Question: Engine missing at light throttle

[JHStevenson]

I have a 1988 4+3, rebuilt engine 20K miles ago (20 over, otherwise stock), with Hypertech Chip, Borla exhaust. Recently, while at light cruise, the engine has developed an irregular miss - such as when using cruise on flat road and at regular highway speeds (65 - 75). With any application of the throttle to accelerate, the miss disappears. Engine is operating at normal temperature, and it makes no difference with or without the A/C running. At WOT, car runs like it should, no miss and puts a smile on my face.

I have pulled the plugs, checked for wear- they appear normal, checked plug gaps – all properly set. Plugs and wires were changed at engine rebuild –stock equivalent. Timing was checked (used book procedures – pain in the rear) found to be spot on.

What should be my next step?

 

[Tuna]

Next Step?

I'd clean the throttle body and put some fuel system cleaner in the tank to clean out the injectors.
Red Line or BG 44K work great for fuel system cleaning.

 

[JHStevenson]

I'd clean the throttle body and put some fuel system cleaner in the tank to clean out the injectors.
Red Line or BG 44K work great for fuel system cleaning.

Thanks - this was done about 3K miles ago, used my old and wise mechanic's favorite, BG products 44k. I'm not adverse to running another can through, so I'll try that and any other suggestions that might be offered.

 

[Tuna]

Thanks - this was done about 3K miles ago, used my old and wise mechanic's favorite, BG products 44k. I'm not adverse to running another can through, so I'll try that and any other suggestions that might be offered.

Don't forget the throttle body - 44K in the tank doesn't clean the TB.

 

[JHStevenson]

Don't forget the throttle body - 44K in the tank doesn't clean the TB.

OK -
Throttle body and fuel treatment with BG products - done. (done before new plugs and wires, after O2 sensor)
New plugs and wires - done (no old plugs had abnormal appearance, and wires no obvious arcing spots)
Replaced O2 sensor (that was fun) - done (had a slight run rich issue - appears to be corrected by new sensor.)

Results... Almost, repeat almost, no missing at light cruise. The car still performs much better in medium to heavy acceleration than I would expect for a near-stock 350. Were HP & Torque under-rated in the late 80's?

 

[Kevo]

Slight miss under light load is usually a lean condition. The egr valve under intake doesn't allow for much testing you could disconnect it and see what happens. Make sure maf is clean of any oily film.

 

[JHStevenson]

Slight miss under light load is usually a lean condition. The egr valve under intake doesn't allow for much testing you could disconnect it and see what happens. Make sure maf is clean of any oily film.

Never thought about the EGR valve -
Lean... and I had a too rich condition before the O2 Sensor replaced... Doing some head scratching now.

Thanks for the comment.