Motor hesitation...I'm at my wits end....

Combo is a 302/5.0 with the Mass-Flo EFI system on it, all Mallory ignition components, Taylor Spiro-Pro 8mm wires, brand new Autolite spark plugs gapped to .050 (low compression, around 9:1). Problem started 2 weeks ago, after a 4 hour road trip. Was running fine before the trip, when I got back, driving around town, at low RPM it felt like the car was pushing back at low RPM; above about 1500, it smoothed out and picked up. Played with the timing a bit, now it shakes REALLY bad at low RPM, definately feels like it's missing. Also, when at low RPM, if I push the throttle more than half way, it starts a really loud clatter, spits out a huge cloud of exhaust, and fights to accelerate. Again, once over 1500 RPM, it smooths out and picks up just fine. From my knowledge, and local mechanic friends, I started small in the ignition system, replacing the plugs first, then wires, distributor cap, and finally coil. I'm put the timing +/- up to 4 degrees from where I had it, no help. I can't think of what else it could be. I don't think bad gas, I've put new gas in it. It's been running pretty rich for a while - could it be messed up O2 sensors? Any help/ideas are much appreciated, I'm at a loss....

It sounds over rich at any rpm under 1,500, look into fuel measuring or pulse width on the injectors...

Test your TPS sensor if you can.

You may also want to make sure that your Catalytic converter isn't clogged.

Crappy tank of gas, low fuel pressure....cloged fuel filter.

Well... changed the O2 sensors, no help. Put in completely new tank of gas, checked fuel filter (no clog), plenty of fuel pressure, still nothing. No cats on the car (1968) so that's not it. The guys at Mass-Flo suggested that it could be one of the modules in the distributor, to start with the TFI. It's a high-end Mallory distributor, though, not driven very hard and less than 2 years old, so I don't know why that would have gone bad. It's the only thing left in the ignition system, though, that I haven't replaced. I noticed now that I can feel it missing at higher RPM also, but it's not as pronounced as at low RPM, where it's very violent. At high RPM, you can't tell on the tach, but at low RPM you can see it jump when it misses/shakes. Any other ideas before I replace the distributor? How do I check the TPS sensor - they seemed to think it wasn't likely, but it could be the problem? All my resources for ideas are running out....

Spark Plug gap is a little to wide.

What auto lite plugs are you using?

Hmmm, after your second description I am skeptical on the TPS sensor as well...

Yeah, what's the whole story on the engine.

Is your distributor dry? Is there moisture getting in the cap causing a low rpm misfire?

Like dfree said are the plugs gapped too wide? How do they look, did you read them?

Could be broken wires, or an intermittant coil as well....

What kind of Mallory distributor are you running? I have a dual point mech advance and I had to throw in a new set of points pretty much right away after I put it in the car, I was having about the same symptoms you are. I know you have EFI so is that the Mallory Uni-Lite? What about the....oh drew a blank...its the circuitry part down below the rotor area, I know its a common thing to go out in HEI distributors....hal switch or something...been awhile since school...I know Mallory has a tech section on their website so maybe check that out. I guess what Im saying is that it could be an internal circuitry of the dist, or cause its Mallory, I had a problem with a brand new one and one of the mechanics I used to work with had a problem with a brand new distributor from them also....

Another common thing on the fuel injected 302 is plenum gasket sucked in. creating a vacumm leak. A vac leak will be worse at idle and better 1500 rpm plus. A vacume leak will tell the computer there is too much oxygen and try to add fuel to compensate. To diag you Can spray at idle with brake kleen propane etc around base of plenem if rpm increases or smothes out there is your problem.,O2 problems and tp sensors and other computer usally will trip a light. I see it is a MAF system another thing that can cause the computer to see the wrong thing is a dirty MAF remove it and make sure nothing is touching the little wires. Look at the wires both sides if one side is shinny and the other dull your maff is very dirty and not telling the computer acurately how much air its getting. Spray with brake Kleen or sensor cleaner and blow with air too clean.

Good luck.