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On my stock 86 blazer with a 2.8, I have been having an issue if I totally take my foot off the accelerator above 35mph.

The engine drops to what feels like almost no rpm, then surges back to normal in cycles. It doesn't do it below 35 or while giving even the tiniest throttle over 35. It has new plugs, wires, cap, rotor, coil, filters and runs fine otherwise.

What should I check? My fear is being broke down somewhere when I could have just replaced the flux capacitor or whatever and prevented it. I recently had the cat removed and put on a flow master 50 series, it did add noticeable power, but the blazer was doing this for a week or two previously. It still had the factory cat on it at 225k miles, though the engine was rebuilt a few years back. I thought maaaaaaybe it would help with the dying/surge issue.
 

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I'd suspect either the idle air control (motor) or the throttle position sensor. Do some net searching, the IAC can be cleaned and a drop of light oil helps, the TPS would need adjusted, or replaced.
 

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Most likely someone's messed with the hard stop screw and disturbed what's called Minimum Airflow which is absolutely necessary to have for proper transition when the IAC goes to retract on decel - with the IAC bottomed out in the bore and disconnected idle speed should be around 450-500rpm
 
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sparkybuoy
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