86 Blazer Missing - only when moving

Hello all.......I am looking at a friends Blazer (V6) for him. It starts to miss at normal operating temperatures when you give it anything but just a little gas, but only while the vehicle is actually moving. While parked it revs just fine and when cold it it does not miss while moving.

I have not started to troubleshoot yet and wanted to get your advice.

Thanks

Tuned up recently?

If missing only under heavy load, could be poor spark caused by bad plugs, bad plug wires, bad coil, bad connections at coil.

I had similar problem when stepping on gas more then little or climbing even minor hills. It was weak coil, shorting out inside itself, putting out little spark.

Hey thanks for the responses......(Cam1959) that all makes sense, but what about it only happening while moving and not at all while parked? Thanks.

Hey thanks for the responses......(Cam1959) that all makes sense, but what about it only happening while moving and not at all while parked? Thanks.

Very little load while revving parked, meaning less resistance to combustion, needing less strong of a spark.

Might also be your compression has dropped, meaning piston rings have gotten ward from age and/or dirty oil. Try 5w30 or 10w30 full syntetic as mine doesn't burn that, meaning it doesn't slip past rings. regular oil it burns past rings a little.

I asked if it had been tuned up recently because most likely the component causing the issue is one that would be replaced during a tune up. (As Cam mentioned) it is probably somewhere in coil to the plugs. When at operating temperature the wires or coil are probably breaking down and giving weak or no spark.

I also would say the fuel filter would be a good idea to change, that could be the problem and it never hurts to change it anyway (plus its pretty cheap and relatively easy)