Just started doing this recently. Sometimes its really bad, sometimes not. I've only noticed it below 2000rpm, in 2nd or 3rd gear. I thought, from some other symptoms I was having, that it was the FPR so I took that off and as Murphy would have it, it was fine. Replaced it anyway, but the problem remains. AFR spikes most of the time from a 'normal' 17-18:1 down to about 12:1 when it does it. I checked the timing, fuel pressure, etc. All seems ok.

Possible vacuum or exhaust leak causing this?

As for the car, its basically a stock 89 305 TPI (close enough anyway) with a Vortech and SLP headers.

 

I had that happen once and it was an exhaust leak. Maybe you can get your car up in the air and let it run while checking for a leak.

 

Exhaust leak can do it as he said.

Also check your overun areas and see what the fuel and timing look like there.

Overrun is like when your doing 75 and let off the gas and your map is reading 20kpa....(Note to self: datalog a MAF car during decel)

Play with the fuel and timing in those areas, going 12:1 your defintiely going rich and dont need that at all on decel. Try making it like 14-15:1 and see what changes. If you pulled alot of timing out already, try adding some back in.

Also, not sure but I believe you also have some kind of decel fuel cutoff on your ecm, might want to see if that happens at the same time or if it is unrelated.

later
Jeremy

 

I had that happen to me recently as well due to a leaking collector.