My 1993 Manual EX has a bad hesitation when the throttle opens under load. It feels like the engine dies and engine-brakes for one rotation then goes again. It's most noticeable when shifting and I push the gas pedal with the clutch more than one third of the way engaged, but if I ride the clutch out until the throttle is opened enough to accelerate there is no hesitation. Not riding the clutch going from a stop will cause a hard stall as if the engine doesn't have enough momentum to rotate through the missed cycle . If I'm cruising at part throttle and the stomp on it, the engine will stumble there too. If i open the throttle plate by hand, it can open slowly, but when whipping it open you can see the engine twist a bit on its mounts as it stumbles. I've had this car for four years and it has only been doing this since this Spring, so I know it isn't me shifting wrong. Once the engine gets over the stumble it Most of the work I've listed has been done this year.

237,000 Miles
Belts and water pump replaced two years ago.
Re-manufactured dizzy installed three years ago.
Last Winter the car struggled to start in the cold as well as stumbling. ECU was throwing #4, #8, #9, #12, #14 error codes. Replaced ECU with a refurbished one from All Computer Resources on Ebay. All codes except #12 have yet to return.
#12 CEL code. EGR Control Solenoid Valve is dead so the EGR valve stays shut. The valve itself works properly. The light comes on after 5-10 minutes of driving. Always comes back after resetting ECU. Ports have been cleaned out.
Cylinder Head replaced ~30,000 miles ago(old one cracked). Head was machined flat after overheating and new head, intake, and exhaust gaskets.
Catalytic Converter blew out it's internals in a cloud of dust several years ago. Muffler just replaced with a Bosal unit after rusting out.
Oxygen Sensor replaced.
Valve lash adjusted.
Spark plugs are new NKGs. No sign of oil on them.
Spark plug wires are in good shape. Resistances are below 12000 ohms.
Spark moving through my spark tester is a dull orange.
Cap and rotor replaced.
Ignition coil replaced.
Ignition control module replaced.
Ignition timing reset to stock.
TPS replaced and calibrated.
Throttle cable tension is good.
Mechanical timing is good. When I replaced the old MLS head gasket with a composite one the timing belt had to be re-tensioned. Crank, cam, and balancer pulleys were aligned.
PCV valve and hose are fine.
Throttle body, IACV, and FITV cleaned with throttle body cleaner and seem to work properly.
MAP Sensor voltages are within specs and responds to vacuum test.
Injectors new-re-manufactured.
Fuel filter replaced.
Fuel rail pressure is a steady 45psi.
Fuel system controller(grey box on driver-side wheel arch) has good voltages.
Black box under intake manifold is in good shape. Replaced the vacuum solenoid on it since it was melted solid.
Vacuum hose running the that flap in one of the air intake tubes upstream of the filter removed and capped. No change.
All vacuum hoses replaced.
Air filter replaced.
Coolant system burped.
Front engine mount replaced. Other mounts are in good shape.
Transmission is in good shape, shifted much smoother after replacing oil.

I may have done more stuff the the car but I don't remember anything else. I think it's an air/fuel problem, but I don't know of anything more to try.

 

Has the Oxygen Sensor ever been replaced?

 

Changed the Oxygen Sensor this Summer. I'll add it to the main post.

 

You've replaced so much stuff that it's hard to say, but these things stand out on your list:

• Re-manufactured dizzy installed three years ago
• Spark moving through my spark tester is a dull orange

Three years is about average for remanufactured distributors. Some fail even quicker than that. At least that has been my experience.

That color spark is weak and will cause erratic ignition and eventually stalling. If your Ignition Control Module (Ignitor) is strong, it should put our a spark that is whiter and therefore hotter.

If you are sure of everything else, I would zero in on that distributor; but check it before buying anything for it. It could be that the rotor button and contacts in it are bad, or gone and the ignition system is getting spark, but struggles when you increase the demand by stepping on the gas.

Likely, the remanufacture involved the cheapest ignitor (the most expensive component in the distribuor) that they could put in there. I stopped using remanufactured distributors after getting stuck with hard starting on bitter cold winter mornings a number of years ago. It was the cheap ICM needing to be hit with current a bunch of times before getting warm enough to produce the spark needed to fire up the engine.

 

Have you timed this engine with a timing light? Mine did this and it turned out that the ignition timing was retarded. I advanced it back to stock and it’s fine now.