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  View original topic: syndrome possibly other hot issue?
howardvw Mon Jun 19, 2017 12:41 pm

2.1
new, temp 2 sensor with thermo, new wires cap rotor, swapped good afm and throttle with good clicking tps. swapped afm wire harness. swapped ecu.

starts perfect every time as did before any swapping or new parts.
when cold has perfect pickup speed, once thermo gauge gets to about half it does the famous running on 3 cyls hesitation etc etc.
if i floor the gas pedal it will catch and smooth itself out- i only realized this since i swapped the the throttle tps. but will return to issue.
and of course restarting will fix itself for the time.

1st i kinda want to know, specifically about the floored gas pedal, has this happened to anyone?
otherwise throw some ideas out there. thanks.

i suspect wiring is old and cracked somewhere in a line, but from what i see or whats connecting are all good wires. i have unplugged oxy but dont think anything happened.

Howesight Mon Jun 19, 2017 1:28 pm

Your symptoms as described strongly suggest that the cable to your oxygen sensor is cracked and shorting out.

When the engine is cold, the ECU ignores the signal from the oxygen sensor until a threshold coolant temperature is reached. Then the ECU begins to note the oxygen sensor voltage and "trims" the air fuel ratio. When the inside signal wire in the coaxial cable shorts out to the surrounding grounded shielding, the oxygen sensor signal is zero volts which the ECU interprets as a signal that the fuel mixture is too lean. The ECU lambda circuit then adds fuel in successive iterations until the actual mixture is so rich that you get the symptoms you describe.

The reason that full throttle application appears to "cure" the problem is that the full throttle switch (part of the TPS) signal to the ECU tells the ECU to ignore the oxygen sensor "reading". In newer cars, a short or oxygen sensor failure will not lead to runaway fuel trim as in the Vanagon because the ECU is programmed to add only so much fuel before a diagnostic trouble code (DTC) is displayed and the ECU goes into "limp home" mode.

Have a look at the pictures in this thread to see where the short in the oxygen sensor wire normally occurs:

https://www.thesamba.com/vw/forum/viewtopic.php?p=6181799

Christopholi Mon Jun 19, 2017 1:32 pm

X2 on O2 sensor. Had a very similar set of circumstances on my 87 Westy. Would go into limp mode, but stomping on the gas would fix it. Turned out to be a bad sensor wire.

Ahwahnee Mon Jun 19, 2017 3:09 pm

Howesight wrote: ...When the engine is cold, the ECU ignores the signal from the oxygen sensor until a threshold coolant temperature is reached. Then the ECU begins to note the oxygen sensor voltage and "trims" the air fuel ratio...

Another clue to a bad sensor wire is that mileage will drop to single digits.

What MPG are you seeing?

howardvw Mon Jun 19, 2017 3:36 pm

Don't really know yet. This only happened cause i changed my clutch. After putting back in and connecting this happened. Been about 3 days troubleshooting with short 5 mile trips. So far.
Gonna check on Oxy tomorrow.

It's an 86 with tan colored outer wiring sleeve covers and orig ide bet. Lots of dry cracking I'm noticing.

howardvw Tue Jun 20, 2017 6:35 am

Did not fix. I cut and put new ends on the green wire. Wire wasn't bad but made sure its good.
Other suggestions wanted.

I can unplug oxy wiring and no difference.

Only happens when engine is warm while driving. I have yet to happen at stand still with 3000 rpm or so. I usually drive till happens come home and then mess with as long as issue is happening.

howardvw Tue Jun 20, 2017 7:34 am

fully disconnected both the green and the click connect, and runs fine.

so assuming,
new oxy, new cat, new muffler, should fix.



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