 1 5 6 8, worst offender is number 6. It doesn't misfire under 2k, consistently reproducible above.

- I've change the for 8 with no change
- I've swapped the coil on 7 and 8 with no change

So fairly certain not a ignition issue. Fuel pressure is a constant good PSI, in range. Doubt vacuum leak cause it gets worse at higher RPM and not surging with the starter fluid test at idle. Also doesn't seem to be one bank or the other, distributed across both banks. Haven't checked any injector related yet but seems unlikely due to only above 2K RPM.

I was thinking I might need to do a Crankshaft Error Relearn (CASE relearn). Anyone done this in these conditions?

I've attached screenshots of freeze frame data from P0300 code as well as misfire counts and fuel trims at low and high RPM (current misfire count on left, history misfire on right)

Twscarp

Hi, don't know if anyone can help, but it would be nice to know the vehicle model, year, engine size, and mileage.

David Purdy

Apologies for leaving that info out

2005 GMC Savana 3500 Base cargo edition
6.0L U Vin Gasoline
About 180K

StanVan

First, I tend to agree with you on the vacuum thought, but you might be a bit confused on one thing, and that is that there is less vacuum at higher RPMS when the throttle is open.

I tend to think it may be injector related. The fuel trim difference between banks 1 & 2 tell a tale.

David Purdy

Right, sorry for the confusion. With less intake vacuum at higher RPM (throttle more open) a vacuum leak would be less problematic and therefore less likely to cause a problem.

Any ideas how to test or verify an injector issue? I could run an injector cleaner through (though that's obviously not going to give any kind of definite diagnosis). I have a pocket scope and lots and lots of time, but not tons of spare money.

StanVan

And the injector cleaner I'd suggest is made by a company called BG. They make lots of stuff, but the 44K is tough to beat.

David Purdy

I'll look into injector cleaner but here's what I did that makes me think that might not be it:

So I attempted to swap the wires from 7 & 8 and in so doing mangled the wire to 7 (the supposed known good). Accordingly I went to AZ and got a fresh set of wires as I think mine are OEM from 180K miles ago....

Problem on 8 still there with new wires on 7 & 8.

- I scoped the fuel injectors on 7 and 8 and both look correct electronically with correct injector pattern looks even with pintle humps on both injecctors
- I scoped both signal wires on the ingition coil on 7 & 8 and they both look good, solid trigger waves
- I scoped the Camshaft Position Sensor and it has a solid 110ms square wave at idle and tightens up linearly with RPM increase, so looks like not Camshaft sensor.

Thoughts? Suggestions?

jimmydriver

Try disconnecting the battery for 20 minutes. That might clear the computer. Check wire connections for corrosion. Look under the hood in the dark for ignition arcing. Your ignition control module might be bad.

David Purdy

For long term solution, Crankshaft Variation Re-learn did solve my issue.

StanVan

Excellent. And thanks for letting us know.