Stumble after Hot Issue

[smaleita]

Hello,
I having an issue with my 87 Vanagon (2.1 Digifant) since replacing the engine that I have spent way too much time and some money trying to fix. I am hoping someone has some further ideas. I have searched the Forum and can't seem to find the exact cause. The problem lies when the bus is warmed up. When it is cold, I put it in gear and it wants to go go go with instant throttle response in first then second and on with a lot of power. After getting hot driving around, it seems to wane on that response when accelerating in first from stop or around corners engaging second gear. I have to floor it and it eventually 'catches' and then starts accelerating like it should. Here are some things I have tried and nothing is working:

1. Replaced the temp 2 sensor twice.
2. New Coil, Wires, Cap, Rotor and Plugs
3. New O2 sensor
4. New Catalytic Converter
5. Soldered Tantalum 22Uf cap to fix Vanagon Syndrome.
6. Ran smoke test with no vac leaks.
7. Adjusted throttle switch and then replaced Throttle Body and switch with one from the old engine that was working before.
8. Checked S Boot - no cracks or leaks
9. Swapped Fuel pressure regulator with one from the old Engine.
10. Tested Fuel pump but I am not against expecting something there intermittently.
11. Replaced Distributor with one from old working Engine.
12. Replaced Negative Battery cable and cleaned and replaced Transmission ground strap.
13. Replaced some grounds in the engine compartment that I could find.
14. New Battery
15. Replaced voltage regulator in the Alternator.
16. Cleaned contacts in AFM - made no adjustments here.

Oh and 1 other thing, The injectors are not new but ones that came with the used motor. They were in the wrapper from the injector repair guy with graphs on their stats. I have not replaced these with new.

Over the course of all these things, I have fixed several issues that I found with some of them but still the Stumble remains after warm. Hoping someone has a silver bullet for me because I am about to pull out what remaining hair I have left.

Much appreciate any advice.
Thanks

[Steve M.]

#13.) Some Grounds...which grounds did you find?

Just want to make sure you got all of them.
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[smaleita]

That's a good question. Besides the main ones at the battery and the transmission, I cleaned and re-cabled these.

The large red wires were both to the block and I think they might be redundant connections that the prev owner installed. These are a few brown ones and a brown green, brown red one as well as the one out of the hall sensor harness.

Sorry that the engine comparment is so dirty. The PO did not clean it and I haven't gotten to the car wash with it since the engine swap.

[Wildthings]

Have you done an electrical diagnostic from the plug the ECU plug? What does it show when you test for a ground on the wire to the O2 sensor?

Your green looking ground wires likely aren't helping things.

[smaleita]

I have checked some things back to the ECU, Like Temp2. I haven't checked o2 to ecu. So are you saying that I should check green O2 wire for ground short? The Green wire in the pic is not going to ground. It is going to the coil. Sorry for the bad pic.

[Wildthings]

[smaleita]

OK, I'll try them all tomorrow. My O2 sensor only has a single wire with insulation -except for the pre-heat wires. I verified no short to ground and continuity to pin 2 on the ECU.

[Howesight]

[Wildthings]

[AceTaylor]

Slow to catch up?

IMHO, the air flow meter. Check all the values you want, but if the vane is slow to react when the throttle is opened the car just won't go. Forget about swapping out with a "known good" used one. You're just swapping old parts out. Bite the bullet and buy a rebuilt one. Sooner or later you're going to anyway and when the engine drives like new, you'll kick your self for not doing it sooner.

[smaleita]

[Wildthings]

[crazyvwvanman]

Right, the red wires are someones idea of a replacement for the original braided ground strap.
A possible PROBLEM is that brown wire sharing the bolt on the head with the 2 reds. It should not share. There is another threaded hole right there and the brown wire could easily be moved to the other hole. That brown wire should attach to engine metal with no other wires sharing that attachment.

That brown wire from the small wire bundle to the distributor is actually a critical part of the O2 sensor circuit. That is why it should attach to engine metal all by itself.
It comes from pin 19 of the ECU and also provides the ground for the O2 wiring green coax shield layer.

Mark

[mikemtnbike]

OP first, Mark knows more about van wiring than anybody else who posts here. Trust him and move that brown wire to it's own ground location.

Second, I'd replace those red wires with a nice thick dedicated ground cable. They come in a wide variety of lengths and are available at any FLAPS by the batteries.
_________________
1991 Vanagon GL 2.1 AT Westfauxlia. "Frankie" Totaled https://www.thesamba.com/vw/forum/viewtopic.php?t=764510&highlight=carnage
1995 Eurovan Camper "Marzivan"
2020 GTI SE manual

[smaleita]

OK guys! Sounds great. I'll give that a shot and let you know.
Thanks!

[smaleita]

So I ripped out those two red grounds in the engine compartment and replaced them with a thick battery type cable to the firewall using one of the nearby bolt holes for the non-existent A/C compressor. I re-terminated the brown O2 ground and cleaned a fresh spot with a new clean bolt. All is hooked up and, I feel, properly grounded. So when I drive it, I have the same issue as before stumble and slow to power when hot. For giggles and grins I pulled over and disconnected the O2 Coax, restarted the car and the problem vanishes. Full power from the stops and changing gears - no problems at all. We drove it around for nearly an hour with no problems. I think that this definitely points to a problem with the O2 circuit, correct??. I will test the wires from the ECU and see what that produces tomorrow. In the meantime, I can drive it, albeit with bad fuel economy and too rich.

What is the procedure for checking that coax cable? Just pin 19 to ground with little or no resistance? Then continuity between pin 2 and the green space connector?

[Wildthings]

I would say at this point this it is not the coax or the problem would have persisted unabated, instead something is throwing your mixture off in a way that the O2 sensor can not account for. I would do simple things first like add FI cleaner to your fuel and change the spark plugs and maybe the spark plug wires. You want every cylinder firing as close as possible to every other cylinder. Pull each wire from the distributor cap in turn and see if the RPM drop is the same for each cylinder.

[smaleita]

I had already replaced the plugs wires, cap and rotor. I wonder if my AFM has been tuned for no O2 sensor. I failed to mention that when I got the bus, with the old engine, the O2 Sensor was not plugged in. It ran great. Heck, I didn't even know what those wires were for back then. I plugged them in with a new sensor trying to put everything right when I swapped the engine. I wonder if the PO turned the bypass screw. What is the procedure to check that? Typically, I would stay away from messing with the metered air bypass because I haven't the tools or know how to adjust it correctly.

[danfromsyr]

unplug the O2 and see if it improves it's behavior?

[smaleita]