Misfire | 1999 Chevrolet 3500 Express Van | 5.7
This Chevy 1 Ton Van came in for an intermittent misfire complaint. The misfire would come and go and has been getting worse. The customer stated that the misfire seemed most noticeable on a cold engine before it hit operating temperature. They also mentioned that the misfire seemed worst at part or mid throttle and did disappear during full throttle. I noticed on my test drive that the misfire was inconsistent and random but it did feel like a sharp ignition miss most of the time that it was misfiring.
With the Tech 2 scanner and Snap-on Modis scope hooked up it was time to go to work. Looking at OBD 2 freeze frame data, there were no codes relating to this problem. The only code in the computer was p0141 B1S2 heater circuit code. There also were not any history or current misfires in the misfre data list. I hooked up my ignition scope and watched the ignition parade. I noticed my spark kv was high at idle averaging between 18kv
and 24kv. I also noticed that cylinder #2 seemed to be having a random but consistent abnormality. My thought was that this van needed a basic ignition tune-up to cure what looked like a #2 cylinder misfire. Since my ignition secondary pattern wasn’t 100% conclusive and the plugs and wires looked to be high quality replacements that appeared in decent shape, I decided to look at a little more data before calling the customer.
Fuel trims were a touch higher then normal for this van but still ok. I normally see the fuel trims on GM 4.3s and 5.7s in -7 range, this truck was at 8. Not part of the
problem but I did still check and clean the MAF and debris off the MAF screen. Everything else in the data look good besides the Cam Retard data PID. It was at 15 degrees. When you check the cam retard data PID, you need to idle the engine up to 1000 rpm for an accurate reading. I was pretty surprised how far this was off without setting a P1345 Cam/Crank Correlation code. I also notice a lot that when the distributor is off on the 4.3 V6s and 5.7 V8s that cylinder #2 is the common cylinder to misfire. You should also note that moving the distributor on this engine does not effect ignition timing and it only changes the gap between the ignition rotor and distributor cap.
I adjusted the distributor back to a cam timing of 0 degrees which may have been
moved after an intake gasket set was installed. The van now does not have any misfires and runs great. I also always take before and after captures, since this is the best way to learn and to verify my repair. The ignition spark kv at idle was now at between 10kv and 15kv. The #2 cylinder abnormality was also gone. Case closed.