I have a 1999 Blazer 4.3L with new intake gaskets, new head gaskets (runs extra good, no oil in the coolant, and no coolant in the oil). The engine idles perfect, starts great. New spark plugs, wires, cap, rotor.
When driving down the road at light throttle (cruise), the engine will misfire and shutter. Under hard acceleration the engine smooths out and runs excellent with normal power. The shutter happens at light throttle either hot or cold and gets slightly better when it is warm.
The engine is getting horrible fuel economy and the spark plugs were checked and they were very dark (too rich).
The error code says Catalytic converter, however the converter was just changed about 6 months ago. A shop recommended changing the EGR valve, but the engine is super clean with only 110,000 miles on it and I don't think it is the EGR valve causing the rich condition and the misfire/shuttering.
Seems like this random misfire issue is becoming more common with the 4.3.
I've got everything new, including a new billet dizzy with a little over 4,000 miles on it and I experience the same issues.
Random misfires only happen when truck is warm (not cold or hot) and has been driven under 6 ish miles. Misfires completely goes away once at normal operating temp. Cap/Rotor looks good, wires are new. No codes are being thrown, no freeze frame data is available with code reader. VCM is basically not seeing this issue.
So, let's throw out a hypothesis and see what you guys think.
We know that when the vehicle is first started and is working it's way up to normal operating temperature, the VCM is running in "open loop" mode.
The O2 sensors have not heated up and the Cat. has not warmed up yet. The base fuel map is being used to operate the engine and most controls are not being used to control the air/fuel ratio etc...
Once the O2 sensors and Cat. have heated up and all other parameters have been met, the system goes into "closed loop" and the air/fuel ratio will be adjusted (fuel trim) for lean/rich conditions based on the free oxygen in the exhaust stream (It gets deeper with short term/long term fuel trims but thats for another class).
Where I'm going with this is, could we be experiencing some anomalies (read that random misfires) when the system is transitioning from open loop to closed loop???
I've pulled my hair out on this one and cannot find the smoking Gun.
All of the ASE tech's PLEASE jump in here...