L98auto

My '86 is suffering from a stumble (un-drivable above idle) issue. The car has about 97,000 on the clock and other than consumables like fuel filters, air filters, plugs, cap, rotor, wires, etc. over the years, its never needed anything.
The car starts up fine and idles fine. It revs ok in neutral. Given throttle under any load, whether driving/moderate normal acceleration or braking against the converter in Drive, the engine with stumble and sometimes backfire through the intake. Return to idle and everything's fine. Happens cold or warmed up. The car will idle down the street in gear fine but as soon as any throttle is needed, it begins stumbling. Which direction should I be looking in? I imagine the fuel pump is probably due soon, but I've only experienced them dying completely, not partially. No codes present.

L98auto

Any ideas?

ULTM8Z

What you're describing sounds like when I had a broken spark plug wire

Recommend taking an ohmmeter to your wires and see if any are bad.

Vets-Vet

Maybe a fuel pump or delivery problem. Idles / starts fine because there is no need for a lot of fuel. Revs OK in neutral, still not a lot of fuel needed. Also check the fuel pressure regulator vac hose to see if the diaphram is leaking gas past it into the vacuum line.

Any codes ?? What is your fuel pressure ??

Have you timed the car recently ??

Make sure timing wire / EST connector is still connected.
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Cruisinfanatic

I agree with previous post. Bad cap, rotor, wire(s) or fouled plugs(s)
I chased the same as you describe for a year. Ended up being a wire not seated correctly on cap

vader86

Sounds like bad plug wire, one is touching the exhaust or isn't snug on the plug.

Incorvettei

Could be all of the above but I would look at timing first.

Vets-Vet

""Sounds like bad plug wire, one is touching the exhaust or isn't snug on the plug.""

That #6 I am never sure about.

ULTM8Z

I'd start with the side that corresponds to the O2 sensor...

When I had mine, it was on the opposite side. I could mildly accelerate, but if I gave it any significant amount of throttle, it would backfire out the intake. But hte ECM wasn't reacting to it because it couldn't "see" what was happening in the exhaust.

However from what the OP is describing, it sounds like the ECM can see it and is probably leaning the mixture out considerably due to unburnt oxygen in the exhaust.... which leads me to speculate that the bum wire/plug is on the O2 side of the engine.

L98auto

Had a warm day today so I got the car out to check it over some more. Plugs look good as does the cap and rotor. The wires, which were properly routed, don't seem to have any bad spots either. I did notice that stabbing the throttle in neutral, from idle to about half pedal or more, would also cause the engine to stumble, buck and backfire through the intake as long as I held it there. Slowly bringing up the revs wouldn't do this. I'm starting to think fuel delivery.