This 2nd Time Around is a recent one, that involved cross-referencing two concurrent threads. Since as a general matter good grounding has great importance to keeping any electronically-controlled car running right, I thought it worth putting here:

A forum poster said:

(The 87 Tintop 2.1 wbx Vanagon) starts smooth and will run nice – able to smoothly increase rpms with throttle – after a few minutes it starts to stutter and run poor – if i add gas it tries to keep going but will eventually stall. When it stalls it will start right back up like nothing happened – poor idle and power loss will resume a few minutes later.

Sounds a bit like a main ground circuit reference voltage offset. Here’s a little test without digging into things that may confirm this: if you warm it up, then restart and let it idle, note how the idle deteriorates and the engine dies and how long that takes. Do it a couple times so you have a feel for the pattern and timing, and to note how it repeats itself. Then restart again and turn the headlights on high beam and heater fan on high. Does the same pattern happen but noticeably faster? If so, then there is a lot of resistance at main grounding points, especially the battery ground wire.

This is causing the voltage signal from the oxygen sensor to look higher to the ECU by the amount of ground voltage offset being created by resistance at one or more ground locations.

Normally, an O2 signal V over about 0.6V corresponds to a rich mixture so the ECU will remove some injector time on every engine cycle to bring it to a leaner mixture, which would normally cause the O2 signal V to dip below the 0.45V midpoint, after which the ECU would add injector time cyclically until it enrichens, and on and on back and forth. But if there is just two or three tenths of a volt offset at a resistive ground, that is added to the actual, isolated O2 signal V, so it never dips below the midpoint V as seen by the ECU, so the ECU keeps on leaning the mixture without seeing an O2 response, and it goes leaner and leaner, running weaker and weaker until it dies.

Turning on large electrical loads increased the current passing the bad ground points, which proportionally increased the voltage loss there, making the increase in O2 signal V at the ECU even higher, so the process happens more quickly. That is, as long as the original no-load V loss at a ground connection was less than the O2 signal V when slightly lean. If the ground offsets were very high, then increasing the load won’t accelerate the leaning-out process, it would already progress as fast as it can happen at idle speed. In that case, raising idle speed manually would accelerate the process since it’s a cycle-by-cycle addition or subtraction of injector open time.

But since the deterioration of ground quality is gradual, when you become aware of the problem the ground voltage offset is still probably low enough that the test of adding loads will probably speed up the leaning-out process.

The last time I recall writing about this was in this thread, I suggest you make all the wiring improvements I describe because they will improve function of all the electrics in the car whether it solves the present problem or not.

[The relevant post of mine that I linked to started by talking about improving the rather weak positive power wiring from the wbx alternator to the starter and to a connection point in a junction box where the engine wiring harness gets its power. The original poster wanted to know if he could connect an accessory power wire to that  point.]

If I were you I would start with separating the alternator trigger wire (blue) from the alternator bundle, and remove the paired wires from the alternator to the starter and the J-box. Replace with a nice fat single 4 or 6ga. red wire from alt to starter (just buy a ready made battery wire at a FLAPS, most have a very wide selection). Then run 8ga. red from the starter post direct to the J-box post. Then you could put as much as 20A extra on the J-box post if you wanted without inducing any supply voltage drops for the engine essentials. If it’s a fan or something that uses more than 20A, just come off the starter or alternator post directly, especially if you don’t do the wiring improvements I’m laying out here.

And while you’re working on wiring the high-current stuff the way it should have been done if VW didn’t have such a lame-ass electrical design dept. (always their weakest work, they never account for the current their systems use when wiring is new, never mind as it ages), add another nice 4 or 6ga. wire to match your hot one, black this time, from the alternator chassis directly to the bodywork. While running, the alternator is the highest-current item in the car, and most of the grounding issues that affect engine operation stem from the lame little engine ground strap carrying a goodly part of the charge current, inducing a big ground reference voltage change. This is extra bad when the trans nose strap is in bad shape, which it usually is because no one knows why it’s there, or even that it is there at all, when its purpose is to handle all the starter and alternator current. Instead take that current to ground by its own, high amperage route, and the engine ground reference voltage is stabilised. Extra points for ganging another 4ga or larger from the top starter mounting bolt to your same body connection as your new alt ground, and the transmission is no longer a major conductor.

To which a well-known and knowledgeable member replied:

The way this is worded makes it sound as if the alternator is a consumer of electrical power instead of the producer of it. I’m confident that tencentlife didn’t intend that. I’ve seen a lot of misconceptions about electricity in this forum from people without an understanding of the subject, and didn’t want this thread to perpetuate the idea of the alternator as an electrical consumer. 

Just to be clear, the alternator is the ultimate source of all the electricity in the vehicle, not a consumer of it. The battery is a “storage reservoir” for electrical power, but its source of power is also the alternator. 

Good points, so my followup:

Fair enough, if you think that needed clarifying for this crowd, then I endorse it. Point is, current is current regardless of direction, the alternator circuit handles the most current while the engine’s running (every amp used anywhere for any reason is produced by the alt and has to travel that circuit), the ground side has to carry every amp the hot side does (ditto), and both sides as wired at the factory were marginal when new and are probably inadequate in many aged vans. 

For my big alternator and starter ground connection , I used an existing hole in the engine bay bulkhead just in front of the airbox. Grind a spot around the hole to bare metal, then an M8 stainless bolt, nut and serrated washer are put there as a post to connect my big alt and starter ground wires. High and dry, easy to maintain, didn’t have to drill any holes.

The alt ground can attach to the end of one of the M5 bolts that sandwich the alt chassis together, so use a small eye there (1/4″ or #10). Use a 3/8″ or 1/2″ eye for the M10 top starter bolt connection. 5/16″ eyes to connect to the M8 ground post. 

That work won’t do the most good unless you attend to the remaining high-current ground, and the most important ground on a vehicle because every amp passes thru it, the battery. The stupidly designed-in weak point there was how they welded a nut in the seat pan, painted the van, then used a thread-cutting M8 bolt to fasten the ground strap. So many problems here: firstly the contact area isn’t good, the eye lays on paint so all contact is exclusively by the threads. The undercoating covered the bolt tip sticking down into the wheel well, but the tit of undercoat breaks off and the tire throws road moisture at the exposed bolt. Corrosion creeps in and is invisible from above, in case anyone wanted to inspect the connection. I’ve seen voltage drop at that point cause the most unbelievable running issues, such as a progressive O2 sensor offset that made the engine run leaner and leaner and leaner until it dies, then it’ll do it all over again after a restart. 

Do a fresh ground strap or wire, make some bare metal for the eye to lay against, and a new stainless bolt and serrated washer there. Refresh the undercoating from below. 

Do all that and you have a very robust grounding network for the highest current items in the van. The starter, headlights (which should have their own direct-to-chassis grounds, not via the overloaded rosettes), everything will get full voltage and work its best, and anything that depends on a zero ground reference (as in every sensor and sender on the car) will be referencing the same value so a lot of odd behaviors disappear.