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Backfiring! Not me...the vehicle

I have a 1992 Chevy C30 Step Van. V-8 with automatic overdrive transmission. When driving in hilly terrain at around 70/75 mph, the van will lose some speed and when it gets to about 60/65 mph it backfires and seems to lack enough power to downshift. I’ve tried not using the overdrive selection and it does seem to do a little better, but stil backfires some. I’ve changed rotor and distributor cap and tried higher octane fuel. Your thoughts?

While you replaced the rotor and distributor cap, did you check the timing? Another probable cause is defective wireset and/or plugs.

For year 1992 I am going to say this is a TBI injected vehicle. I suggest you somehow get a look at a factory service manual and take a look at the flow chart for your symptons. When troubleshooting in a manner that has a stratagey (the prefered way) you need to look at what has come out over the years in the area of known defects with the systems on your car. I would hope you are not still on the original fuel pump? Fuel pumps are are part that can check out fine (as they are not acting up when checked) but still be the cause. I would get a manifold vacuum reading. It sure would be nice to have compression raedings so if this turns into a sparkplug change, keep this in mind. In reality the work getting compression readings probably will not bear fruit, but just perhaps.I am not ready to point you towards suspecting a partial plugged catalytic converter as your symptons just do not match good enough.

In short,verify fuel injection type and follow flow chart for your symptons. Backfiring through intake can at the minimum indicate too much fuel being sent down the pipe or in the extreme indicate a fault in valve timing(as in chain slop) or sealing. Perhaps just the manifold vacuum reading will be enough to give you a “state of mechanical health” determiniation for your engine.