URS4 - Idle issue

[PRY4SNO]

Did a motor swap (with no prior issues) into a clean chassis and car won't run without a bit of throttle. The idle will wander downwards and then die. When I try to gas the shit out of it and drive, it'll hesitate and jerk a bit before cutting power and coming back on. If it doesn't just stall.

Tried unplugging the ISV and no change.

There was a 1/4 tank of gas left, it's about a year old, so I added 25L of fresh fuel. Didn't have a chance to drive much and mix the fuel around, but the problem persists.

Where should I start looking tomorrow?

Is it possible the after-run pump and ISV harnesses got swapped? My friend Rob (the PO) suggested that may be a culprit.

Any ideas are appreciated!

(FWIW, the motor ran strong before and didn't have any issues like this.)

[PRY4SNO]

Tried maf it started and died a half second later.

Checked engine bay grounds and all three had juice.

Just pulled plugs... all were black with a .025 gap, oe bosch. Its oily but I knew the VC needed done, never had any problems tho. Wanted to run the gas out and do the oil change, vc plugs and fuel filter together.


Disconnected the hose that goes from under the isv into the intake manifold and we have a strong high idle.

It kinda hunts/blips itself around 2k rpm. If I touch the gas it wont lose revs.

Isv tested as good before on a friends car (morris400) so I'm leaning toward the wiring harness?

[PRY4SNO]

Talking to myself lol

Swapped the coolant after run pump switch and ISV switch with a buttery idle as a result. Went for a rip around the block, but there's no power...

N75? Evap canister?

[chaloux]

N75, major boost hose not connected, etc... You're on the right track lol. Do the stock Motronic ecus have limp mode?

[PRY4SNO]

Yeah I'd guess the old school Motronic has limp mode ala Nick's 3b swap...

I'll look into the N75 tmrw more closely. And the boost/vac tract integrity. Note, the inlet boost hose (TIP @ MAF) is held on by a zip tie lol. Albiet, a very tight, industrially-sourced, zip tie haha

In the meantime, my cayenne cq's clutch or trans took a shit on the way home so I have 99 car problems at them moment

[chaloux]

Damn. At least you can still drive the s on spring pressure. I thought for a while that my wiring for the n75 was somehow messed up (swapped engine problems) but it turned out my n75 was just dead. Make sure it didn't somehow come unplugged lol

[PRY4SNO]

Good call.

Will a stock b6a4 n75 work? I might have one somewhere...