Misfire loss of power when warm

[Inlet]

1984 WBX, I'm having an issue with a very rapid misfire that starts around 2.5k rpm all the way up, under load, about 3-4 mins of driving after a cold start. This is a bit hard to troubleshoot because it idles fine and doesn't present itself when just reving the engine (or at least I can't really tell).

I noticed a couple misfire instances like a hiccup about a month ago but I didn't think to much of it as it only happened a couple times over a 500mile trip, my gas milage was excellent 19ish mpg. Since then the misfiring has become pretty severe and its hard to get past 3.2k rpm once it starts. It doesn't seem to be running rich, no unusual smells, or smoke.


Things I've done:
I've checked for vacuum leaks buy spraying all the obvious areas.
Unplugged the o2 sensor, no effect, checked spray of all injectors,

Parts in the last 6000-10,000 miles:
fuel tank, hall sensor, capacitor trick to AFM, spark plugs, temp2 sensor, o2 sensor, catalytic converter,

Parts replaced about 400 miles before the first misfire:
Coil, Cap, Rotor, Air Filter, Fuel Filters, Plug Wires, intake boot, intake manifold runner boots, checked compression with good numbers.

Parts swapped out after misfire with no improvement:
Coil, Fuel pump.

Any ideas?

[Howesight]

A rapid misfire under load, but not at idle, suggests a weak spark in one cylinder. A heavy (or heavier) load increases the voltage required for the spark to jump the gap (ionization to be exact).

If this is the problem, then what might cause it?

a. Defective distributor cap: (Inspect for signs of arcing on one carbon post)
b. Defective rotor;
c. Defective spark plug wire or some way that wire is compromised (split, cut, tear, etc). Test all wires with your ohm meter.
d. Rotor not seated properly;
e. Dist. Cap not seated properly;
f. Faulty spark plug;
g. Fawlty Towers; (ask Basil)

It may be that the spark failure is occurring in only one cylinder, which the above comments are based upon. But it may also be occurring in more than one cylinder. To isolate this, you could create the problem on a lonely road, and after forcing the symptom to go on for, say, a minute, put in the clutch and turn off the engine IMMEDIATELY and remove and read the spark plugs to see which has tell-tale signs of a misfire.

If you cannot isolate the cylinder, it may be that the misfire is "roving" around all cylinders in which case, you should suspect the ignition coil. If you have access to a known-good coil, swap it in to avoid all the above diagnostics and check if this cures the misfire.
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'86 Syncro Westy SVX

[75 Westy FI noob]

i've recently got mine running again.. it starts easily and idles well. but, after about 3-4 minutes as it comes up to op temperature.. it will misfire here and there.. and it seems to misfire more as it gets warmer. hard to tell, but seems to be a single cyl. I've turned it off at that point.

Coil, ign wires, and plugs are new. I'm not thrilled with the wires tho ( STI brand ).

Are ignition wires the most likely culprit? Engine is warm but I wouldn't call it it hot. Valves are adjusted a bit on the loose side, so I don't think any valve is sticking open.

[Ahwahnee]

One trick that sometimes reveals faculty wires is to look at the running and misfiring engine in the dark... you may be able to see leakage.

But even poorer quality wires should be adequate when they're new.

[75 Westy FI noob]

It’s consistent. Is this a symptom of fuel pressure regulator? Maybe the fuel pump is only running on start..? Cold timing v warm timing..? Its. mech advance dist.

Thinking my Torque Converter or Auto Trans may be binding .. or bad generator..

Gonna charhe the battery overnight and recheck.

[Pchill2]

Could be something as simple as a cracked distributor cap.
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87 Syncro Westy
Honda K24 swap

[Wildthings]

Could be the Digital Idle Stabilizer, ignition control module, or the coil.

[75 Westy FI noob]

I just found this thread … looks like the goods


https://www.thesamba.com/vw/forum/viewtopic.php?t=550235

[75 Westy FI noob]

So, it was a vacuum leak.. i wrapped every hose connection with a quality rubber electrical tape. And now it idles fine when warm. Thanks to the Samba community for the guidance.